Compare one variable at a time
Hold tire diameter and engine RPM constant while comparing teeth. Ratio is independent of both, but a speed shown beside that ratio is not. After changing tire size, regenerate the speed columns rather than reusing an old printed table. A chart is most useful when it includes combinations you can actually fit.
R = rear/front; MPH = engine RPM × π × diameter(in) / (1,056 × R)Follow the worked comparison below for the input units and assumptions.
Work through an example
At 3,600 rpm and 13-inch nominal diameter, 12/60 produces about 27.85 MPH; 12/72 gives about 23.20 MPH. Both values assume direct drive and no slip. The speed ratio is 5/6.
Read a row as a controlled comparison
Choose one front-tooth column and move through rear-tooth rows. More rear teeth increase reduction. If the chart shows theoretical MPH, check its RPM and tire diameter first: those assumptions apply to every speed cell, even though the tooth-count ratio itself needs neither input.
Save the assumptions with your gearing chart
Record tire dimensions, engine RPM, any converter ratio and jackshaft stages next to the chart. A saved chart without these inputs cannot be reliably compared with a later setup. Use the interactive chart to regenerate speeds when an input changes.
Turn a gear chart into a repeatable comparison
This compact chart uses a fixed 12-tooth driver, 3,600 engine rpm and 13-inch nominal tire diameter. It isolates rear sprocket choices. The speed column assumes a direct chain drive without converter slip or an extra stage; adding either changes the complete engine-to-wheel reduction.
R = rear/front; MPH = engine RPM × π × diameter(in) / (1,056 × R)- Check whether the row lists rear teeth or front teeth before reading the crossing cell.
- Write RPM, nominal diameter and total additional reduction above the chart.
- Read across one row or down one column to change one sprocket at a time.
- Regenerate speed cells whenever the operating RPM, tire dimensions or converter ratio changes.
| Front / rear | Reduction (:1) | Axle RPM | Theoretical MPH |
|---|---|---|---|
| 12/60 | 5 | 720 | 27.85 |
| 12/66 | 5.5 | 654.5454545454545 | 25.31 |
| 12/72 | 6 | 600 | 23.2 |
A chart containing both ratio and MPH answers two different questions. Ratio depends only on teeth; MPH also depends on wheel travel and engine RPM. Two combinations with the same ratio can share speed cells while having different physical sprocket sizes and chain-length requirements.
Check the inputs before interpreting a difference
A printed chart is easy to reuse after its assumptions have changed. Keep a dated copy with the setup it describes. Do not interpret a speed cell as proof that the engine will achieve the chosen RPM on the longest straight.
Before you apply the result
Do not apply a direct-drive chart unchanged to a converter or compound jackshaft. Add every reduction, and label the operating converter ratio. Keep setup notes with the chart so comparisons remain reproducible.
Try the calculation
Enter your own values in the related calculators. Compare one input at a time and keep units and operating conditions visible with the result.
Frequently asked questions
Why does my kart not reach the chart speed?
The chart evaluates an entered RPM with a no-slip geometric model. Engine load, converter behavior and tire rolling dimensions can prevent the observed speed from matching it.
Can two sprocket pairs have the same ratio?
Yes. A 12/60 pair and a 14/70 pair both give 5:1. Their physical size, chain path and fit can still differ.
Does changing tire size change the sprocket ratio?
No. It changes distance traveled per wheel revolution, so the speed columns must be recalculated.
Why does a gear-ratio chart need engine RPM?
The ratio cells do not need RPM. A chart needs engine RPM only when it also converts the ratio into axle RPM or theoretical road speed.
Reference and method
Angular speed and tangential speed. Gear reductions follow tooth engagement and conservation of rotation count.
OpenStax — University Physics: rotation with constant angular acceleration ↗These explanations and examples describe the documented model. No manufacturer testing, named expert authorship or independent professional review is claimed. Read the method policy or report a correction.