Here's what's what.
The P-51 D5 uses the class "FuelTankGun_Tank75gal2"
This version of the 75 gal tank (there are more than one, it happens over the years... decades) comes at a weight value of 205 kg worth of fuel. This is a correct amount, this is what 75gal gasoline weigh, give/take.
This class indeed was among those reworked. Previously, as stated before, it only contained 156 kg of fuel. So it is increased by ~ 50kg of fuel, per tank.
What is not taken into account is any other inconsistency regarding the engine or your in flight settings. Is your bird a terrible guzzler?
Anyway, the amount of fuel provided is as good as it can be though. Sorry, that's about what I can do.
About historical range in the game.
The game has severe limitations. It gets fuzzy when you really try to build 12 hour round trips or some such. There aren't even maps big enough. AI can go from Iwo Jima to Japan, fight the baddies, and back on their own terrible patterns on the B-29 alley map in my test mission. To me, that's about as good as the game needs to be really. If you want to enter this any further, you need to go into the java and the engine data and for all birds concerned.