Speedometer Error Calculator
Actual speed = speedometer reading × new tire diameter ÷ old tire diameter. A taller tire turns fewer times per mile, so the speedometer shows less than you are really driving. Going from 265/70R17 to 285/70R17 (3.5% taller), 60 mph on the speedometer is about 62.1 mph.
Speedometer difference and engine speed
The calculator needs JavaScript. The formulas and tables on this page work without it: sidewall = width × aspect ratio ÷ 100, and diameter = rim × 25.4 + 2 × sidewall (in mm).
Accepts 225/45R17, P225/45R17, LT285/75R16, 225/45ZR17 94W and 33x12.50R15.
Result
Next: speedometer error · diameter · plus sizing
How to use it
- Enter your original size as A and the new size as B.
- Choose mph or km/h.
- Read the actual speed for each speedometer reading.
- Optionally enter axle ratio, gear ratio and speed for engine rpm.
Speedometer table: 265/70R17 to 285/70R17
| Speedometer shows | Actual (mph) | Speedometer shows | Actual (km/h) |
|---|---|---|---|
| 20 mph | 20.7 mph | 30 km/h | 31.0 km/h |
| 30 mph | 31.0 mph | 50 km/h | 51.7 km/h |
| 40 mph | 41.4 mph | 70 km/h | 72.4 km/h |
| 50 mph | 51.7 mph | 90 km/h | 93.1 km/h |
| 60 mph | 62.1 mph | 100 km/h | 103.5 km/h |
| 70 mph | 72.4 mph | 110 km/h | 113.8 km/h |
| 80 mph | 82.8 mph | 130 km/h | 134.5 km/h |
Assumes the speedometer was exactly right on the original size. In practice it may already read a little high; see the note on UN Regulation No. 39 below.
Engine speed and effective gearing
Engine rpm = road speed × tire revolutions per mile (or km) ÷ 60 × axle ratio × gear ratio. A taller tire lowers engine speed at the same road speed, which works like a numerically lower (taller) axle ratio. The tool shows the effective ratio and roughly which axle ratio would bring the engine speed back to where it was.
Speedometer rules
In countries that apply UN Regulation No. 39, paragraph 5.4 (EUR-Lex) (including the EU member states), a vehicle type is approved only if its speedometer never shows less than the true speed, and at the test speeds shows at most 10% plus 4 km/h more. That rule is about how new vehicles are approved. It does not say which tire sizes you may fit, and rules for changing tires differ between countries and states. Check locally, and ask a tire shop or the vehicle maker.
Recalibrating
Many vehicles can have the speedometer recalibrated after a size change, by a dealer or with a programmer for the vehicle. Whether that is possible, and needed, depends on your vehicle. Odometer and trip computer readings change by the same percentage as the speedometer.
Frequently asked questions
Will bigger tires make my speedometer wrong?
Yes. With a taller tire the speedometer reads lower than your actual speed, by the same percentage as the diameter change. With a smaller tire it reads higher.
How do I calculate actual speed after a tire change?
Multiply the speedometer reading by the new diameter and divide by the old diameter. 60 mph × 32.7 ÷ 31.6 is about 62.1 mph. The calculator does this for every speed in the table.
How much speedometer error is allowed?
For approving new vehicles, UN Regulation No. 39 says the speedometer must never show less than the true speed and at most 10% plus 4 km/h more at its test speeds. Rules after a tire change vary between countries, so check locally.
Does the odometer change too?
Yes. The odometer counts wheel turns, so it is off by the same percentage. With 3.5% taller tires, the vehicle covers about 103.5 km for every 100 km shown.
Can I recalibrate my speedometer?
Often, yes, by a dealer or with a programmer for your vehicle. Some vehicles need a different part or software setting. Ask a dealer or a tire shop for your model.
How do tire sizes affect gear ratio?
A taller tire acts like a taller gear: the engine turns slower at the same road speed. The effective axle ratio is the real ratio times old diameter divided by new diameter.
Last updated: October 15, 2026