I'm thinking of starting of converting an old car to an electric car. I have a question, but google won't answer me my very long not very well explained question.

Take this scenario. I would take a small 2 HP electric motor that can run up to 5000 RPM, say I put a gear for reducing revolutions (e.g. 6:1 reduction gear). So the question would be, is there any formula that would give me the output HP at the output of the gear?

I hope you understood my not very well put question.


2 Answers 2


tl;dr: Yes. The horsepower shouldn't change but the torque should go up by a factor of 6.

The thing to remember is that what you usually want is actually the torque of your motor. That's what actually does the useful work for you. In this case, though, you've been handed a horsepower number at a particular RPM.

We can get the equivalent torque at that RPM using the standard equation:

horsepower = (torque * rpm) / 5252

So, if your cited horsepower is 2 HP at 5000 RPM, we expect that the torque output is 2.1 foot-pounds (yeah, ick, English measurements - blame the British empire).

At this point, we can introduce the reduction gear. The thing to remember is that we don't expect the output gear to affect the power of the motor at all. It will continue putting out its output torque at its previous angular velocity. The reduction gearing will then trade angular velocity for more torque at a ratio of six to one.

If you apply your reduction gear, you'll change the rpm in the equation. Plugging in 2 HP and 833 RPM (at a 6:1 reduction), we expect to see a torque of about 10.5 foot-pounds.

At the end of all this, you'll have a system with much more pulling power and a much lower top speed.

  • 1
    Also of note, the 833 RPM Bob talks about is the output RPM ... the input RPM would logically remain the same (5k). Mar 10, 2014 at 1:21

Bob's answer explains it well.

Just wanted to add that a 2hp motor will be very slow on anything heavier than a bicycle. And you'll be lucky to get even 25mph (on a flat road without a tailwind).

  • So what minimum power should I be looking for if I want to reach a maximum of 100 KM/H at a reasonable speed increase? Mar 10, 2014 at 7:40
  • The smallest motor that I could find that makes sense is 13kW (or about 19HP). But that was for a very light-weight car. I wouldn't suggest anything less than 40 to 50 kW (or 64 to 80 HP) if you're going to put it in an old car. Even though electric motors have lots of torque, having very little horsepower means you'll never go very fast. Mar 10, 2014 at 10:25

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