Tire size calculator
Enter your current size and the one you are considering. Everything is computed in your browser — nothing is sent anywhere.
225/45R17 → 235/40R18
+1.7%
rolling diameter · +10.9 mm
- Ride height
- +5.5 mm
- per side
- Tread width
- +10 mm
- 235 mm section
- Sidewall
- −7.3 mm
- 94.0 mm tall
- True speed at 60
- 61.0
- mph when the dial reads 60
›Speedometer table and full specifications
Speedometer error
| Dial reads | True speed | Error |
|---|---|---|
| 50 mph | 50.9 | +0.9 mph |
| 60 mph | 61.0 | +1.0 mph |
| 70 mph | 71.2 | +1.2 mph |
225/45R17
- Diameter
- 24.97″
- Circumference
- 78.5″
- Revs / mile
- 808
235/40R18
- Diameter
- 25.40″
- Circumference
- 79.8″
- Revs / mile
- 794
How the numbers are worked out
A metric size gives you everything you need. Sidewall height is the section width times the aspect ratio, and it appears twice in the overall diameter — once above the rim and once below.
sidewall = width × (aspect ÷ 100)
diameter = (rim × 25.4) + (2 × sidewall)
circumference = π × diameter
revs/mile = 1,609,344 ÷ circumference
225/45R17 → sidewall = 225 × 0.45 = 101.25 mm
diameter = 431.8 + 202.5 = 634.30 mm (24.97″)Comparing two sizes is then a matter of the diameter ratio. That single ratio drives the speedometer error, the odometer error and the change in ride height — which is half the diameter change, because radius is what sits between the axle and the road.
Questions
›How much can tire size differ from standard?
The industry rule is 3% of rolling diameter. Inside that, speedometer error stays small and wheel-speed-derived systems such as ABS and traction control keep working from their original calibration. Beyond 5% you are modifying the car, not substituting a tyre.
›What do the numbers in 225/45R17 mean?
225 is the section width in millimetres. 45 is the aspect ratio — the sidewall height as a percentage of that width, so 101.25 mm here. R means radial construction, and 17 is the rim diameter in inches.
›Why does a bigger tire make my speedometer read low?
The speedometer counts wheel revolutions and assumes a fixed distance per turn. A larger tyre covers more ground per revolution, so you travel further than the dial assumes and your true speed is higher than the number shown.
›Does a wider tire always need more clearance?
Not vertically — that is set by diameter, not width. But extra section width needs room at the inner arch and the suspension leg, and it shows up at full steering lock long before it shows up driving straight.
›Is the calculation the same for inch-based sizes?
No. Flotation sizes such as 31x10.50R15 state the overall diameter directly in inches, so no aspect-ratio maths is needed. This calculator covers the metric format.