Tire size and speedometer error: how to calculate it
By FitMyTires · Published · Updated
A taller tire covers more distance per revolution. If the speedometer was accurate with the current tires, the estimated speed after a change is the indicated speed multiplied by proposed diameter divided by current diameter.
Worked example: 225/45R17 to 245/40R18
The nominal diameters are 634.3 mm and 653.2 mm. The proposed tire is 2.98% larger. At an indicated 60 mph, the estimate is 61.79 mph.
| Indicated | Estimated actual |
|---|---|
| 30 mph | 30.89 mph |
| 60 mph | 61.79 mph |
| 70 mph | 72.09 mph |
Compare the speedometer change
Truck example: 265/70R17 to 285/70R17
The nominal diameters are 31.61 in and 32.71 in (802.8 mm and 830.8 mm). The proposed tire is 3.49% larger, outside the calculator's 3% informational band. When the speedometer shows 65 mph, the estimated speed is 67.27 mph. At an indicated 45 mph it is 46.57 mph, and at 75 mph it is 77.62 mph. An odometer showing 1,000 miles would have covered about 1,034.9 miles.
Compare 265/70R17 with 285/70R17
How far off is the speedometer for each percent of change?
Each 1% of diameter change moves the estimate by 1% of the indicated speed: 0.6 mph at an indicated 60 mph. Find the percentage for your sizes in the calculator, then read across. The table assumes the speedometer was accurate with the current tires.
| Diameter change | Indicated 30 mph | Indicated 60 mph | Indicated 75 mph |
|---|---|---|---|
| -3% | 29.10 mph | 58.20 mph | 72.75 mph |
| -2% | 29.40 mph | 58.80 mph | 73.50 mph |
| -1% | 29.70 mph | 59.40 mph | 74.25 mph |
| +1% | 30.30 mph | 60.60 mph | 75.75 mph |
| +2% | 30.60 mph | 61.20 mph | 76.50 mph |
| +3% | 30.90 mph | 61.80 mph | 77.25 mph |
| +5% | 31.50 mph | 63.00 mph | 78.75 mph |
| +7% | 32.10 mph | 64.20 mph | 80.25 mph |
| +10% | 33.00 mph | 66.00 mph | 82.50 mph |
Which direction is the error?
With a larger proposed tire, the estimate is higher than the indicated speed. With a smaller proposed tire, it is lower. Compare the tire diameters, not just the rim diameters: a larger rim combined with a shorter sidewall can leave overall tire diameter nearly unchanged.
The same geometric ratio describes distance if the odometer also uses the original rolling calibration. In the 225/45R17 to 245/40R18 example, an indicated 100 miles corresponds to about 102.98 estimated miles. This is not a correction for an odometer's existing bias or a substitute for calibrated measurement.
Can published revolutions per mile improve the estimate?
When both exact tire models have published rev/mile figures, use indicated speed × current rev/mile ÷ proposed rev/mile. For example, 820 current rev/mile and 800 proposed rev/mile give 61.50 mph at an indicated 60 mph. Note that this ratio runs in the opposite order to the diameter ratio.
FitMyTires uses supplied rolling figures only when both are present. Otherwise it uses the diameter ratio and labels the basis. Published rolling values depend on measurement conditions; tire wear, inflation, and load can change operating behavior. Michelin's tire glossary describes rolling measurements.
How is the reading corrected after a size change?
On vehicles with electronic speedometers, the gauge is not adjusted directly. A control module converts speed-sensor pulses into speed and distance using a stored tire size or revolutions-per-mile value, and correcting the reading means changing that value. What can be changed, and by whom, depends on the vehicle.
- Dealer reprogramming. Some manufacturers offer a calibration for specific sizes. A General Motors service bulletin filed with NHTSA says GM supports calibrations only for tires sized and tested for the vehicle, that the service may cost extra, and that it is not covered by warranty. It also notes that changing the tire size stored in one module does not necessarily correct the speedometer.
- Aftermarket programmers and calibrators. Plug-in programmers and inline calibrators can change the stored value on some vehicles. Confirm that the tool supports the exact vehicle and software, and keep the original settings so they can be restored. Discount Tire recommends recalibrating after any tire size change and notes that some vehicles allow owner adjustment.
- Check the result. Compare the speedometer with GPS speed at a steady speed on a straight, level road, or time a measured distance. An FCA bulletin for an oversized-tire calibration describes this kind of road test and adjusting the setting if the reading is still off.
Why the stored value matters beyond the gauge
The same value can feed other systems, so an uncorrected change can affect more than the speed shown on the dash.
- Odometer. With a larger tire, recorded mileage falls behind actual distance. The odometer then reaches mileage-based service intervals and warranty limits after the vehicle has actually traveled farther.
- ABS, traction, and stability control. These systems compare wheel speeds. The GM bulletin above ties the tire size stored in the ABS module to wheel-speed sensor calibration.
- Transmission shift points. Many automatic transmissions use calculated vehicle speed when choosing shifts, and Discount Tire lists shift points among the things that change with overall tire height. A taller tire also lowers engine speed at a given road speed; recalibrating the reading does not change that gearing.
Does a small percentage establish compatibility?
No. A diameter comparison does not establish the correct load and speed ratings, wheel compatibility, or vehicle-system requirements. The 3% band in the calculator is an informational comparison threshold. Consult vehicle-specific guidance for AWD, staggered setups, and permitted replacements. Check the complete size marking and the vehicle specification before choosing a tire.