Tyre Size Speed Calculator
Fit a taller tyre and your speedometer starts reading low. Fit a shorter one and it reads high. This tyre size speed calculator takes the original and new tyre specs in standard metric format (width / aspect R wheel, e.g. 225/70R16), works out overall diameters, and returns your actual ground speed, the speedometer error as a percentage, and the corrected MPH or km/h the gauge should be showing.
How the tyre size speed calculator works
A tyre's overall diameter comes from three numbers on the sidewall: section width in millimetres, aspect ratio (sidewall height as a percentage of width), and rim diameter in inches. The formula is:
diameter (inches) = rim diameter + 2 x (width x aspect / 100) / 25.4
For a 225/70R16: sidewall = 225 x 0.70 = 157.5 mm, diameter = 16 + (2 x 157.5) / 25.4 = 16 + 12.40 = 28.40 inches. For a 245/70R16: sidewall = 245 x 0.70 = 171.5 mm, diameter = 16 + (2 x 171.5) / 25.4 = 29.51 inches.
Your speedometer measures driveshaft turns, not ground speed, and converts using the factory tyre diameter. When you fit bigger tyres, each wheel revolution covers more ground but the gauge still assumes the stock distance, so the displayed number drops below the real speed. The speed correction factor is new diameter / original diameter. Multiply the indicated speed by that factor to get actual speed.
Plus-sizing rule of thumb
| Change | Diameter impact | Speedo error (indicated 60) |
|---|---|---|
| +10 mm width, same aspect | +0.55 in (+1.9%) | Actual 61.2 mph |
| +1 inch rim, lower profile (plus-one) | +/- 0.3 in | Within 1% (correct) |
| Stock 225/70R16 to 245/70R16 | +1.11 in (+3.9%) | Actual 62.3 mph |
| Stock 265/70R17 to 315/70R17 | +2.75 in (+8.8%) | Actual 65.3 mph |
Staying legal and safe
Most jurisdictions (US, UK, EU, Australia, NZ) allow a speedometer to read up to 10 percent high but never low. After a tyre upsize the gauge shifts toward reading low, so you can quickly end up driving faster than you think. Use this tyre size speed calculator before a fitment change and plan to stay under a 3 percent diameter increase if you do not want to recalibrate. Above 3 percent you should retune the speedometer through the ECU or swap the driven gear on older manual gearboxes.
What to measure if the sidewall is worn off
If your tyre has no readable spec (common on trailers and older vehicles), measure overall diameter with a tape at the centre of the tread, then a second time at 90 degrees to catch any out-of-round. Take the average. You can feed a custom diameter into the plain speedometer calculator, which supports both tyre size and axle-gear inputs. Note that a worn tyre is typically 0.5-1.0 inch smaller than a new one, which adds 2-4 percent to the speedo reading before you even change sizes.