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 Knowledge Jeep Math 101 |
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You've probably seen these Tire Size / Gear Ratio charts before, but have you ever wondered where all of the numbers come from? I did, so I did the math. Here are the results.
Let's start with some basic facts that we'll need to use in our calculations. Since we are working with a tire measured in inches not miles, we're going to need to know how many inches are in a mile. And, if we want to know our RPMs at 65 miles per hour we need to know how many inches are in 65 miles.
Next, we need to be able to calculate the circumference of our tires in order to know how far they travel per rotation. This is expressed as an equation we should all recognize:
Pi... Hmmm.. OK, I can't explain Pi. We'll just trust that Pi or 3.1416 works for us. Ready for an example? Good. Let's start with a 30" tire and 3.73 gears. Calculation #1 - Finding our tire circumference.
Calculation #2 - Finding how many times the tire rotates in 65 miles.
Calculation #3 - Finding how many times the tire rotates in 1 minute assuming we ran the 65 miles in 1 hour. (remember, we're looking for RPMs).
Calculation #4 - Finding the engine revolutions in 1 minute. Here's where we use the axle gear ratio.
Conclusion: With 30" tires and 3.73 gears, at 65 MPH our RPMs should be 2716. This can really be boiled down into a single equation that looks like this: RPMs =((4118400 / ([Tire Size] *3.14159))/60)* [Gear Ratio] |
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