If you go to a 3.5 or 3.9 final drive (from a 3.3) will it affect what the car puts down on a dyno? You would think that if you getting more power to the ground it would make your rwhp go up. Experts feel free to discuss.

 

I think only really if you have a light clutch/flywheel and dyno in 3rd or 4th gear - i guess 5th too since it wouldn't be a 1:1 ratio anymore

 

It will not add hp. But, what it will do is shift your dyno curve to the left (EDIT) if you use mph as the lower axis number. Giving you more hp at each mph. I posted a overlay graph of before and after the 3.9. Do a search. Its from last Sept-Oct ish.

 

I doesn't matter what gear or gear ratio you use. A dyno should overlay the graphs no matter what gearing is used. Assuming it's compensating properly for the intertia of the drivetrain. If the drivetrain inertia is set incorrectly the dyno will read slightly different in the various gears, but the overall shape of the curve will be the same.