Thinking about bigger or different tires? This tire size calculator compares your current tire with a new size and shows the overall diameter, sidewall height, revolutions per mile, the percentage difference, and how fast you are really going when your speedometer shows a given speed.
- Current diameter 0 in
- New diameter 0 in
- Current sidewall 0 in
- New sidewall 0 in
- Revolutions per mile (current → new) 0
- Actual speed at your reading 0 mph
How to Read a Tire Size
Take 225/45R17 as an example:
| Part | Meaning |
|---|---|
| 225 | Tire width in millimeters |
| 45 | Aspect ratio: sidewall height is 45% of the width |
| R | Radial construction |
| 17 | Fits a 17-inch wheel |
Your car's original tire size is printed on the tire sidewall and on the placard inside the driver's door jamb.
Tire Diameter Formula
Overall diameter = wheel diameter + 2 × sidewall height, where sidewall height = width × aspect ratio. For 225/45R17, the sidewall is 225 × 0.45 = 101.25 mm (about 3.99 inches), so the diameter is about 24.97 inches. A 235/45R17 is about 25.33 inches, or 1.42% larger.
Speedometer Error From a Different Tire Size
Your speedometer is calibrated for the original tire. A larger-diameter tire covers more ground per turn, so you travel faster than the speedometer shows; a smaller tire makes it read high. The formula is actual speed = indicated speed × new diameter ÷ old diameter. With the 235/45R17 example, 60 mph on the speedometer is really about 60.9 mph. The odometer is affected by the same percentage.
The 3% Rule
A common guideline is to keep a replacement tire within about 3% of the original overall diameter. Bigger differences can affect speedometer accuracy, ABS and stability control, gearing, and clearance in the wheel well. Always check load index, speed rating and fitment with a tire professional.
Safety note: Never install a tire with a lower load index or speed rating than your vehicle requires, and make sure the tire does not rub the suspension or fenders when turning or over bumps.
Related guides: learn how long tires last, read our guide to winter tires, and see suspension repair costs.
Tire Size Calculator FAQ
How Do I Calculate Tire Diameter?
Multiply the width by the aspect ratio to get sidewall height, double it, and add the wheel diameter. Convert millimeters to inches by dividing by 25.4.
How Much Bigger Can I Go on Tire Size?
A common guideline is to stay within about 3% of the original overall diameter, and to confirm fit and ratings with a tire professional.
Do Bigger Tires Affect the Speedometer?
Yes. Larger-diameter tires make the speedometer read lower than your actual speed; smaller tires make it read higher.
What Does 225/45R17 Mean?
A tire 225 mm wide, with a sidewall 45% as tall as the width, radial construction, for a 17-inch wheel.
Can I Put Wider Tires on My Car?
Often a slightly wider tire fits, but it must match the wheel width, clear the suspension and fenders, and meet load and speed ratings.
Does Tire Size Affect Fuel Economy?
It can. Larger, heavier or wider tires often reduce fuel economy slightly.