I think average on 6.0's is 14-16 range. So I would estimate you're going to lose 3 or 4 right off the bat. 400hp 6.0 vs 400hp 5.3 ..... 6.0 would probably have better mileage, just because you wouldn't need as extreme a cam, etc to make the power.


The gear/stall is going to be a bigger difference then the engine swap. Can't imagine you're going to have a stock stall, with a 3.23 gear behind a 400hp motor.