How good a plane was the P-40, really? (1 Viewer)

[Shortround6]

The timing, however, I believe my recollection is more or less correct, as he discussed the Battle of Britain as interfering with his ambitions--whether that's a self-serving hindsight, a faulty recollection or something else...I leave up to individual interpretation.

There was some discussion of putting a 2 stage Merlin in the P-40 in Dec 1941 and early 1942. Both LTC Bogart, chief of the technical staff section at Wright Field and LTC Carroll, chief of experimental engineering section at Wright Field were on record in writing as of Jan 19, 1942 as being against it.
This is 1 1/2 years (almost) after the BoB so it is a little hard to blame the BoB. That is assuming that the Merlin 61 was actually hardware in Aug/Sept 1940. Spits were getting Merlin XIIs and Hurricanes started getting Merlin XXs by Sept 1940. first flight of a Wellington VI with Merlin 60 engine was in Oct 1941 (?).

June 30th 1941 saw the first flight of the XP-40F (modified P-40D airframe) with a Merlin 2 speed engine. Deliveries of the P-40F started Jan 3rd 1942.
I think somebody many have confused 2 speed and 2 stage.

The P-40 always seemed to be 1-2 years behind in the state of it's engines. A P-40 in the summer/fall of 1940 had an engine almost as good as Spitfire I had in the spring of 1939.
The P-40E in the summer of 1941 had an engine that was not quite as good as the Merlin XII in the MK II Spit in Aug/Sept 1940.
Unfortunately the Allison got kind of stuck and the late model Allisons in the P-40M and N were nowhere close to the Merlin 45 engines in Spitfire Vs of early 1941.
The P-40F & Ls got the equivalent of the Merlin XX used in the Hurricane of late of 1940.
Both engines crossed over a bit with emergency power or WEP.
The P-40 suffered due to weight for the installed power. It is seldom given credit for it's streamlining. Perhaps because it is compared to the P-39 (a smaller airplane) and the P-51 which was a generation later than just about anything else that flew in WW II.
The P-40 was very, very good at the speed it archived for the power it had given how much it weighed. It just wasn't as good as the P-51.

The P-40s did a lot of good work in NA and Italy. In part because they were given top cover by Spitfires and P-38s, pretty much starting around the time of Operation Torch.
P-40s had done a lot during much of 1942.

 

[Geoffrey Sinclair]

Using Rolls Royce figures, Crewe began production with Merlin III in June 1939, switched to Merlin X in October, restarted III production in March 1940. The first production Merlin XX were built at Crewe in July 1940. Glasgow started production in November 1940, Ford in August 1941, both with Merlin XX. Derby did all the Merlin I, II, VIII, XII and 30.

Derby was producing Merlin 60 from November 1941 to April 1942 with 75 built. The Ministry of Aircraft Production reports 14 Merlin 60 in stock in December 1941. Merlin 61 were produced starting May 1942, ultimately 724 built, Merlin 62 from April, with 95 built.

The first 19 Spitfire IX were produced in June 1942, there had been 102 Merlin 60 series engines built by end May. There were 29 Wellington VI built January to June 1942. The period June to December 1942 saw 781 Merlin 60 series engines built, Spitfire production in the time period was 7 VII, 14 VIII and 388 IX. Another 35 Wellington VI built October 1942 to February 1943.

The P-39, P-40 and Hurricane had their service lives prolonged due to a number of factors starting with problems building enough of the newer designs, the P-51 was considered in short supply during 1944. The rise of the fighter bomber, the way the older fighters could still be a threat to enemy bombers and the need to stock the training system also contributed.

The 12th Air Force P-39 and P-40 groups were transferred to India or had P-40 replaced with P-47 December 1943 to July 1944, MTO Merlin P-51 operations began in April 1944, in the end the 12th had 6 P-47 groups, the 15th 3 P-38 and 4 P-51. The 8th Air force had removed its final P-38 fighters in mid September 1944, the 9th Air Force changed one of its P-38 groups to P-47 in February 1945, another to P-51 in March leaving it with 1 P-38 group.

Even the US had problems equipping its combat units with the latest versions, for example 31 March 1945, 15th Air Force B-24 combat unit strength return, 38 G, 129 H, 345 J, 217 L, 137 M. The strength return of 24 June 1944 reported 166 P-51B, 66 P-51C and 53 P-51D, on 10 January 1945 the figures were 65 B, 99 C, 210 D, on 30 April 1945 it was 46 B, 62 C, 251 D. The 15th Air Force only did 1 P-51 fighter bomber operation, on 19 April 1945, dropping 1 ton of bombs.

