Thinking about bigger wheels, wider tires or a different size for winter? This tire size calculator compares two sizes side by side: overall diameter, sidewall height, width, circumference, revolutions per mile, and how much your speedometer will be off. Enter the numbers from the sidewall of your current tire (for example 225/45R17) and the size you’re considering.
Tire Size Comparison Calculator
Current tire
New tire
How to Read a Tire Size
Take 225/45R17 as an example:
- 225 is the section width in millimeters, measured from sidewall to sidewall.
- 45 is the aspect ratio: the sidewall height is 45% of the width.
- R means radial construction, which almost all modern car tires use.
- 17 is the wheel (rim) diameter in inches.
You'll often see more characters after the size, such as 94W. The number is the load index and the letter is the speed rating. Any replacement tire should meet or exceed the load index and speed rating listed on your vehicle's tire placard (usually on the driver's door jamb) or in the owner's manual.
The Math Behind the Calculator
- Sidewall height = width × aspect ratio ÷ 100 (in mm), then ÷ 25.4 for inches.
- Overall diameter = rim diameter + 2 × sidewall height.
- Circumference = π × overall diameter.
- Revolutions per mile = 63,360 inches ÷ circumference.
- Actual speed = speedometer reading × (new diameter ÷ old diameter).
These are calculated dimensions. Real tires can measure slightly differently by brand, tread depth and inflation, so treat the results as close estimates.
Why Diameter Matters Most
Your speedometer and odometer are calibrated to your original tire's diameter. A taller tire turns fewer times per mile, so the speedometer reads lower than your true speed. A shorter tire does the opposite. That's why "plus sizing" (bigger wheels with lower-profile tires) tries to keep the overall diameter close to stock.
Before You Change Tire Sizes
- Check clearance: wider or taller tires can rub on suspension parts or fender liners when turning.
- On AWD vehicles, many manufacturers require all four tires to match closely in diameter. Check your manual.
- Keep the load index and speed rating at or above your vehicle's specification.
- Expect ride and handling changes: lower-profile tires usually ride firmer.
- After any tire change, check pressures with a good gauge. See our tire pressure gauge picks and how to check tire tread and pressure.
More free tools: all car calculators, including the true cost of ownership calculator. Related reading: wheel alignment cost and tire repair cost.
Frequently Asked Questions
How much can tire diameter change safely?
Many tire shops use about 3% as a rule of thumb for staying close to the original diameter. The right answer depends on your vehicle, so check the owner's manual or ask a tire professional.
Will bigger tires make my speedometer wrong?
Yes, if the overall diameter is larger. A taller tire makes the speedometer read lower than your actual speed. The calculator shows the difference at any speed you enter.
What does 225/45R17 mean?
It's a tire 225 mm wide, with a sidewall 45% as tall as the width, radial construction, made for a 17-inch wheel. Its overall diameter is about 25 inches.
Can I put a wider tire on the same wheel?
Often, but only within the range of widths approved for that wheel's rim width. Tire manufacturers publish approved rim-width ranges for each size, so check before buying.