Tire Size Calculator
Check how a new tire size changes diameter and speedometer reading.
Your numbers
Your results
Tire Size Calculator by TheCalcTools
How the Tire Size Calculator works
Enter the three numbers printed on the tire, for example 225/45R17.
Overall diameter = rim diameter + 2 × sidewall height, where sidewall = width × aspect ratio.
A larger diameter covers more road per turn, so the speedometer reads low.
The formula
Worked example
Using the default values shown in the calculator:
| Current width (mm) | 225 |
|---|---|
| Current aspect ratio (%) | 45 |
| Current rim (in) | 17 |
| New width (mm) | 245 |
| New aspect ratio (%) | 40 |
| New rim (in) | 18 |
- 225/45R17: 17 × 25.4 + 2 × 101.25 = 634.3 mm.
- 245/40R18 is 653.2 mm, 2.98% different, so 60 mph indicated is really 61.8 mph.
Result:
| Current diameter (mm) | 634.3 |
|---|---|
| New diameter (mm) | 653.2 |
| Diameter difference | 2.98% |
| Real speed when the speedometer shows 60 mph | 61.8 |
| New sidewall height (mm) | 98.0 |
Frequently asked questions
How much diameter change is safe?
Many manufacturers recommend staying within about 3% of the original diameter so speedometer, ABS and traction systems still work correctly.
Is this a fitment guide?
No. Check clearance, load rating and your vehicle manual or a tire shop before changing size.
Sources and further reading
Related calculators
More in Automotive & Vehicle Costs · All calculators
This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.