In 1944 deliveries of P-40 were 1,373 for the USAAF, 261 Australia, 41 Brazil, for P-39 it was 815 USAAF, 75 France, 811 USSR.

In hand waving terms,
P-40N-1 400 built starting Mar-43, armament reduced to 4 guns, later restored
P-40N-5 1,100 built starting May-43, to 27 inch wheels, provision for wing bombs and fuel tanks
P-40N-10 99 built starting Aug-43, winterisation equipment added
P-40N-15 377 built starting Aug-43
P-40N-20 1,523 built starting Sep-43, to V1750-99 engine
P-40N-25 500 built starting Feb-44
P-40N-30 500 built starting Mar-44
P-40N-35 500 built starting Jun-44
P-40N-40 220 built starting Oct-44, to V1750-115 engine, back to 30 inch wheels, metal covered ailerons.

 

[tomo pauk]

These airframes were ordered after the first P-51B series, yes, but it is also worth acknowledging the teething troubles the P-51B series was going through at this time (as is typical for any new production type), and that it was never going to be a primary service airframe in the MTO during this timeframe. Besides that, arguably, the P-51B-10 and similar were not especially well-suited to the MTO theatre, whereas the P-40 was never going to thrive in the ETO as a pursuit aircraft or escort.

There was a lot of escort job to be done in the MTO, too. WAllies were hard pressed to make the long-range escort work, even the A-36s were pressed in that job (covering the B-25s) during 1943. Having the P-51B around in 1943 would've meant that there is an all-altitudes long-range fighter around that is not worried about the enemy bouncing it from above, a thing that it was a problem for the P-40s.
Unavailability of the capable LR escort fighters meant that invasion of Italy could not encompass Naples that have had the excellent port facilities.

Basically - any fighter job the P-40 did, the P-51 did even better, while vice-versa is not true.
Main problem with the P-51B was it's unavailability when it was needed during 1943 (yes, it also needed sorting out the undercarriage cover system).

Anyways, for a slight tangent, but I felt it might be worthwhile to share a general characteristics scan I took for a P-40N-1 several years ago. You'll have to forgive the low quality of the image, but I felt this may be an intriguing, if fairly banal, item for you folks.

Facsimile of an original document is always well regarded on this forum

 

[Think Tanker]

Using Rolls Royce figures, Crewe began production with Merlin III in June 1939, switched to Merlin X in October, restarted III production in March 1940. The first production Merlin XX were built at Crewe in July 1940. Glasgow started production in November 1940, Ford in August 1941, both with Merlin XX. Derby did all the Merlin I, II, VIII, XII and 30.

Derby was producing Merlin 60 from November 1941 to April 1942 with 75 built. The Ministry of Aircraft Production reports 14 Merlin 60 in stock in December 1941. Merlin 61 were produced starting May 1942, ultimately 724 built, Merlin 62 from April, with 95 built.

The first 19 Spitfire IX were produced in June 1942, there had been 102 Merlin 60 series engines built by end May. There were 29 Wellington VI built January to June 1942. The period June to December 1942 saw 781 Merlin 60 series engines built, Spitfire production in the time period was 7 VII, 14 VIII and 388 IX. Another 35 Wellington VI built October 1942 to February 1943.

The P-39, P-40 and Hurricane had their service lives prolonged due to a number of factors starting with problems building enough of the newer designs, the P-51 was considered in short supply during 1944. The rise of the fighter bomber, the way the older fighters could still be a threat to enemy bombers and the need to stock the training system also contributed.

The 12th Air Force P-39 and P-40 groups were transferred to India or had P-40 replaced with P-47 December 1943 to July 1944, MTO Merlin P-51 operations began in April 1944, in the end the 12th had 6 P-47 groups, the 15th 3 P-38 and 4 P-51. The 8th Air force had removed its final P-38 fighters in mid September 1944, the 9th Air Force changed one of its P-38 groups to P-47 in February 1945, another to P-51 in March leaving it with 1 P-38 group.

