Guys most of you are forgetting to use the Pony's greatest strengths.
You say its impossible to outdive a 109 from 20k?
Bull puckey
If your at 20k, and even if the 109 is GLUED to your 6 at 300 yards, if you can survive the first 3 seconds you are home free.
Your first reply is gonna be that the 109 can accelerate faster and therefore win the dive with the P-51.
Your right, but your wrong.
Yes the 109 will easily out accelerate the P-51, and thats exactly how you can escape the fight

Confused yet?
When you decide to disengage, roll into the very steep vertical, and watch your speed gauge. If you can survive these first 3 seconds then you can escape. Some lazy rolls help here to survive.
As soon as you reach around 375-400 (the 109 is probably much faster due to his superior acceleration) roll hard 90 or 180 degrees' and level out into a shallow dive. The 109 can't stay with you due to its problems with compression. So right here you should gain horizontal seperation, and breathing space.
If he stays in the dive with you he will be totally uncontrolable by this point and will probably lawndart eventually. Also by this time you should be able to reach your higher top speed and start to pull away.
9 out of 10 times you have disengaged by this point and you can run home to mamma

One time outta 10 though this will happen.
Here is the trick though, if your fighting a VERY good 109 pilot. The smart 109 pilot will pull up to retain control and plan on catching you in a few minutes with superior E. So you need to watch your rear, and as soon as the 109 levels, you should level too and keep that E in the bank.
This is so that once you both bleed off that dive speed to normal level flight speeds he doesn't have alot of E on you and catch you eventually with another dive.
If he does catch you and you still have sufficent altitude, just rinse and repeat the above method.
Very very rarely can a 109 catch a P-51 with enough altitdue under him to dive away, due to its far superior high speed handling.
Actually, you can use the same exact manuever against 109's in the FW190, the La5FN, and the F4U-1.
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Vermillion
WB's: (verm--), **MOL**, Men of Leisure,
"Real men fly Radials, Nancy Boys fly Spitfires

"