Tools · Karting
Kart gear ratio & rollout calculator
Enter your sprocket tooth counts and the rear tyre's rolling circumference to get the final drive ratio, the rollout per engine revolution, and the theoretical road speed at a chosen engine speed. Compare a proposed sprocket change against your current gearing, and scan nearby axle sprockets in one table.
This is arithmetic, not advice: it doesn't know your track, engine, class rules, or grip. Use it to see what a change does on paper, then test one change at a time on track.
Single-speed, direct-drive karts only. The math assumes the engine sprocket drives the axle sprocket directly. On a gearbox kart — KZ and similar — engine speed reaches the axle through the primary drive and the selected gear as well, so the rollout and speed figures on this page don't apply to them.
Current gearing
The prefilled numbers are examples, not recommendations — replace them with your own. Engine speed starts empty on purpose: ratio and rollout don't need it, and the speed figures appear once you enter the rpm you actually want them for.
Final drive ratio
7.09:1
Rollout per engine rev
118.5 mm
Speed — enter an engine speed
—
Compare a sprocket change
Same tyre and engine speed as above; only the sprockets change.
Proposed ratio
7.27:1
+0.18 vs current
Proposed rollout
115.5 mm
-3.0 mm vs current
Speed — enter an engine speed
—
Nearby axle sprockets
Your current engine sprocket, tyre, and engine speed, across axle sprockets four teeth either side.
| Axle (T) | Ratio | Rollout (mm) | Speed (km/h) |
|---|---|---|---|
| 74 | 6.73:1 | 124.9 | — |
| 75 | 6.82:1 | 123.2 | — |
| 76 | 6.91:1 | 121.6 | — |
| 77 | 7.00:1 | 120.0 | — |
| 78current | 7.09:1 | 118.5 | — |
| 79 | 7.18:1 | 117.0 | — |
| 80 | 7.27:1 | 115.5 | — |
| 81 | 7.36:1 | 114.1 | — |
| 82 | 7.45:1 | 112.7 | — |
How the numbers are calculated
- Final drive ratio = axle sprocket teeth ÷ engine sprocket teeth. 78 ÷ 11 = 7.09:1 — the engine turns 7.09 times per axle revolution.
- Rollout = tyre rolling circumference ÷ ratio — the distance the kart travels per engine revolution, assuming no clutch slip and no tyre slip.
- Speed = rollout × engine speed, converted to km/h. It's the theoretical speed at that rpm on a single-speed, direct-drive kart with the assumptions above — real speed differs with slip, tyre growth, and load.
Measuring rolling circumference
Measure it rather than trusting a nominal size: with the tyre at running pressure, mark the tyre and the floor, roll the kart one full wheel revolution in a straight line, and measure the distance between floor marks. A tape around the tyre, or π × measured diameter, gets close but ignores how the tyre flattens under load — the rolled measurement is a practical static approximation — tyre growth with speed and load still moves the real figure.
What this can't tell you
The calculator has no idea which gearing suits your track, engine, class, weather, or driving — and no single number here is "right". Class rules may also restrict which sprockets you're allowed to run; the rules document for your class governs, not this page. Treat the output as a way to size a change before you make it, change one thing at a time, and let track testing decide.
Where to go next
- How to run a kart setup test — the worksheet for testing one bounded change, gearing included.
- When a faster lap does not mean the setup improved — before you credit the new sprocket for the lap time.
- Kart chain care: alignment, tension, and lubrication — a sprocket change means re-checking all three.
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