Gearing: road speed, rev drop and tyre size

Everything about gearing comes from one chain: the engine turns, the gearbox divides, the final drive divides again, and the tyre turns that into distance.

wheel rpm = engine rpm ÷ (gear ratio × final drive)
road speed = wheel rpm × π × tyre diameter

In inches and miles per hour, with 63,360 inches in a mile:

mph = wheel rpm × π × diameter × 60 ÷ 63360

Tyre diameter from the sidewall

A metric size like 205/50R15 is width in millimetres, aspect ratio as a percentage of that width, and wheel diameter in inches. The overall diameter is the wheel plus a sidewall top and bottom:

diameter = wheel + 2 × (width × aspect ÷ 100) ÷ 25.4

So 205/50R15 is 15 + 2 × 102.5 ÷ 25.4 = 23.07 inches.

That is the nominal figure. A loaded tyre rolls slightly smaller than it measures, which is most of the reason a speedometer reads a little high from the factory.

The number that matters: rev drop

A top speed per gear is easy to compute and tells you almost nothing. What decides whether a gearbox suits an engine is where the needle lands after an upshift, because road speed cannot change across the shift:

landing rpm = rev limit × (next gear ratio ÷ current gear ratio)

Take a stock GSR box, 3.230 into 1.900, on an 8,000 rpm limit. You land at 8,000 × 1.900 ÷ 3.230 = 4,706 rpm, a drop of nearly 3,300. On an engine whose second cam lobe comes in around 5,500 that shift drops you out of the power band entirely — which is the whole argument for a close-ratio set, and the reason a car can be slower with taller gears despite a higher top speed in every one of them.

Note the final drive does not appear in that formula. It changes every speed and every ratio equally, so it moves the whole curve without changing its shape. Final drive sets where the gears land; the ratios set how far apart they are.

Changing tyre size

A taller tyre is a taller final drive by another name — it lengthens every gear at once. It also puts the speedometer out, since the gauge was calibrated for the original rolling circumference:

error % = (new diameter − old diameter) ÷ old diameter × 100

A taller tyre means the car is travelling faster than the dial says, which is the direction that costs a licence.

About the calculator below

It plots road speed against engine speed, one line per gear, and shows the landing rpm and drop for each shift. Two setups can be compared side by side, because comparing is the only reason to do this at all.