Calculate your speedometer error and true speed after a tire size change or from a GPS speed check, plus the odometer impact of new tires.
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Speedometer Error Formula
AS = IS * (ND / OD)
E% = ((IS - AS) / AS) * 100
- AS is your actual (true) road speed
- IS is the speed shown on the speedometer
- ND is the overall diameter of the new tire
- OD is the overall diameter of the original tire the speedometer was calibrated for
- E% is the speedometer error as a percentage of true speed
A speedometer counts wheel or driveshaft revolutions and converts them to speed using the tire diameter the factory calibrated it for. If you install tires with a different overall diameter, each revolution covers a different distance, so the displayed speed drifts away from your true speed. The tire size change mode of the calculator applies the ratio ND/OD to your speedometer reading to find your true speed, the error percentage, and a correction factor.
When you enter tire sizes as sidewall codes such as 225/65R17, the calculator first converts each code to an overall diameter:
D = R + (2 * W * (A / 100)) / 25.4
- D is the overall tire diameter in inches
- R is the rim diameter in inches
- W is the section width in millimeters
- A is the aspect ratio (sidewall height as a percent of width)
The GPS speed check mode compares the speed on your dash to the speed reported by a GPS device at the same steady moment. Here the error is E% = ((IS – GPS) / GPS) * 100, and the correction factor GPS/IS lets you estimate your true speed at any other reading. The tire size mode also reports odometer impact, because the same ratio applies to distance: actual distance = indicated distance * (ND / OD).
Speedometer Error by Tire Size Change and Accuracy Standards
The first table shows how much a change in overall tire diameter shifts your true speed when the dash shows 60 mph. A positive diameter change means larger tires, which make the speedometer read low.
| Diameter change | True speed at indicated 60 mph | Speedometer error |
|---|---|---|
| -6% | 56.4 mph | Reads 6.4% high |
| -4% | 57.6 mph | Reads 4.2% high |
| -2% | 58.8 mph | Reads 2.0% high |
| 0% | 60.0 mph | No tire-related error |
| +2% | 61.2 mph | Reads 2.0% low |
| +4% | 62.4 mph | Reads 3.8% low |
| +6% | 63.6 mph | Reads 5.7% low |
| +8% | 64.8 mph | Reads 7.4% low |
The second table summarizes the accuracy rules speedometers and odometers are actually held to. This is useful for interpreting your result: a speedometer that reads a few percent high is normal and intentional, while one that reads low on larger tires falls outside how factory speedometers are set up.
| Standard | Applies to | Allowed error |
|---|---|---|
| UNECE Regulation 39 (Europe, UK, Australia) | Passenger vehicles | Must never read below true speed; may read up to 10% + 4 km/h high |
| US federal rules (passenger cars) | Passenger vehicles | No current federal accuracy requirement; manufacturers typically target within about 3% |
| 49 CFR 393.82 (FMCSA) | US commercial vehicles | Accurate within plus or minus 5 mph at 50 mph |
| Typical odometer practice (SAE J678) | Odometers | Generally within about plus or minus 4% |
Tire size errors also flow into your odometer. On tires 6% larger than stock, every 10,000 miles shown is about 10,600 miles of real driving, which quietly shifts lease mileage limits, warranty mileage, resale mileage, maintenance intervals, and any fuel economy you calculate from the odometer.
Example Problems
Example 1: Tire size change
You replace 225/65R17 tires with 245/70R17 tires and want your true speed at an indicated 60 mph.
First convert each size to an overall diameter. Original: D = 17 + (2 * 225 * 0.65) / 25.4 = 28.52 in. New: D = 17 + (2 * 245 * 0.70) / 25.4 = 30.50 in.
The ratio is 30.50 / 28.52 = 1.070, so at an indicated 60 mph your true speed is 60 * 1.070 = 64.2 mph. The error is (60 – 64.2) / 64.2 * 100 = -6.5%, meaning the speedometer reads 6.5% low. Every 10,000 miles on the odometer is about 10,700 miles of actual travel.
Example 2: GPS speed check
Your dash shows 70 mph while a GPS app reads a steady 66 mph.
The error is (70 – 66) / 66 * 100 = 6.1%, so the speedometer reads 6.1% high. The correction factor is 66 / 70 = 0.94, so when the dash shows 60 mph your true speed is about 60 * 0.94 = 56.6 mph. In km/h terms the reading is 112.7 against a true 106.2, which is inside the UNECE limit of 10% + 4 km/h, so this is normal factory behavior.
Frequently Asked Questions
Why does my speedometer read higher than my GPS?
Most factory speedometers are deliberately set to read slightly high. Rules such as UNECE Regulation 39 make it illegal for the speedometer to ever read below true speed, so manufacturers build in a margin of a few percent to cover tire wear, tire pressure, and manufacturing tolerances. Tire wear adds to this: as tread wears down, the tire diameter shrinks slightly, which pushes the reading even higher relative to true speed. A GPS reading a few percent below the dash at steady speed is normal.
Do bigger tires make my speedometer read faster or slower?
Bigger tires make the speedometer read slower than you are actually going. A larger diameter tire covers more ground per revolution, but the speedometer still converts revolutions using the smaller original diameter, so it under-reports speed. This is the riskier direction of error, because you can be speeding while the dash shows a legal speed. Smaller tires do the opposite and make the speedometer read high.
Does a tire size change affect the odometer too?
Yes, and by the same percentage as the speed error. The odometer counts the same revolutions, so larger tires make it under-record distance and smaller tires make it over-record. Over tens of thousands of miles this matters for lease mileage caps, warranty limits, resale value, service intervals, and fuel economy figures calculated from odometer miles. The calculator’s odometer impact result shows the real distance behind every 10,000 indicated miles.
