Which final-drive ratio do I need?
A higher-number final drive multiplies torque and RPM; a lower-number one stretches every gear. Start with the behaviour you want to change and end with the calculation that proves the direction.
What are you trying to improve?
Choose the problem you can measure. “Faster” by itself is not enough, because a ratio that helps one part of the drive can hurt another.
What does the engine do at cruise now?
What happens when you apply full power in the lower gears?
How did the new overall tyre diameter change?
A taller tyre travels farther per wheel revolution and acts like a lower-number final drive. A shorter tyre does the opposite.
Where is the engine at the finish or target speed?
Model a lower-number final drive
A lower numerical ratio — for example 3.90 to 3.55 — reduces engine RPM at every road speed and extends the speed available in each gear. The percentage change is nearly proportional: 3.55 ÷ 3.90 = 0.91, so cruise RPM falls about 9% and each gear becomes about 10% longer.
Confirm the engine can still pull the taller ratio. If road load places cruise below a stable operating range or top gear below useful power, the quieter number can create more downshifts, heat and throttle opening rather than a better drive.
Your current gearing may already be too tall
Low tachometer RPM is not automatically efficient. If the engine lugs, the transmission unlocks its converter or the car repeatedly downshifts on small grades, the operating point is below the useful load range.
A modestly higher numerical final drive can hold a more stable gear and reduce the throttle needed. Check the speed/RPM table and the manufacturer's transmission and axle options before changing hardware.
Keep the current ratio as the baseline
You do not have evidence that the final drive is the problem. Record tyre size, actual cruise RPM, the individual gear ratios and a target speed, then compare candidate finals against the current one.
Change only enough to solve a measured issue. A final-drive swap moves every gear at once, so the complete speed table matters more than one attractive result cell.
Fix traction before adding more multiplication
A higher-number final drive multiplies wheel torque, which makes an already traction-limited launch harder to use. It may feel more aggressive while producing no better acceleration.
Work on tyre, differential behaviour, launch technique and suspension control first. Once the car can apply the available torque, model whether another ratio keeps the engine in the power band without adding a wasteful shift.
Model a higher-number final drive
A higher numerical ratio — for example 3.55 to 3.90 — raises engine RPM at a given road speed and multiplies more torque at the wheel. The trade is more cruise RPM, less speed per gear and potentially an additional shift.
Use the complete ratio stack and target speed. The best result keeps the engine inside its useful power band and avoids placing a shift at the exact point you are trying to accelerate through.
Inspect the ratio spacing before the final drive
A final-drive change moves every gear but does not change the percentage RPM drop between adjacent gears. If every upshift falls below the power band, the spacing inside the gearbox is the real issue.
Calculate the landing RPM with new RPM = shift RPM × next ratio ÷ current ratio. A closer gear pair, a different shift point or a broader power curve solves a drop that the differential ratio cannot.
A taller tyre needs a higher-number final to restore gearing
Tyre diameter and final drive oppose one another. To preserve the original overall gearing, multiply the current final by new diameter ÷ old diameter. A move from 650 to 700 mm is 7.7% taller, so a 3.73 final needs roughly 4.02 to restore the previous engine RPM and wheel-torque multiplication.
First confirm the actual mounted diameter and speedometer effect. Label sizes are nominal, and tyre construction, pressure and load move the rolling circumference.
A shorter tyre needs a lower-number final to restore gearing
A shorter tyre raises engine RPM and shortens every gear. Use new final = old final × new diameter ÷ old diameter to preserve the original overall relationship. Because the diameter ratio is below one, the correcting final-drive number is lower.
You may prefer some of the shorter effective gearing, so calculate the correction rather than assuming you must remove all of it.
The current final drive already places the finish well
Crossing near peak-power RPM in the intended gear is the outcome ratio planning tries to create. A change risks adding a shift, reaching the limiter early or moving the engine below useful power.
Keep the current final as the reference and look for time elsewhere: launch, shift execution, power consistency, air conditions and vehicle mass.