How to use the speedometer calculator
- Enter the original tire size your speedometer is calibrated for (the size on the door-jamb placard).
- Enter the new tire size, metric like 285/70R17 or flotation like 33x12.50R17.
- Enter the speed your speedometer shows. Choose mph or km/h; the error percentage is the same in both.
- If you also changed axle gears, enter the old and new axle ratios (for example 3.73 and 4.10).
- Read your actual speed, the percentage error, the odometer correction, and the full speed table.
How to calculate speedometer error
Actual speed = speedometer reading × new tire diameter ÷ original tire diameter. If you changed gears too, multiply by the old axle ratio ÷ new axle ratio. For a truck going from 265/70R17 (31.61 in) to 33x12.50R17 (33.00 in):
- Correction factor: 33.00 ÷ 31.61 = 1.0441 (+4.4%)
- At an indicated 60 mph: 60 × 1.0441 = 62.6 mph
- At an indicated 70 mph: 70 × 1.0441 = 73.1 mph
Bigger tires and the speedometer
Taller tires make the speedometer read low: you are going faster than it shows, and the odometer under-counts miles. Shorter tires do the opposite. Within about ±3% the difference is small, but popular off-road upgrades go well past that:
| Stock 265/70R17 → new | Error | Shows 60 mph | Shows 70 mph |
|---|---|---|---|
| 245/70R17 (30.5 in) | −3.5% | 57.9 mph | 67.6 mph |
| 33x12.50R17 (33.0 in) | +4.4% | 62.6 mph | 73.1 mph |
| 35x12.50R17 (35.0 in) | +10.7% | 66.4 mph | 77.5 mph |
| 35x12.50R17 + 3.73 → 4.10 gears | +0.7% | 60.4 mph | 70.5 mph |
Re-gearing for bigger tires
Taller tires also make the engine turn slower at a given speed, which costs acceleration and towing power. That's why big tire upgrades are often paired with numerically higher axle gears. With a 0.69 overdrive top gear at 65 mph:
- 265/70R17 with 3.73 gears: about 1,779 rpm
- 35-inch tires with the same 3.73 gears: about 1,607 rpm (sluggish, and the speedometer reads +10.7% low)
- 35-inch tires with 4.10 gears: about 1,766 rpm, and the speedometer error drops to +0.7%
Engine speed = mph × 63,360 ÷ 60 ÷ tire circumference × axle ratio × transmission gear ratio.
How to fix a speedometer after changing tire size
- Electronic speedometers (most vehicles since the 2000s): program the new tire size or revolutions per mile with a handheld programmer or tuner, or at the dealer.
- Cable-driven speedometers (older vehicles): swap the speedometer driven gear in the transmission for one matched to the new tire and axle ratio.
- Until it's fixed: use this table or a GPS app to know your true speed, and adjust mileage-based fuel economy and service intervals by the same percentage.
Related tire and wheel tools
- Tire size comparison: height, width, sidewall and ride height of your current vs new tires.
- Tire size calculator: dimensions of any size, a tire size chart, and equivalent sizes.
Frequently asked questions
Do bigger tires make your speedometer read faster or slower?
Slower. A taller tire travels farther each turn, but the speedometer still counts turns as if the original tire were fitted, so it shows less than your real speed. Going from 265/70R17 (31.6 in) to 33x12.50R17 (33.0 in), an indicated 60 mph is really 62.6 mph.
How do you calculate speedometer error from tire size?
Actual speed = indicated speed × new tire diameter ÷ old tire diameter. With 33-inch tires replacing 31.6 in stock tires: 60 × 33 ÷ 31.61 = 62.6 mph, an error of +4.4%.
How much does a 35 inch tire affect the speedometer?
On a truck that came with 265/70R17 tires, 35-inch tires are +10.7% taller, so the speedometer reads 66.4 mph when it shows 60 and 77.5 mph when it shows 70. The odometer under-counts by the same percentage.
Does changing the axle gear ratio affect the speedometer?
Yes, if the speedometer reads from the transmission or driveshaft. A numerically higher ratio (for example 3.73 → 4.10) turns the driveshaft more for each wheel turn, so the speedometer reads higher. Re-gearing to 4.10 with the 35s above brings the error from +10.7% down to +0.7%. Enter both ratios in the optional fields to include a gear change. Vehicles that read speed from the wheel sensors ignore gear changes.
Do smaller tires make the speedometer read faster?
Yes. With a shorter tire you travel less distance per turn, so the speedometer reads high. Fitting 245/70R17 (30.5 in) instead of 265/70R17, an indicated 60 mph is only 57.9 mph.
Does the odometer change with bigger tires?
It's off by the same percentage as the speedometer. With the 33s above, every 100 miles on the odometer is really 104.4 miles, so fuel economy calculated from the odometer looks worse than it is, and service intervals come around later than the odometer suggests.
How do I recalibrate my speedometer after new tires?
On most vehicles since the 2000s, enter the new tire size (or revolutions per mile) with a handheld programmer or tuner, or have a dealer do it. Older vehicles with a cable-driven speedometer need a different driven gear in the transmission. Some trucks allow a tire size setting in the instrument cluster menu.
Are speedometers accurate from the factory?
Most read slightly high by design: regulations in many countries forbid showing less than your true speed, so manufacturers build in a small over-read, often 1-3%. A speedometer that reads low after new tires has lost that margin, which is when tickets happen. A GPS app is a quick way to check.
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