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The motor torque in lb-ft is simply HP*5252/RPM, so your motor will supply about 22 lb-ft continuously and a maximum of 2-3 times that, or about 50 lb-ft. Now you need to figure the drive train ratio. You can do this by reading the tachometer and speedometer in each gear, and you might as well use 3600 RPM because that's your motor spec. So, I think my car will probably go 20 MPH at 3600 RPM in first gear. You have to measure the tire diameter to determine the axle RPM, so I'll guess at 2ft diameter which is about 6ft circumference. So every rotation will go 6ft, and a mile is 5280ft, so it will spin 20*5280/6 Rev/hr or about 290 RPM. So the drive train ratio is 3600/290 or 12.3/1.

Now the torque you need to climb a hill is related to the vehicle weight and the incline of slope. For a 20% grade, and a 3000 lb vehicle, you need a force of 3000*0.2 or 600 pounds. Since the tire has a radius of 1 foot, that is also 600 lb-ft of torque. The motor needs 600/12 or 50 lb-ft of torque. So, your 15HP motor will just barely meet that, and it will be overloaded so it will probably only be able to climb that hill for no longer than about 5 minutes (at 20 MPH max). Since you want some extra torque so you can accelerate, the vehicle will be severely limited. It may be OK in KS or FL, but not in WV.

And if your 3000 lb does not include batteries, you must add that weight as well. And my top-of-the-head calculations do not factor efficiencies and wind resistance.