Road speed in every gear from engine RPM, gear ratios, final drive and tyre size, for up to three complete configurations at once. The number most gearing calculators leave out is the overall drive factor, which is what tells you whether a wheel diameter change and a final drive change have cancelled each other or not. Rolling radius and final drive are the same lever: both scale every gear by the same amount and neither changes the spacing between them. So if you have gone down a wheel size for handling reasons, this page tells you which available final drive puts the gearing back where it was, or deliberately past it.
Presets
Ford Fiesta ST150 · standard IB5
Configuration Slots
Each slot holds a full set: gear ratios, final drive and tyre. Load a preset into it or type your own numbers. Blank gear fields drop out of the chart and the table. The page loads with the FST-150 gearbox history in A, B and C, so it doubles as a worked example: A is the standard car, B is the Quaife box as fitted now on 17in wheels, C is the proposed 15in wheel with a taller fifth and a longer final drive to pay for the smaller rolling radius.
Gearing Chart
One line per gear per active configuration, from 1,000 RPM up to that slot's shift RPM. Move the pointer across the chart on a desktop, or tap it on a phone, to read the speed, the RPM in the gear you would be using, and the RPM the engine drops to if you upshift there.
Comparison Matrix
Two configurations with the same drive factor give the same road speed at the same RPM in the same gear, whatever the wheel size. A higher drive factor is shorter overall gearing. It is the single number that tells you whether a tyre change and a final drive change have cancelled out, because it contains both and nothing else. It says nothing about the spacing between the gears, which only the ratios themselves control.
So dropping from a 17in to a 15in wheel shortened the gearing, going from 4.380 to 4.083 on the final drive more than paid it back, and the taller fifth is what turns a small net gain into a useful one at the end of a straight. The cost is on the four to five shift: B lands at 6,627 rpm and C lands at 6,350 rpm, so C gives away 277 rpm on every upshift into top.
Solve For The Ratio You Need
You know the speed you want at a given RPM in a given gear. This gives you the final drive that delivers it, and separately the gear ratio that delivers it if the final drive is fixed. Where the loaded configuration has a list of ratios you can actually buy, the nearest real options either side are shown with the speed each one would give.
Optimal Shift Point From A Torque Curve
The fastest upshift point is not the rev limiter and it is not peak power. It is the RPM where the wheel torque you are making in the current gear equals the wheel torque you would be making in the next gear immediately after the shift. Below that RPM you are better off staying where you are. Above it you are better off having shifted already.
Scanned upward in 50 RPM steps with torque interpolated linearly between your points. Any consistent torque unit works, because both sides of the equation use the same one. Flywheel torque, not wheel torque, and it does not need correcting for transmission losses as long as you treat them as constant across the range.
No default curve is supplied. A made-up torque curve gives a made-up shift point, and the answer is only worth having if the curve came off a dyno or a decent load-based log of your own engine.
Accuracy And Method
Two per cent of rolling radius is two per cent of every speed on this page. That is larger than most of the gearing changes people agonise over, so if you are comparing two configurations that run different tyres, measure both rather than calculating both. The measurement method is on the Tyre Size Calculator.
combined ratio = RPM × circumference_m × 60 / (speed_kmh × 1000)
That gives you gear ratio multiplied by final drive together. Do it in two different gears at the same road speed and the ratio between the two answers is the ratio between those two gears, with the final drive and the rolling circumference cancelling out entirely. That second method is the useful one, because it does not care whether your rolling circumference is right.
What You Need To Do This Properly
Gearing Questions
Load the old setup into one slot and the new wheel into another, keeping the final drive the same in both, then read the drive factor difference. That percentage is how much shorter the smaller wheel has made you. Multiply your current final drive by one minus that fraction and you have the final drive that cancels it exactly.
You will almost never be able to buy that number. That is what the reverse solver is for: it shows the nearest ratios that actually exist either side of the ideal, and what each one gives you, so you can choose which way to miss. Missing on the tall side costs you drive out of slow corners. Missing on the short side costs you revs at the end of the straight and may put you on the limiter before the braking point.
No. The final drive multiplies every gear by the same amount, so the ratio between any two gears is untouched and the RPM you drop to on each upshift is exactly the same. All that changes is the road speed the whole set sits at.
Rolling radius behaves identically. This is why the two are interchangeable as a lever and why the drive factor combines them into one number. If the gaps are wrong, the final drive cannot fix them and neither can the wheels. That is a gearset change.
Only if the crossover point is above it. With a close ratio set the RPM drop on each upshift is small, so you land high on the torque curve and the optimal shift point is often below peak power. With a wide set, particularly a standard box with a long fourth to fifth step, the drop is large enough that hanging on to the limiter is genuinely faster.
The solver above answers this per upshift rather than for the whole box, because the answer is usually different for each one. Feed it a real torque curve. Without one you are guessing, and guessing here costs more than it saves.
Most gearing spreadsheets multiply the free tyre diameter by pi and call it rolling circumference. A loaded tyre does not roll on its free diameter. This page uses effective rolling radius, which sits between the free radius and the static loaded radius, and comes out around 2.5% smaller than the free radius for a typical fitment. So every speed here reads roughly 2 to 3% lower than a free-diameter spreadsheet at the same RPM.
This page is the closer of the two, but neither is a substitute for measuring. The same error also corrupts logger wheel speed if you enter a free circumference as your calibration constant, which is covered on the Data Channel Reference.