Even the US had problems equipping its combat units with the latest versions, for example 31 March 1945, 15th Air Force B-24 combat unit strength return, 38 G, 129 H, 345 J, 217 L, 137 M. The strength return of 24 June 1944 reported 166 P-51B, 66 P-51C and 53 P-51D, on 10 January 1945 the figures were 65 B, 99 C, 210 D, on 30 April 1945 it was 46 B, 62 C, 251 D. The 15th Air Force only did 1 P-51 fighter bomber operation, on 19 April 1945, dropping 1 ton of bombs.

In 1944 deliveries of P-40 were 1,373 for the USAAF, 261 Australia, 41 Brazil, for P-39 it was 815 USAAF, 75 France, 811 USSR.

In hand waving terms,
P-40N-1 400 built starting Mar-43, armament reduced to 4 guns, later restored
P-40N-5 1,100 built starting May-43, to 27 inch wheels, provision for wing bombs and fuel tanks
P-40N-10 99 built starting Aug-43, winterisation equipment added
P-40N-15 377 built starting Aug-43
P-40N-20 1,523 built starting Sep-43, to V1750-99 engine
P-40N-25 500 built starting Feb-44
P-40N-30 500 built starting Mar-44
P-40N-35 500 built starting Jun-44
P-40N-40 220 built starting Oct-44, to V1750-115 engine, back to 30 inch wheels, metal covered ailerons.

I believe this hits the nail on the head precisely. These legacy airframes remained in service in a variety of fronts because of the unavailability of the newer types. Of course, there were also compounding problems at the entry of the latter, but it's the former which was the greatest contributor.

Though I would note, from what I recall, the P-39 was never particularly a popular stand-in in any US theatre, and were phased out much earlier than the other legacy types. Partially, it helped to keep the supply lines dedicated for the Soviets, who achieved fairly good performance out of the P-39 type (and, indeed, it helped that all production runs for those derivatives of the Allison V-1710 were unique to the P-39, as the Soviets had many frustrations in the limited engine supplies of other lend-lease types). Though I also believe the P-39's fairly anemic range was a contributing factor. The P-40 may have short-range compared to the P-51B and the like, but it was handily superior to the Spitfire and P-39 types. It was about on-par with the P-47C when both were in 'slick' configurations. Not enough to perform long-range escort duties, but certainly enough to make for a useful escort of the Mediterranean medium bomber fleets in most circumstances.

Though I believe the most poignant note which you made is the supply limitations of all powers in the War. Attrition wasn't low for any of these types, be it from training, operational accidents, or combat losses, and having relatively modern pursuit fighters on-hand to plug the gaps in other theatres was an invaluable thing indeed. Even when you look to the aerial supremacy of the ETO in late 1944, there were still never enough escorting fighters to provide thorough cover for all the 8th's and 9th's operations. Much less for all the other overlooked theatres of the War.

As an aside, thank you for sharing your numbers, do you have any references for where more information may be found of these specific quantities? I've found plenty of secondary sources to such numbers, but never the original reports, as it were.

 

[Frog]

I believe this hits the nail on the head precisely. These legacy airframes remained in service in a variety of fronts because of the unavailability of the newer types. Of course, there were also compounding problems at the entry of the latter, but it's the former which was the greatest contributor.

Though I would note, from what I recall, the P-39 was never particularly a popular stand-in in any US theatre, and were phased out much earlier than the other legacy types. Partially, it helped to keep the supply lines dedicated for the Soviets, who achieved fairly good performance out of the P-39 type (and, indeed, it helped that all production runs for those derivatives of the Allison V-1710 were unique to the P-39, as the Soviets had many frustrations in the limited engine supplies of other lend-lease types). Though I also believe the P-39's fairly anemic range was a contributing factor. The P-40 may have short-range compared to the P-51B and the like, but it was handily superior to the Spitfire and P-39 types. It was about on-par with the P-47C when both were in 'slick' configurations. Not enough to perform long-range escort duties, but certainly enough to make for a useful escort of the Mediterranean medium bomber fleets in most circumstances.

Though I believe the most poignant note which you made is the supply limitations of all powers in the War. Attrition wasn't low for any of these types, be it from training, operational accidents, or combat losses, and having relatively modern pursuit fighters on-hand to plug the gaps in other theatres was an invaluable thing indeed. Even when you look to the aerial supremacy of the ETO in late 1944, there were still never enough escorting fighters to provide thorough cover for all the 8th's and 9th's operations. Much less for all the other overlooked theatres of the War.

