Bigger tires make your speedometer read slow. Enter your stock tire size and the new one, and the calculator shows your true speed at any speedometer reading, how far off the odometer is, how much ground clearance the axles gain and, if you add your axle ratio, the ratio that brings back stock gearing. Sizes can be metric (285/70R17), flotation (35x12.50R17) or just the height in inches (35).
Speedometer correction
Actual speed after a tire size change
Why a taller tire makes the speedometer read slow
The speedometer does not measure road speed. It counts how fast the transmission output shaft or the wheels turn and converts that to miles per hour using the circumference of the factory tire. A taller tire covers more ground with every turn, so the vehicle moves faster than the gauge shows and the odometer logs fewer miles than you drive.
The correction is a simple ratio: actual speed = speedometer reading × new tire height ÷ stock tire height. An LT285/70R17 is 32.7 inches tall and a 37x12.50R17 is 37.0 inches, so 37.0 ÷ 32.7 = 1.131. At an indicated 60 mph the truck is doing 60 × 1.131 = 67.9 mph, and the odometer records 88.4 miles for every 100 driven. A shorter tire works the same way in reverse: the speedometer reads fast.
Ground clearance under the axle housings rises by half the height difference, because the axle sits at the center of the tire. Going from 32.7 to 37.0 inches lifts the axles about 2.1 inches. Clearance under the frame and body changes by the same amount only when the suspension stays the same.
Speedometer error for common tire upgrades
Calculated heights for popular swaps, with the true speed when the speedometer shows 60 and 70 mph and the miles the odometer logs for every 100 driven.
| Stock tire | New tire | Height change | Actual at 60 mph | Actual at 70 mph | Odometer per 100 mi |
|---|---|---|---|---|---|
| 225/75R16 29.3 in | 265/75R16 31.6 in | +8.1% | 64.8 mph | 75.6 mph | 92.5 mi |
| 245/75R16 30.5 in | 285/75R16 32.8 in | +7.8% | 64.7 mph | 75.4 mph | 92.8 mi |
| 245/75R17 31.5 in | 285/70R17 32.7 in | +3.9% | 62.4 mph | 72.8 mph | 96.2 mi |
| 245/75R17 31.5 in | 33x12.50R17 33.0 in | +4.9% | 62.9 mph | 73.4 mph | 95.4 mi |
| 245/75R17 31.5 in | 35x12.50R17 35.0 in | +11.2% | 66.7 mph | 77.9 mph | 89.9 mi |
| 255/75R17 32.1 in | 35x12.50R17 35.0 in | +9.2% | 65.5 mph | 76.4 mph | 91.6 mi |
| 265/70R16 30.6 in | 285/75R16 32.8 in | +7.3% | 64.4 mph | 75.1 mph | 93.2 mi |
| 265/70R17 31.6 in | 285/70R17 32.7 in | +3.5% | 62.1 mph | 72.4 mph | 96.6 mi |
| 265/70R17 31.6 in | 33x12.50R17 33.0 in | +4.4% | 62.6 mph | 73.1 mph | 95.8 mi |
| 275/65R18 32.1 in | 35x12.50R18 35.0 in | +9.1% | 65.5 mph | 76.4 mph | 91.6 mi |
| 275/60R20 33.0 in | 35x12.50R20 35.0 in | +6.1% | 63.7 mph | 74.3 mph | 94.3 mi |
| LT285/70R17 32.7 in | 35x12.50R17 35.0 in | +7.0% | 64.2 mph | 74.9 mph | 93.5 mi |
| LT285/70R17 32.7 in | 37x12.50R17 37.0 in | +13.1% | 67.9 mph | 79.2 mph | 88.4 mi |
| 35x12.50R17 35.0 in | 37x12.50R17 37.0 in | +5.7% | 63.4 mph | 74.0 mph | 94.6 mi |
| 37x12.50R17 37.0 in | 40x13.50R17 40.0 in | +8.1% | 64.9 mph | 75.7 mph | 92.5 mi |
How to correct the speedometer
- Late-model vehicles: the tire size is a setting in the vehicle computer. A speedometer calibrator or a programmer writes the new size, which also corrects the odometer, shift points on many automatics and the stability control math.
- Older vehicles with a cable speedometer: the fix is a different speedometer drive gear in the transmission or transfer case, chosen by tire size and axle ratio.
- After any change: check the result against a GPS speed reading at a steady highway speed.
Bigger tires and gearing
A taller tire has the same effect on the drivetrain as a numerically lower axle ratio. With 4.10 axles, going from 32.7-inch to 37-inch tires feels like 3.62 gears on the stock tires: the engine turns about 12 percent slower at any road speed, pulls harder to get the truck moving and the automatic shifts more often on grades. To restore the stock feel, multiply the axle ratio by new height ÷ stock height (4.10 × 1.131 = 4.64) and choose the closest ratio made for your axles, such as 4.56 or 4.88.
To compare every gear, the crawl ratio and cruise RPM before and after a change, use the gear ratio calculator. Browse ring and pinion gears and installation kits for the parts, and the tire size calculator to convert between metric and flotation sizes.
About these numbers
Metric heights are calculated from the size (2 × width × aspect ratio ÷ 100 ÷ 25.4 + wheel diameter), and flotation sizes use the stated height. A tire under load rolls a little shorter than its unloaded height, and two brands of the same size can differ by a few tenths of an inch, so the real error can land slightly above or below the calculated figure. For more on tire size math, read 325/60R20 in Inches: Tire Size Math Made Simple, and see off-road tires and lift kits to make a bigger tire fit.