How to read a tire size
The size is molded on the sidewall, for example 225/45R17:
- 225 is the section width in millimeters.
- 45 is the aspect ratio: the sidewall height is 45% of the width.
- R means radial construction.
- 17 is the wheel (rim) diameter in inches.
Tire size formula
Diameter (in) = rim (in) + 2 × sidewall
Circumference = π × diameter
Revolutions per mile = 63,360 ÷ circumference
Worked examples
Plus-one wheel upgrade: 225/45R17 to 245/40R18
The stock tire is 24.97 in tall and the new one is 25.72 in, a difference of +2.98%. When your speedometer shows 60 mph you will really be doing about 61.8 mph, and the car sits about 0.37 in higher. That is inside the usual 3% guideline.
Bigger truck tires: 265/70R17 to 285/70R17
Diameter goes from 31.61 in to 32.71 in, +3.49%. At an indicated 60 mph the truck is doing about 62.1 mph. This is a popular upgrade, but check for rubbing at full steering lock and consider recalibrating the speedometer.
Speedometer error explained
Your speedometer counts wheel revolutions. A taller tire covers more ground per turn, so the car moves faster than the needle shows and the odometer under-reports miles. A shorter tire does the opposite.
| Diameter change | Real speed at an indicated 60 mph |
|---|---|
| -3% | 58.2 mph |
| -1% | 59.4 mph |
| +1% | 60.6 mph |
| +3% | 61.8 mph |
| +5% | 63.0 mph |
Before you change tire size
- Stay within about 3% of the original diameter to keep the speedometer, ABS and traction control accurate.
- Match or beat the load index and speed rating shown in your door jamb placard.
- Check the wheel width. Each tire width fits a range of rim widths; a tire shop can confirm.
- Check clearance at full lock and full suspension compression, especially with wider tires.
Planning a long drive on new tires? Price it with the fuel cost calculator, and track the change in economy with the gas mileage calculator.
Frequently asked questions
How much bigger can I go on tire size?
Most tire shops recommend staying within 3% of the original overall diameter. Larger changes can cause rubbing, speedometer error and problems with ABS, stability control and automatic transmission shift points.
Will bigger tires make my speedometer read wrong?
Yes. Taller tires make the car go faster than the speedometer shows. A 3% taller tire means you are doing about 61.8 mph when the speedometer reads 60 mph.
What does 225/45R17 mean?
The tire is 225 mm wide, the sidewall height is 45% of the width (about 3.99 in), it has radial construction, and it fits a 17 inch wheel. Its overall diameter is about 24.97 inches.
How do I calculate tire diameter?
Multiply the width in millimeters by the aspect ratio divided by 100 to get the sidewall height in millimeters, divide by 25.4 to convert to inches, double it, and add the rim diameter.
Do bigger tires affect gas mileage?
Usually yes. Larger and wider tires are heavier and have more rolling resistance, which typically lowers fuel economy. The odometer also under-reports distance, so measured MPG looks worse than it really is.
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Last updated October 2026.