As an aside, thank you for sharing your numbers, do you have any references for where more information may be found of these specific quantities? I've found plenty of secondary sources to such numbers, but never the original reports, as it were.

Just my 2c, do not forget its contribution to the war effort as an advanced trainer in the USA (along with the P-39).

 

[swampyankee]

There was some discussion of putting a 2 stage Merlin in the P-40 in Dec 1941 and early 1942. Both LTC Bogart, chief of the technical staff section at Wright Field and LTC Carroll, chief of experimental engineering section at Wright Field were on record in writing as of Jan 19, 1942 as being against it.
This is 1 1/2 years (almost) after the BoB so it is a little hard to blame the BoB. That is assuming that the Merlin 61 was actually hardware in Aug/Sept 1940. Spits were getting Merlin XIIs and Hurricanes started getting Merlin XXs by Sept 1940. first flight of a Wellington VI with Merlin 60 engine was in Oct 1941 (?).

June 30th 1941 saw the first flight of the XP-40F (modified P-40D airframe) with a Merlin 2 speed engine. Deliveries of the P-40F started Jan 3rd 1942.
I think somebody many have confused 2 speed and 2 stage.

The P-40 always seemed to be 1-2 years behind in the state of it's engines. A P-40 in the summer/fall of 1940 had an engine almost as good as Spitfire I had in the spring of 1939.
The P-40E in the summer of 1941 had an engine that was not quite as good as the Merlin XII in the MK II Spit in Aug/Sept 1940.
Unfortunately the Allison got kind of stuck and the late model Allisons in the P-40M and N were nowhere close to the Merlin 45 engines in Spitfire Vs of early 1941.
The P-40F & Ls got the equivalent of the Merlin XX used in the Hurricane of late of 1940.
Both engines crossed over a bit with emergency power or WEP.
The P-40 suffered due to weight for the installed power. It is seldom given credit for it's streamlining. Perhaps because it is compared to the P-39 (a smaller airplane) and the P-51 which was a generation later than just about anything else that flew in WW II.
The P-40 was very, very good at the speed it archived for the power it had given how much it weighed. It just wasn't as good as the P-51.

The P-40s did a lot of good work in NA and Italy. In part because they were given top cover by Spitfires and P-38s, pretty much starting around the time of Operation Torch.
P-40s had done a lot during much of 1942.

If one looks at the overall performance of the P-40 vs the Bf109, it's pretty obvious that Curtiss did a good job with its aerodynamics. In other non-news, the P-40 reputedly had quite good handling, which is critical for new pilots to get the most out of their aircraft.

That said, the P-40 was aerodynamically less efficient than the Mustang. This is not surprising; every other fighter with pistons also was.

 

[GrauGeist]

The fact that the P-40 out lasted it's contemporaries (P-35, P-43, etc.) and soldiered on to the end of the war, while remaining an effective adversary to the enemy, is certainly a testament.

 

[Geoffrey Sinclair]

The P-39 had a handicap in the Pacific, many early ones were P-400 with unusable British oxygen systems. The P-39 was 609 out of 2,155 front line USAAF fighters in December 1941, 1,116 out of 5,301 in December 1942, 2,019 out of 11,328 in December 1943, 746 out of 15,100 in December 1944. In percentage terms peaked at 32% of the force in May 1942, dropped below 20% in October 1943, below 10% in June 1944.

P-40 peak (wartime) was 47% end November 1941, below 40% in March 1943, below 30% in July 1943, below 20% December 1943, below 10% February 1945. P-39+P-40 combined, peaked at 78% in February 1942, below 60% in May 1943, below 50% in September 1943, below 40% in December 1943, below 30% in May 1944, below 20% in October 1944, below 10% in April 1945. Not sure if the P-400 were counted as P-39 or are under "other" in the Statistical Digest table, probably "other".

A good indicator of the problems in starting up and tearing down a production line is P-39 ended production in August 1944, its P-63 successor started production in October 1943, October 1943 to August 1944 inclusive production was 3,943 P-39Q and 966 P-63A. Also despite receiving most of the P-63 production with bulk deliveries beginning in June 1944 in the US, the Red Air Force is reported to have only started using the type in combat near the end of the war in Europe.

