I would like to offer another scenario, 3-5% is too high. Here are some logical assertions from a previous post of mine:
"An observation - I had a viper t56 on the bench and turned the input shaft by hand in all the gears. There is a substantial difference in the amount of force required to rotate the shaft between the different gears. At higher rpm's the those differences have to show up on the dyno. Although not the differential, there should be some correlation to changing the rear-end ratio and a comparison between different transmission gears.
On the dyno there is no doubt that there would be a recordable difference if you dyno'ed with a transmission gear numerically lower than fourth. On the other hand using a gear higher than fourth would put undue stress on the driveline. For those that start their dyno runs at much lower than 3000 make sure you don't stand to close to the car.
Losses of 15% are real world in a car with half-shafts. If you a assume a max loss of 4% beyond the rear-end, that leaves 11% losses in the transmission, drive shaft and rear end. The 3.55 gears require a 14% increase in rpm's to offset the change in ratio to obtain the same speed. There are going to be some fixed losses that do not change with rpm, therefore, the max additional losses would be 14% of 11%, or 7 hp. Understanding that there are fixed losses the net change on a dyno should result in something less than 7 hp, probably in the 3-5 hp range."
