Glossary
Gear Ratios and Final Drive Explained: Formulas, Overall Ratio & Selection
A gear ratio is the number of input turns for one output turn. The gearbox ratio and the final drive (differential) ratio multiply together into the overall ratio, which with the tyre's rolling diameter fixes road speed for a given engine RPM. Ratio selection aims to put the engine in its power band through the most important corners and to reach the redline just before the braking point at the end of the longest straight.
| RPM | Overall ratio | Road speed |
|---|---|---|
| 5,000 | 3.90 | 157 km/h |
| 6,500 | 3.90 | 204 km/h |
| 7,500 | 3.90 | 235 km/h |
| 7,500 with 4.10 final drive | 4.10 | 224 km/h |
Picking a final drive
Start with top gear: at the end of the longest straight the engine should reach peak power RPM or just touch the limiter. If it hits the limiter early, lengthen the final drive (numerically lower); if it never gets there, shorten it. Then check each corner: the car should exit the slowest corners in a gear that puts the engine in its torque band without an immediate upshift. Setup Genius's gear ratio and final drive calculator produces the speed-per-gear table from your ratios, tyre diameter and RPM limit.
Tyre diameter and gearing
A larger rolling diameter effectively lengthens every gear. On ovals, stagger between left and right rear tyres makes the effective diameter the average of the two. Tyre growth at speed adds a few percent on top. Use the loaded rolling diameter, not the unloaded tyre size, for calculations.
Frequently asked questions
What is a close-ratio gearbox?
One where the steps between gears are small, keeping the engine in a narrow power band at the cost of a smaller overall spread between first and top.
Should I gear for top speed or acceleration?
Lap time usually favours slightly shorter gearing than pure top speed, because the car spends more time accelerating than at terminal velocity. Hitting the limiter for a second or two at the end of the straight is common.
How do I calculate the RPM drop between gears?
Divide the lower gear ratio by the higher gear ratio and multiply by the shift RPM. A 1.35 to 1.00 shift at 7,500 RPM lands at 5,556 RPM.
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