When it comes to talking range the internal tankage of P-40 versions varied between 120 (P-40L, N-1) and 180 US gallons (P-40) according to Francis Dean, the early Spitfires had 104.5 US gallons giving the mark I a greater range than the Bf110, the Spitfire VIII had 149 US gallons. Using the Francis Dean yardstick ranges the early P-47C and D on internal fuel range was around 900 miles, the P-40 on internal fuel 650 to 750 miles, if this is on par then the Spitfire range was on par with the P-40.

Britain: The pre 1942 Merlin figures are from the Rolls Royce Historical Trust.

UK Statistical Digest of the War, also called Fighting with Figures, appendix 4 of British War Production by H M Postan, monthly production January 1938 to December 1944. National Archives files AIR 19/524, AIR 8/278, AVIA 46/146, AVIA 10/315, AVIA 49/229.

Royal Air Force Contract Cards and Serial Registers from the RAF Museum.

Air Historical Branch/British Archives. The Ministry of Aircraft Production (MAP) monthly Statistical Bulletin. The Ministry of Supply monthly Statistical Bulletin. Ministry of Aircraft Production Statistical Review.

The National Aerospace Library at Farnborough. Rate of attainment of peak output after first delivery, report no 5. 11/313.

USA: USAF Material Command reports,

Airplane and Glider Acceptances, Factory Deliveries and Departures from U.S, by type model and country, January 1940 - December 1942 inclusive
USAAF/USAF Aircraft Procurement Program report, WS-301A/RC-301
USAF Aircraft (and Guided Missiles) Production report, Monthly, January 1949 to August 1950
WS-302/605 Aircraft deliveries and allocations.

USAAF/USAF Statistical Digest, 1945, 1945 Supplement, 1946, 1947, 1948, 1950, available on line.

AFHRA 134.23-8, Monthly US Aircraft Factory Acceptances and Deliveries, April to July 1943.
AFHRA 134.23-16, Monthly US Aircraft Factory Acceptances and Deliveries, August 1943 to January 1944, July to September 1945, November 1945 to February 1946, March 1946 to October 1947, December 1947 and January 1948.
U.S. AIRPLANE FACTORY DELIVERIES BY TYPE, ALLOCATION AND COGNIZANCE, JAN 42 TO DEC 43, AFHRA Call number 134.65-433 (Reel A1110, frame 1514, PDF page 1419).

U.S. National Archives files
RG 149B Entry C2743, U.S. Military Aircraft Acceptances 1940 - 1945 Aircraft, Engine and Propeller Production, U.S. Department of Commerce, Civil Aeronautics Administration
RG 179 Entry 1 Boxes 1086, 1087, 1089, 1092, 1094, 1096 and 1097, War Production Board Aircraft Reports, various months July 1940 to July 1945.
RG 179 Entry 249B Box 719, Bulky central decimal files 452.1, Air Technical Services Command Special Historical Report of Airframe Weight 1940 to 1944.

Other files mentioned at World Ace List and Other Data

 

[tomo pauk]

When it comes to talking range the internal tankage of P-40 versions varied between 120 (P-40L, N-1) and 180 US gallons (P-40) according to Francis Dean, the early Spitfires had 104.5 US gallons giving the mark I a greater range than the Bf110, the Spitfire VIII had 149 US gallons. Using the Francis Dean yardstick ranges the early P-47C and D on internal fuel range was around 900 miles, the P-40 on internal fuel 650 to 750 miles, if this is on par then the Spitfire range was on par with the P-40.

A wee bit of conversion errors are there
Spitfire in most cases was 84 imp gals = 101 US gals. Spitfire VIII with 120 imp gals = 144 US gals. Most of the 'short-nosed' P-40s were with 160 US gals (indeed less if one of the tanks was removed on some marks).

What 'mark I' was with the greater range then the Bf 110, that was supposed to do 1040 km on ~520 km/h?

If the drop tanks were carried, Spitfires can match the P-40s range with the 90 imp gal (108 US gal) tank carried, while the P-40s often had 52 or the 75 US gal tank. Unfortunately, seems like that P-40 never carried a more substantial drop tank on the combat missions (although I'm not sure that Spitfires carried the 90 gal tank that often).

 

[Think Tanker]

Just my 2c, do not forget its contribution to the war effort as an advanced trainer in the USA (along with the P-39).

