Higher mAh = More Top Speed
 

On Traxxas website they say that with the VXL if you use a 4000mAh you hit 65+mph but if you use 8000mah you hit 70+. Wouldn't they be the same just the 8000mah have more run time

http://www.traxxas.com/products/elec...peed_chart.jpg

it delivers more constant amps and therefore when you pull more amps you get less voltage drop. voltage X kv = speed

Think of it in terms of power. Power = voltage x current. The faster you go, the more power is required. As tashpop said, a pack with a higher capacity and with the same "C" rating will be able to deliver more current at the same voltage drop as a lower capacity pack, which gives more power.