Tire Size Calculator
Overall diameter and sidewall from a tyre size such as 225/45R17, and how a different size changes the diameter, what the speedometer reads and the odometer.
Result
Change in overall diameter: 1.72%
Dimensions only. Fitment, clearance, load index and speed rating need checking with a tyre specialist.
- The speedometer will read
- 68.8 mph
- Odometer reading per 100 true miles or km
- 98.31
- Current overall diameter
- 634.3 mm (25 in)
- New overall diameter
- 645.2 mm (25.4 in)
- Current sidewall height
- 101.3 mm (4 in)
- New sidewall height
- 94 mm (3.7 in)
- New rolling circumference
- 79.8 in (2,027 mm)
https://calcgallery.com/vehicle-specs/tire-size-calculator/
Results are arithmetic on the figures you enter. The site stores no vehicle data, fuel prices, mileage rates, depreciation rates or residual values, because they vary by model and place and change over time. Real fuel economy varies with driving, weather and load. Nothing here is a fitment, safety or financial recommendation. Read the full disclaimer
How to use the Tire Size Calculator
- [object Object]
- Enter the size you are thinking of fitting.
- Read the change in overall diameter, what your speedometer will read at a true speed, and the odometer error.
The formulas
sidewall height = width × aspect ratio ÷ 100overall diameter = rim diameter + 2 × sidewall heightspeedometer reads = true speed × current diameter ÷ new diameterThe rim is in inches and the width in millimetres, so the rim is converted to millimetres first: 17 inches is 431.8 mm. Adding inches to millimetres directly is the classic mistake, and it gives a diameter of a few hundred millimetres too small.
What this does not tell you
Whether a tyre fits. Wheel-arch clearance, offset, load index, speed rating and the effect on stability and braking systems are outside what the dimensions can show. These are nominal, unloaded dimensions; a tyre on a car rolls slightly smaller.
With your numbers
- Current: 17 in + 2 × 101.25 mm = 634.3 mm
- New: 18 in + 2 × 94 mm = 645.2 mm
Quick reference
Dimensions only. Fitment, clearance, load index and speed rating need checking with a tyre specialist.
| Example | Change in overall diameter |
|---|---|
| 225/45R17 to 235/40R18 | 1.72% |
| 205/55R16 to 225/45R17 | 0.38% |
| 275/35R20 against 255/40R20 | -1.62% |
| 265/70R17 to 285/70R17 | 3.49% |
| 195/65R15 to 205/55R16 at 100 km/h | -0.41% |
| Same size | 0% |
Common questions about the Tire Size Calculator
- How do I read a tyre size?
- In 225/45R17, 225 is the tread width in millimetres, 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 here is 101.25 mm, and the overall diameter is the rim plus two sidewalls: 431.8 + 202.5 = 634.3 mm.
- Why does a bigger tyre make my speedometer read low?
- The speedometer counts wheel turns. A larger tyre travels further each turn, so at the same count you are going faster than it shows. Going from 225/45R17 to 235/40R18 is 1.72% larger: at a true 70 mph the speedometer shows about 68.8.
- How much diameter change is acceptable?
- A figure of about 3% is widely repeated, but it is a rule of thumb, not a standard. Clearance, load rating, speed rating, stability systems and local rules all matter, and this page cannot check any of them. Ask a tyre specialist before changing size.
- Does the odometer change too?
- Yes, by the same ratio. With the larger tyre in the example, 100 true miles show as about 98.3 on the odometer, so it under-records distance by about 1.7%.