The trainers of WW2 are often overlooked--but it is doubtless one of the most important contributions to an Air Force. I'd argue in this regard though that the basic trainers are even more overlooked than the 'flashier' advanced trainers like the Warhawk. The P-40 was an interesting contrast in this regard, that it was not exactly a novice-friendly aircraft (lack of automation preceding the P-40N, strong roll tendency at take-off and high-speed dives, etc.) but once the aircraft was learned, it turned out to be quite the pilot's airplane. Agile, quick to respond and light on the controls even at higher speeds.

I'd wager this, in part, helped to contribute to its success as a fighter-bomber against infrastructure targets--i.e. bridges--as the relative maneuverability of the type lent it well to these sorts of dive-bombing operations (this was around the time where 1,000-lb munitions started being requisitioned from the 5th Air Force's B-24 stockpiles, of note). Especially in regions where the terrain didn't lend itself well to lower-altitude level-bombing strikes from tactical fighters.

 

[Think Tanker]

A wee bit of conversion errors are there
Spitfire in most cases was 84 imp gals = 101 US gals. Spitfire VIII with 120 imp gals = 144 US gals. Most of the 'short-nosed' P-40s were with 160 US gals (indeed less if one of the tanks was removed on some marks).

What 'mark I' was with the greater range then the Bf 110, that was supposed to do 1040 km on ~520 km/h?

If the drop tanks were carried, Spitfires can match the P-40s range with the 90 imp gal (108 US gal) tank carried, while the P-40s often had 52 or the 75 US gal tank. Unfortunately, seems like that P-40 never carried a more substantial drop tank on the combat missions (although I'm not sure that Spitfires carried the 90 gal tank that often).

A wee bit of conversion errors are there
Spitfire in most cases was 84 imp gals = 101 US gals. Spitfire VIII with 120 imp gals = 144 US gals. Most of the 'short-nosed' P-40s were with 160 US gals (indeed less if one of the tanks was removed on some marks).

What 'mark I' was with the greater range then the Bf 110, that was supposed to do 1040 km on ~520 km/h?

If the drop tanks were carried, Spitfires can match the P-40s range with the 90 imp gal (108 US gal) tank carried, while the P-40s often had 52 or the 75 US gal tank. Unfortunately, seems like that P-40 never carried a more substantial drop tank on the combat missions (although I'm not sure that Spitfires carried the 90 gal tank that often).

Indeed, conversion from Imperial Gallons to US Gallons is always one of those conversion fault-lines. Though, in combat configurations the P-40 almost universally had greater range than its contemporary Spitfires (especially if we compare the most-prolific types of each, Mk. V/IX & P-40N-5-20), there were nonetheless ferry configurations (in some cases not ostensibly marked as such, but 'long-range' configurations) for both aircraft which could greatly extend the range of both types.

There were the 90-Gallon tanks as you mentioned for the Spitfire, and the fairly-universal 75-Gallon tanks by the time of the P-40N (phase-down of the 52-Gallon tank appears to be circa August of 1943). Though I would note the P-40N did have the long-range 170-Gallon tanks (either carried on the centerline or one under each wing, by September 1943). It appears, as you allude, that these were not combat drop-tanks--or at least were never frequent in such a role. The more 'common' references for Spitfire drop-tanks that I am aware of are the 37-IMP.GAL and 48-IMP.GAL tanks. I am not aware of instances where the 90-IMP.GAL was carried outside of ferry flights, ala the 170-Gallon tank, but I would be glad to find images of either being used in 'combat configurations,' so to speak.

 

[tomo pauk]

There were the 90-Gallon tanks as you mentioned for the Spitfire, and the fairly-universal 75-Gallon tanks by the time of the P-40N (phase-down of the 52-Gallon tank appears to be circa August of 1943). Though I would note the P-40N did have the long-range 170-Gallon tanks (either carried on the centerline or one under each wing, by September 1943). It appears, as you allude, that these were not combat drop-tanks--or at least were never frequent in such a role.

That the P-40N didn't have the 150 or 165 US gal tank was probably a mistake on the part of the war planners in the USAAF, rather than the actual inability of the P-40 to use such a tank. After all, P-40Ns were at least tested with two 225 US gal tanks for the ferry purposes, so the carrying capacity was there; we also know that the P-40N was rated for the very heavy bomb load after all.
Even the 108 US gal tank would've been a boon.

The more 'common' references for Spitfire drop-tanks that I am aware of are the 37-IMP.GAL and 48-IMP.GAL tanks.

That would be the 30 and 45 imp gal tanks.