im planning and saving for a 4.10 rear swap with my 3.9 (S5 auto swapped into a 5 speed, retained 3.9 diff).

question is will the change in gear ratio change the results of a dyno run?

basically i dont want to dyno my car before the diff swap if im only gonna have to dyno it again coz the numbers will change. however, if it won't then ill do my dyno now then my diff swap.

 

it shouldnt make a difference, your ratio's will only effect what speed your at in a given gear.

 

The numbers will not change. Your rear end ratio is used in the calculations to correct your power output.