Gear Ratio, RPM & Speed
Road speed at any rpm in every gear from tire diameter, gear ratios and final drive; rpm at a speed; the final drive that restores gearing after a tire change.
A per-gear table of overall ratio, speed at your rpm and rpm at your speed, km/h per 1,000 rpm, and the corrected final drive for a new tire size.
Example: A 28 in tire with a 3.73 final drive in a 1:1 gear runs 71.9 km/h (44.7 mph) at 2,000 rpm; 31 in tires make the gearing 10.7 % taller, and a 4.13 final restores it.
Every gear,
one tire.
How rpm becomes road speed through the gearbox, final drive and tire, what a tire change does to the effective gearing, and how this differs from a machine-drive ratio.
From rpm to road speed
The engine turns at rpm; the gearbox divides by the gear ratio and the axle by the final drive, so the wheel turns at rpm ÷ (gear × final). Each turn covers one tire circumference, π × diameter. So speed = rpm × 60 × π × d ÷ (gear × final) — in km/h with d in metres and ÷ 1,000. The page tabulates every gear at your rpm, the rpm at your chosen speed, and km/h per 1,000 rpm, the figure people quote for a top gear.
Tire change and the final drive
A taller tire lengthens every gear by the same fraction as its diameter grows — 31 in tires on a truck geared for 28 in make the gearing 10.7 % taller, lowering the rpm at any speed and raising the load on the engine. The final drive that restores the original overall gearing is the old ratio × new diameter ÷ old diameter; the page reports it, and the nearest available ratio is a parts question.
Not the manufacturing Pulley & Gear Ratio
That utility relates two shafts in a machine drive — pulleys, belts, gear pairs. This page is the vehicle drivetrain with the tire in the chain, which is why it takes a tire diameter and produces road speed. Use Tire Size for the diameter from a size marking.
What the number does not settle
Kinematics only: no tire slip, no torque-converter slip at low speed, nominal (not loaded) tire diameter, and no judgement of whether the engine can pull the gear at that speed. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.