Here's some indisputable facts about the AH2 P-51D and Bf 109K-4. These have been exhaustively tested and retested... There's no doubt about the figures.
Max speed at sea level, 25% fuel, full magazines.
P-51D: 367 mph
109K-4: 368 mph
Now, let's look at variations in load out. You are flying fully armed 109K-4 with about 50% of internal fuel remaining. You run into a P-51D with 25% fuel and only 1/3 of his ammo remaining. He elects to avoid an engagement. He dives for the deck with you chasing. You both level off at abut 50 feet doing around 450 mph.
You will never catch him until he runs out of WEP.
Why? Simple really, a P-51D with 25% gas and 1/3 ammo can maintain 368 mph, and with a little less gas or ammo and it'll do 369 mph. Reducing weight reduces induced drag, reducing induced drag increases speed.
Another factor: The P-51D retains speed better than the 109K-4. Again, this has been tested and verified. Due to its greater mass and lower total drag, the P-51D will bleed down from 450 mph to 368 mph considerably slower than the lighter, draggier 109K-4. What this means is that in between 450 mph and 368 mph, the Mustang will actually be pulling away from the 109 because the 109 is bleeding off speed faster.
In sum, you will have to chase the Mustang long enough to run it out of WEP. Even then, it will take about 45 second for the P-51D to bleed down to its 355 to 356 mph MIL power speed. So after nearly 6 minutes of chase, you have a 12 mph advantage. However, more time is needed to close to gun range. That could take a few minutes more. Meanwhile, the Mustang's WEP is recharging. So, after 9 or 10 minutes of chasing the P-51, you are probably looking at a low fuel condition and if you have exhausted your WEP, the Mustang will walk away yet again.
Keep all of the above in mind when chasing P-51s. If you're not closing on them at a significant rate, don't waste your time. Instead, go find a target that isn't so fast.
My regards,
Widewing