Problem with the P-40 is that it gained weight, via installation of heavier armament, protection, structure and fuel system, while there was no gain in engine power. We cannot blame only the heavier fuel system for what it lost in the RoC vs. the lighter predecessors.
The European counterparts gained, sometimes, armament weight, a bit of protection, but they received more engine power every year, if not half a year, like sometimes the Bf-109 received. The Spitfire V with two cannons and 4 Brownings (= interceptor) should weight more than a Spit V with Brownings only and an empty self-sealing 30 imp gal rear tank (= longer range fighter), and would be also a bit draggier.
Just using the P-40 to help illustrate the point. But lets try something, still using the P-40D/E since it is a bit easier to get numbers for, granted in climbs like a pregnant hog but it's speed is pretty good for it's power which means it's drag isn't that far off. AND the P-40D/E has 123imp gallons of internal fuel and can take 43.3imp or 62.5imp drop tanks. Or use the P-40F.
Now figure out you combat radius;
1. Warm up and take off allowance.
2. Climb to altitude and form up allowance.
3. Combat allowance (5min military power and 15min max continuous)
4. reserve allowance (20-30min at most economical).
These are pretty much fixed and won't change much from mission to mission.
For a particular mission (and like the 8th airforce, lets assume you are escorting in relays) you need to figure:
5. cruise to meeting point with bombers.
6. Cruise speed while escorting bombers (if the bombers are doing 180-200 the fighters
have fly faster and weave or else they will be flying to slow to quickly respond) and air miles flown vs ground distance covered due to weaving.
7. Separation point from bombers. Worst/best case is over the bomber target, Fighters defend and then break for home as Fresh fighter squadrons show up to escort bombers on return leg. What cruise speed and altitude is needed to minimize risk/losses as fighters withdraw from enemy airspace? It will NOT be most economical cruise.
8. At what point can you throttle back from "combat cruise" to something less as the danger of being bounced by fresh enemy fighters fades (like crossing the channel).
Some of these change depending on
when in the war we are talking about. In 1940-41 the Channel was somewhat contested. You are NOT safe as soon as you are over water. Lets also remember that while the Channel was pretty narrow in the Dover - Calais area over much of it's length it is closer to 100 miles wide and for the British to hit Germany, they have to fly over Belgium or Holland. Germans have it a bit easier once they take France/Low Countries.
Now for the P-40 in US gallons 1 2 come up to about 40-50 gallons (lighter planes should do better, and perhaps in summer less warmup is needed?).
3. is about 38 gallons
4. is 11 gallons for 20 minutes.
So we are up to about 89 gallons already.
Now at about 233mph at 12,000ft it takes a P-40 23-24 gallons to fly 133 miles (distance from Ipswich to the Hague. And since you need to go both ways that is 135 gallons total, Granted the trip over will be done on the drop tank Or at leas t partially. Yo know have 13 gallons left plus drop tank for actual
penetration into enemy territory while escorting bombers. The P-40 can do 280mph at 15,000ft on 60 gallons an hour or 4.33miles per gallon (engine is running lean, much faster and it has to go to rich. With a 52 US gallon drop tank you can theoretically penetrate about 140 miles past the The Hague but that does not count weaving.
IF you decide your fighters need to cruise faster for better response to an attack the P-40 does 297mph at 15,000ft but burns 85 gallons an hour running rich for 3.49miles per gallon. P-40D/E can do 316mph at 12,000 as long as the fuel holds out (max continuous) but is burning 112 gallons an hour or 2.82mpg.
Obviously other planes may differ a bit, especially as to altitude and max lean cruising speed but the general performance area is there. For the British an extra 20-30 US gallons inside and a US 50 gallon drop tank are barely going to get you to the German Border if you are formation flying and weaving over bombers and maybe not even that. Germans are a bit better off as they don't have to cross 100-150 miles of formally neutral countries to get to important targets.