"Typically, and speaking in generalities, for each thousand foot gain in elevation, horsepower drops by three-percent. Given that, at 6,000 feet a 150 horsepower motor will lose 18-percent of its hp (6 x .03 = 0.18). So, this 150 hp, with 18-percent less horses (27 less) will slap a mountain face with 123 ponies. Add to that, an approximate 40 percent hp loss through the transmission — mostly through track rotation — this now 123 HP motor (123 X .40) is delivering 73.8 hp at the rear axle. The approximate 40-percent hp loss is charged to rotating mass, as well as binding, slippage, redirecting the twisting forces and resistance."

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Source: https://www.snowmobile.com/how-to/seeing-the-light-on-lite-1056.html