The new tire changes overall diameter by +5.42%. With a speedometer calibrated to the original tire, that same percentage is how far the speedometer and odometer read off — a taller tire means your true speed is higher than indicated and the odometer under-counts distance.
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 42.2 km/h | 25 mph | 26 mph |
| 60 km/h | 63.3 km/h | 37 mph | 39 mph |
| 80 km/h | 84.3 km/h | 50 mph | 52 mph |
| 100 km/h | 105.4 km/h | 62 mph | 66 mph |
| 120 km/h | 126.5 km/h | 75 mph | 79 mph |
How this math works
Every result is governed by an exact equation — no guessed numbers, no hidden constants:
- Sidewall height:
sidewall (mm) = width × (aspect ÷ 100) - Overall diameter:
diameter (mm) = rim (in) × 25.4 + 2 × sidewall— A tire’s outer diameter determines how many times it rotates per mile. - Circumference:
circumference = π × diameter - Speedometer error:
true speed = indicated speed × (new diameter ÷ old diameter)— The speedometer is calibrated to the original tire, so a taller tire makes you travel faster than it shows.
Worked example
Going from 225/45R17 (≈634 mm / 25.0 in) to 235/45R18 (≈686 mm / 27.0 in) increases overall diameter by about 8%. At an indicated 100 km/h you would actually be doing about 108 km/h, and your odometer would under-count distance by the same 8%.
FAQ
How much bigger tire can I fit without a big speedometer error?
A change in overall diameter of roughly 3% or less keeps the speedometer error small (about 3 km/h at 100 km/h) and is generally considered acceptable. Anything beyond that starts to noticeably affect the speedometer, odometer and, on some vehicles, ABS/traction calibration.
Does a taller tire make my speedometer read high or low?
Low. A taller tire covers more ground per revolution, so you are actually travelling faster than the speedometer indicates. A shorter tire does the opposite — the speedometer reads higher than your true speed.
Will bigger tires affect my odometer and fuel economy readings?
Yes. A taller tire means fewer wheel revolutions per mile, so the odometer under-counts distance and the calculated fuel economy (miles per gallon / litres per 100 km) shifts accordingly. The percentage error matches the diameter difference this tool shows.
Can I type a full tire size like 225/45R17?
Yes — paste the size into the quick-entry box and it fills the width, aspect ratio and rim fields automatically. It also understands flotation sizes such as 33x12.5R17 (common on trucks and off-road vehicles), where the first number is the overall diameter in inches.
Can the wrong tire size trigger a warning light or fault code?
It can. A large enough diameter change makes the wheel-speed and vehicle-speed signals disagree with the vehicle’s calibration, which on some cars sets ABS, traction-control or speed-sensor trouble codes (for example wheel-speed-sensor codes in the C0035–C0050 range, or P0500 vehicle speed sensor). Recalibrating the speedometer/TPMS after a big change avoids this.
Related trouble codes
Popular tire sizes
Jump to a specific size for its full dimensions and closest alternatives:
- 225/45R17
- 235/45R17
- 205/55R16
- 225/40R18
- 245/45R18
- 255/55R18
- 265/70R17
- 275/55R20
- 285/75R16
- 215/55R17
- 235/55R19
- 265/65R17
More calculators
- Gear Ratio, Tire Size & Engine RPM Calculator — Find engine RPM at any speed from gear ratio, final drive and tire size.
- Engine Compression Ratio Calculator — Calculate static compression ratio and displacement from engine dimensions.