Found this article about how to calulate transmission loss on any car, seems to work pretty well too.
The 1.8 Vauxhall Astra GTE (new shape from 1985 on) has a frontal area of 20.5 sq feet and a Cd of 0.31. With two people and some test equipment on board (which is how most reputable magazines do their tests) the car weighs about 2460 lbs. The engine is rated at 115PS (about 113 bhp) and the tested top speed is about 123 mph. lets see how much net power is required to achieve that speed.
Rolling resistance power is 0.013 x 2460 x 123 / 375 = 10.5 bhp
Air resistance power = 20.5 x 0.31 x 0.00256 x 123 cubed / 375 = 80.7 bhp
Total bhp at the wheels must be about 91.2 bhp to achieve that speed. If we apply my formula for FWD cars to the quoted flywheel power we get (113 x 0.9) - 10 = 91.7 bhp at the wheels. Hmmm - so you gonna step outside and fight me about 0.5 bhp or is this starting to make some sort of sense?
Lets try a more powerful car.
The 2WD Sierra Cosworth was rated at 205 PS (about 202 bhp). Top speed was in the 145 mpg region according to most magazines. Test weight with 2 people and 50 lbs of equipment on board is around 3060 lbs. Frontal area is 21 sq feet and Cd is 0.35.
Rolling resistance power is 0.013 x 3060 x 145 / 375 = 15.4 bhp
Air resistance power is 21 x 0.35 x 0.00256 x 145 cubed / 375 = 153 bhp
Total net power required is 168.4 bhp. Apply the RWD formula to 202 bhp and we get (202 x 0.88) - 10 = 167.8 bhp.
The conclusion here is pretty obvious. If transmission losses were as high as 30% then there just wouldnt be enough power left at the wheels to achieve the top speeds that the cars actually show. The Cosworth would only have 141 bhp at the wheels if this were the case and its top speed would therefore only be around 136 mph. You can work that out for yourself by applying the formulae above. The Vauxhall would only have 79 bhp at the wheels and be capable of around 117 mph.
So does anybody have the figures for the 172????