Tire Size Speedometer Error Calculator
Compare two tire sizes, such as 225/45R17 and 245/40R18: diameter change, speedometer error, true speed in mph or km/h and odometer distance. How it works
Use the Tire Size Speedometer Error Calculator
About the Tire Size Speedometer Error Calculator
Changing from 225/45R17 to 245/40R18 makes the tire 2.98% taller, 634.3 mm to 653.2 mm, so a speedometer reading of 60 mph is a true 61.8 mph. True speed = indicated speed × new diameter ÷ stock diameter, and the speedometer reads slow when the new tire is bigger and fast when it is smaller.
Speedometer reading and true speed, 225/45R17 to 245/40R18
| Speedometer reads (mph) | True speed (mph) | Difference (mph) |
|---|---|---|
| 30 | 30.9 | +0.9 |
| 45 | 46.3 | +1.3 |
| 55 | 56.6 | +1.6 |
| 60 | 61.8 | +1.8 |
| 65 | 66.9 | +1.9 |
| 70 | 72.1 | +2.1 |
| 75 | 77.2 | +2.2 |
| 80 | 82.4 | +2.4 |
The change is 2.98% at every speed. It assumes the speedometer was accurate on the stock tires.
Other size changes at a 60 mph reading
| Stock size | New size | Stock diameter (mm) | New diameter (mm) | Diameter change | True speed at 60 mph |
|---|---|---|---|---|---|
| 205/55R16 | 225/45R17 | 631.9 | 634.3 | +0.38% | 60.2 |
| 225/45R17 | 235/40R18 | 634.3 | 645.2 | +1.72% | 61.0 |
| 225/45R17 | 245/40R18 | 634.3 | 653.2 | +2.98% | 61.8 |
| 225/45R17 | 255/35R19 | 634.3 | 661.1 | +4.23% | 62.5 |
| 265/70R17 | 285/75R16 | 802.8 | 833.9 | +3.87% | 62.3 |
Nominal diameters from the size markings. The calculator marks a change above 3% with a warning; the last two rows are above it.
How to use this tool
- Type the stock size as printed on the sidewall, such as 225/45R17, P225/45R17 or 35x12.50R17, or fill in width in mm, aspect ratio in percent and rim diameter in inches.
- Enter the new tire the same way, then the speed your speedometer shows; the mph and km/h buttons switch the unit.
- Read the true speed, the difference, the two overall diameters and the percentage error, then the odometer’s true distance per 100 indicated and the wheel turns per mile or km.
Frequently asked questions
- How do I calculate speedometer error after changing tire size?
- Divide the new overall diameter by the stock one and multiply the speedometer reading by the result. For 225/45R17 to 245/40R18 the ratio is 1.0298, so a 30 mph reading is a true 30.9 mph.
- What do the numbers in a tire size like 225/45R17 mean?
- 225 is the section width in millimeters, 45 is the sidewall height as a percentage of that width, R means radial construction and 17 is the rim diameter in inches. The sidewall of a 225/45 tire is 225 × 0.45 = 101.25 mm tall, and the tire’s overall diameter is the rim plus two sidewalls. A lower second number on the same rim means a shorter sidewall and a smaller tire.
- Will bigger tires make my speedometer read slow or fast?
- Slow. A taller tire covers more ground per turn, so the true speed is higher than the reading. A smaller tire does the opposite: going back from 245/40R18 to 225/45R17, a 60 mph reading is a true 58.3 mph, 2.89% lower. The odometer is off by the same percentage: 100 indicated miles on the taller tire are 102.98 real miles.
- What does the 3% warning mean?
- It appears when the new overall diameter differs from the stock one by more than 3%. That is this calculator’s own display threshold, not a legal or manufacturer limit. A larger change moves the speedometer and odometer further from the true values, so check the vehicle or tire maker’s guidance before fitting it.
- Does the calculator work in km/h?
- Yes. Press km/h and a reading already typed is converted (60 mph becomes 96.6 km/h). The percentage error is the same in any unit, so a 100 km/h reading on 245/40R18 is a true 103.0 km/h, and the odometer shows 103.0 true km per 100 indicated. Speeds run from 1 to 200 mph or 1 to 320 km/h.
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