Driving · Tires & gears

Speedometer Calculator

Fitted bigger tires or changed your gears? Enter your original and new tire sizes to see how far your speedometer is off and your true speed at every reading — or use the speedometer gear calculator below to find the driven gear that puts it right.

True speed

62.1 mphwhen the speedometer shows 60 mph

Speedometer error: 3.4% low — you are going faster than shown.

Original vs new tire
Spec	Original	New
Overall diameter	31.61 in	32.71 in
Sidewall height	7.30 in	7.85 in
Circumference	99.3 in	102.8 in
Revolutions per mile	638	617

Speedometer reading vs true speed

Speedometer reading vs true speed on the new tires
Speedometer shows	20	30	40	50	60	70	80
True speed	20.7	31.0	41.4	51.7	62.1	72.4	82.8

Metric & flotation tire sizes
True speed at any reading
Diameter & revs per mile
Speedometer gear calculator
mph or km/h

Tire size

How Tire Size Affects Your Speedometer

A speedometer doesn’t measure speed directly. It counts how fast the wheels or driveshaft turn and assumes a tire of the original size.

Fit a taller tire and every revolution covers more road, so you travel faster than the speedometer thinks: bigger tires make your speedometer read low, and smaller tires make it read high. The error is almost exactly the percentage change in overall diameter, which is why this tire size speedometer calculator compares diameters first.

The odometer is affected by the same amount, so after a 4% upsize your truck clocks roughly 96 miles for every 100 it drives. Even tread wear matters a little: a worn tire is a few tenths of an inch shorter than a new one, which nudges the reading up.

Speedometer error for common tire upsizes
Tire change	Diameter	Speedometer reads	True speed at 60
245/75R16 → 265/75R16	30.5″ → 31.6″	3.7% low	62.3 mph
265/70R17 → 285/70R17	31.6″ → 32.7″	3.4% low	62.1 mph
265/70R17 → 33x12.50R17	31.6″ → 33.0″	4.2% low	62.6 mph
275/55R20 → 275/65R20	31.9″ → 34.1″	6.4% low	64.1 mph
255/75R17 → 35x12.50R17	32.1″ → 35.0″	8.4% low	65.5 mph

Calculated from nominal sizes. Actual diameters vary slightly by brand and tread depth.

Cable speedometers

Speedometer Gear Calculator

On vehicles with a cable-driven speedometer, the reading is set by a small driven gear in the transmission. Change the tires or the axle ratio and you need a different gear.

Driven gear you need

18teeth · exact 18.39

Tire revolutions per mile: 617

A 18-tooth driven gear reads 2.1% high. Based on 1001 cable turns per mile.

Formula

Driven teeth = (axle ratio × drive teeth × tire revs per mile) ÷ 1001

Tire revolutions per mile ≈ 20,168 ÷ tire diameter in inches. GM-style speedometers are calibrated for 1,001 cable turns per mile.

Finding your gear counts

The driven gear’s tooth count is usually moulded on the gear itself, where it sits in the transmission tail housing. The drive gear is on the output shaft inside the transmission — check your transmission’s specs or look when the housing is off.

This speedometer gear ratio calculator suits classic cars and trucks with cable speedometers, including those with GM transmissions such as the TH350, TH400 and 700R4. Driven gears only come in whole teeth, so the calculator shows the nearest gear and how far off it will still read. If no gear gets you close enough, an inline ratio adapter on the speedometer cable can make up the difference.

Modern vehicles with electronic speedometers don’t have a gear to swap: the tire size is set in the engine or body computer. Seehow to calibrate a speedometer for the options, and verify the result with theGPS car speed test.

FAQ

Speedometer Calculator FAQ

Do bigger tires affect the speedometer?

Yes. Your speedometer counts wheel (or driveshaft) revolutions, and a taller tire covers more ground per turn. With bigger tires the speedometer reads low — you are going faster than it shows — and the odometer records fewer miles than you actually drive. Smaller tires do the opposite.

How much will my speedometer be off with bigger tires?

By roughly the same percentage as the change in overall diameter. Going from a 31.6″ to a 32.7″ tire (265/70R17 to 285/70R17) is about 3.5% taller, so an indicated 60 mph is really about 62 mph.

How do you calculate speedometer error from tire size?

Work out each tire’s overall diameter — rim diameter + 2 × (width × aspect ratio ÷ 25.4) for metric sizes — then true speed = indicated speed × new diameter ÷ original diameter.

How do I calculate which speedometer gear I need?

Driven gear teeth = (axle ratio × drive gear teeth × tire revolutions per mile) ÷ 1001, where revolutions per mile ≈ 20,168 ÷ tire diameter in inches. Round to the nearest gear you can buy.

Does tire size affect the odometer too?

Yes, by the same percentage. Bigger tires make the odometer under-record your mileage, which matters for lease limits, warranties and service intervals.

What are tire revolutions per mile?

How many times a tire turns in one mile. Theoretically it is 63,360 ÷ (π × diameter in inches); real tires turn about 2–3% more because they flatten under load. Manufacturers publish measured figures for each size.

Related Tools