Tire Size Calculator
Enter your current tire size and the size you are thinking of, such as 225/65R17 and 235/65R17. The calculator compares diameter, width, sidewall and revolutions, shows how much your speedometer will be off, and below it estimates the fuel and cost of a trip.
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How to use it
- Enter the size of your current tires as printed on the sidewall, for example 225/65R17.
- Enter the new size you are considering, and the speed you want to check, such as 60 mph.
- Read the actual speed, the difference in diameter and the full comparison table.
- Optionally, enter a distance, fuel economy and fuel price to see what a trip costs.
Why use this tool
Any tire size
Metric sizes such as 205/55R16, P-metric and LT sizes, and flotation sizes such as 31x10.50R15.
Speedometer error
See your real speed and how far off the speedometer and odometer will be with the new size.
Full comparison
Width, sidewall, rim, overall diameter, circumference and revolutions per mile or km, side by side.
3% check
A warning when the new size differs by more than 3% in diameter, the limit most tire shops use.
Trip cost
Fuel and cost of a drive in mpg, l/100 km or km/l, one way or round trip, split between passengers.
Private
Everything is calculated in your browser. Nothing you enter is sent or stored.
How to read a tire size
In 225/65R17, 225 is the section width in millimeters, 65 is the aspect ratio (the sidewall height is 65% of the width), R means radial construction and 17 is the rim diameter in inches. A P in front means passenger and LT means light truck.
Flotation sizes such as 31x10.50R15 give the overall diameter (31 inches), the width (10.5 inches) and the rim diameter (15 inches) directly.
Tire size formulas
Sidewall height = width × aspect ratio ÷ 100. For 225/65R17 that is 225 × 0.65 = 146.3 mm.
Overall diameter = rim × 25.4 + 2 × sidewall. For 225/65R17: 17 × 25.4 + 2 × 146.3 = 724.3 mm, or 28.5 inches.
Circumference = π × diameter, and revolutions per mile = 1,609,344 ÷ circumference in mm.
Speedometer: actual speed = shown speed × new diameter ÷ old diameter.
Common plus-sizing options
Going up in rim size while lowering the aspect ratio keeps the overall diameter close to the original. A few common combinations:
| Original | Plus size | Diameter change | At 60 mph you drive |
|---|---|---|---|
| 205/55R16 | 225/45R17 | +0.4% | 60.2 mph |
| 205/55R16 | 225/40R18 | +0.8% | 60.5 mph |
| 225/65R17 | 235/55R19 | +2.3% | 61.4 mph |
| 225/65R17 | 235/65R17 | +1.8% | 61.1 mph |
| 265/70R17 | 285/70R17 | +3.5% | 62.1 mph |
| 195/65R15 | 205/55R16 | -0.4% | 59.8 mph |
Why the 3% rule matters
Most tire shops keep the new overall diameter within about 3% of the original size. Beyond that the speedometer and odometer drift further, the transmission may shift at different points, and ABS, traction control and stability control get wheel-speed readings they were not calibrated for.
Larger tires may also rub on suspension parts or the wheel arch when you steer or the suspension compresses, so always check clearance, load index and speed rating before buying.
How to save fuel on a trip
Fuel economy depends as much on how you drive as on the car:
- Keep tires at the pressure on the door sticker; under-inflated tires waste fuel.
- Drive at a steady speed; fuel use rises quickly above about 55 to 60 mph (90 to 100 km/h).
- Remove roof racks and heavy cargo you don't need.
- Plan fuel stops where prices are lower.
- Wider and heavier tires usually raise fuel use slightly.
- Share the ride: split the cost by the number of passengers.
Frequently asked questions
What does 225/65R17 mean?
The tire is 225 mm wide, the sidewall is 65% of the width (146 mm), R stands for radial, and it fits a 17-inch rim. Its overall diameter is about 724 mm or 28.5 inches.
Can I put bigger tires on my car?
Often yes, if the overall diameter stays within about 3% of the original, the tire fits without rubbing, and the load index and speed rating are at least those on your door sticker. Check with your tire shop.
Will bigger tires make my speedometer wrong?
Yes. With a taller tire the wheel turns fewer times per mile, so the speedometer shows less than your real speed. A 3% larger tire at a shown 60 mph means you are actually doing about 61.8 mph.
Is 235 wider than 225?
Yes, by 10 mm (about 0.4 inches). With the same aspect ratio the sidewall also gets taller, so the overall diameter grows too.
How do I convert tire sizes to inches?
Divide millimeters by 25.4. Enter the size here with inches and mph selected to see the width, sidewall and diameter in inches.
How is the trip cost calculated?
Fuel = distance × fuel use, converted between mpg, l/100 km and km/l, and cost = fuel × price. For a round trip the result is doubled, and you can split it between passengers.
Is my data sent anywhere?
No. Everything is calculated in your browser. The page only remembers whether you use metric or US units on this device.
How to Choose the Right Tire Size
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