A good starting go-kart gear ratio is around 6:1 for many small four-stroke kart engines on a tight track, with 4:1 or 5:1 for faster, longer-straight tracks. The ideal figure depends on the engine, tyre size, driver weight and track, so treat any number as a starting point and follow your engine or kart maker's guidance.
This guide explains what a gear ratio is, how to calculate it, how it changes speed and acceleration, and works through an example. It is aimed at petrol karts with a chain drive, which are for private land or a circuit only.
What is a go-kart gear ratio?
The gear ratio is how many times the engine turns for each single turn of the rear axle. On a simple chain-driven kart it is set by two sprockets: a small one on the engine or clutch, and a larger one on the axle.
Divide the number of teeth on the axle sprocket by the number of teeth on the engine sprocket. A 60-tooth axle sprocket and a 12-tooth engine sprocket give 60 ÷ 12 = 5:1.
Higher or lower ratio: what changes?
A numerically higher ratio, such as 6:1, multiplies engine torque more, so the kart accelerates harder but reaches a lower top speed. A numerically lower ratio, such as 4:1, gives a higher top speed but slower acceleration.
| Change | Acceleration | Top speed | Engine load |
|---|---|---|---|
| Higher ratio (bigger axle sprocket or smaller engine sprocket) | Stronger | Lower | Engine revs higher at a given speed |
| Lower ratio (smaller axle sprocket or bigger engine sprocket) | Softer | Higher | Engine works harder to pull the gearing |
Sprocket maths: a worked example
Take an engine turning at 3,600 rpm at the clutch and rear tyres 11 inches in diameter. The circumference of the tyre is about 34.6 inches (diameter x 3.14). These figures are illustrative only.
- Axle speed = engine rpm ÷ ratio. At 6:1 that is 3,600 ÷ 6 = 600 rpm.
- Distance per minute = 600 x 34.6 inches = about 20,760 inches.
- Convert to miles per hour: 20,760 x 60 ÷ 63,360 = roughly 19.7 mph.
| Ratio | Axle rpm | Theoretical speed at 3,600 rpm |
|---|---|---|
| 6:1 | 600 | About 20 mph |
| 5:1 | 720 | About 24 mph |
| 4:1 | 900 | About 29 mph |
These are theoretical figures that ignore clutch slip, tyre growth and drag. They show the direction and size of the change.
Two-stage and jackshaft gearing
Some karts use a jackshaft with two chain stages. Multiply the two ratios together: (36 ÷ 12) x (45 ÷ 15) = 3 x 3 = 9:1. A torque converter works differently, since its ratio changes automatically with load and speed.
What ratio suits what?
- Tight, twisty track or small paddock: a higher ratio such as 5:1 to 6:1 for quicker acceleration out of corners.
- Long straights: a lower ratio such as 4:1 for higher top speed, provided the engine can pull it.
- Heavier driver or kart: a higher ratio to help acceleration.
- Larger tyres: a larger tyre acts like a lower ratio, so you may need to go higher to compensate.
How to choose and adjust safely
- Start with the ratio recommended for your engine and kart. Do not guess from another kart.
- Change one tooth at a time and test. A single tooth on the engine sprocket makes a bigger difference than one on the axle.
- Check the chain length and tension after any sprocket change.
- Watch for bogging or a hot engine, which suggest the ratio is too high or too low for the engine.
Common mistakes
| Mistake | Why it matters |
|---|---|
| Chasing top speed | A low ratio on a short track gives a slow, lazy kart out of corners. |
| Ignoring tyre size | A different tyre changes the effective ratio. |
| Changing too much at once | You cannot tell which change helped. |
| Ignoring the clutch | A clutch that slips or engages at the wrong rpm hides what the gearing is doing. |
Looking for a kart rather than gearing advice?
If you are shopping for a kart rather than tuning one, our petrol kart, the OneKart PX3S 98cc four-stroke, is listed with automatic gearing, and our electric karts need no gearing choices at all. Petrol karts are for private land only, with adult supervision and a correctly fitting helmet.
Browse the go-karts collection or the kids' petrol ride-ons. We offer free mainland UK delivery.
Frequently asked questions
What is a good go-kart gear ratio?
Around 6:1 is a common starting point for small chain-driven kart engines on tighter tracks, with 4:1 or 5:1 for longer straights. Check your engine and kart maker's recommendation first.
How do I calculate a go-kart gear ratio?
Divide the teeth on the axle sprocket by the teeth on the engine or clutch sprocket. For example, 60 teeth divided by 12 teeth is 5:1.
Does a higher gear ratio make a go-kart faster?
A numerically higher ratio gives stronger acceleration but a lower top speed. A lower ratio does the opposite, as long as the engine can pull it.
What is a go-kart gear ratio chart?
It is a table of speed for different sprocket combinations at a set engine rpm and tyre size. You can build your own with the formula above.
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