Here's the consumption figures achieved under Australian testing of the Spitfire VIII with Merlin 66:
This aircraft had 123 gallons internally, and a 90 gallon drop tank. The drop tank remained attached at all times, hence the "ferry" designation.
It should be noted that Hop usually waves about this leaflet from an Australian archives, which is claiming 10 miles per gallon achievable. That of course, is by far the highest mileage you`d find for any Spitfire test - it appears to be a single abberant test result, with dozens of other fuel consumption trials showing around 6-6.5 mpg being realistically achieavable. Hop knows those very well.
Here`s for the Mk IX. Mk IX and VIII had the same engine :
These full endurance trials showed 6.76 mpg being achievable at 20000 feet, 1800 rpm.
If you`d add up the claimed mileage in Hop`s paper with the fuel tankage, ie. 10 mpg and 120 gallons available, this would give you something like 1200 miles (!!!) of range for the Spitfire. If this would be true, there would be never any need for the Mustang as Spitfires would easily reach out to Berlin and back. Nonsense, of course, and all MkVIII data sheets give only 740 miles or so range, which points to the 10 mpg data is simply result of an abberant test, perhaps, but agrees well with the ~ 6.5 mpg given everywhere else.
The original comment on the Spitfire`s lack of penetrative range is quite correct. Despite many attempts, the Spitfire, as the war progressed, become increasingly a defensive fighter, lacking the range for any serious escort job.
Simply adding fuel droptanks did not solve the problem, since your penetrative range in any case is determined how much internal fuel you`re left with after you dropped the fuel tank. In other word, you can`t use practically a 90 gallon droptank to bring you over to enemy territory, if all you have is 85 gallons (some of which you`ll use up quickly during combat, where fuel consumption skyrockets at max power) to return on.
The rear tanks, fitted irregularly to some planes at the very end of the war were similiar to droptanks, as they could be used for ferry (one-way) missions only, like Malta. The reason was that with the rear aux. tanks the Center of Gravity shifted so far back that the plane was outright difficult and dangerous to fly, not to speak air combat. Therefore the rear tank, like a droptank, had to be emptied first, and then return on the normal internal tankage, again 85 gallons on the vast majority of Spits.
The third factor was that the rapidly increasing fuel consumption of the Merlin made the Spitfire shorter and shorter ranged. The confidental document titled 'Development of the Spitfire Fighter' from the Australian archives gives some idea :
The Spitfire I, while cruising very slowly - hardly an option over enemy territory - could go as far as 575 miles at 188 mph, but only 350 miles if it travels at 250 mph.
The Spitfire V was shorter ranged, at slow economic cruise it would manage 480 miles at 185 mph, and 335 miles at 310 mph cruising speed.
The Spit IX was even thirstier, at economic cruise it would manage 450 miles on slow economic cruise, as seen above.
The Spit VIII was the only variant which had some range at least, with the internal fuel being increased by 50% to 120 gallons with 740 miles being possible at slow economic cruise, and much less if any reasonable speed was maintained. However few were build and most of those went either to the PTO or MTO, where with the large overseas missions the short range of the Mk V or IX would simply not do.
Enter the MkXIV and the Griffon engined fuel hogs.. range was down again, fuel tankage was 120 gallon, as in the MkVIII, but the Griffon engine was not any shy using it up... at enourmous rates. The range at an economic, slow cruising range was down again to about ca 450 miles. That is, with 50% more fuel carried than the Mk I, and having 100 miles less range. That of course would refer to the luxury of cruising at slow speed over enemy territory, asking to be bounced by an enemy fighter.
Of course this is on interal fuel, but for escort or any two-way mission, it makes sense to look at the droptank range. The absolute maximum range of a mission was defined by the range of what you could get back on your limited internal tankage, ie. after dropping your external tank when engaged and using up considerable amounts of fuel at high power during air combat. A % of allowance had to be also given for navigational etc. errors, or just circling above your own airfield and wait for your turn to land.
There`s a good reason why US escort fighters were introduced, and while they featured very large internal fuel tankage.