Final Drive Ratio, wheel, and WHP changes

Originally Posted by SS_Firehawk

I'm still waiting on the diff work to be complete, but I was doing some more research and this is what I came upon.

If you change your final drive, it should not reflect on a dyno when using a 1:1 gear. If you're running a lower final drive any dyno in 4th; your numbers will be lower because the rpm delta is greater when you calculate 4th gear with the lower final drive gear.

formula: rpm = (mph x final gear ratio x 336*) / tire diameter
(you get the gear calculation by multiplying the transmission gear ratio with the final drive gear ratio)

Using 4.08 as my final drive
with a 1:1 gear; 75 x 4.08 x 336 / 26.9 = 3822rpm
with a 1.27:1 gear; 75 x 5.1816 x 336 / 26.9 = 4854rpm
delta = 1032rpm

Using 3.69 as my final drive
with a 1:1 gear; 75 x 3.69 x 336 / 26.9 = 3456rpm
with a 1.27:1 gear; 75 x 4.6863 x 336 / 26.9 = 4390rpm
delta = 934rpm

Hopefully this helps make a bit more sense. There was a post on a forum talking about how the dyno calculates power, but I can't find it in my search history. It was a lot of math. But the calculations above at least make sense to my why anything but 1:1 reads lower.