🛞 Tire Size Calculator
Enter a tire size like 225/45R17 for diameter, sidewall and revs per mile, then compare a new size to see speedometer error and clearance change.
Use the size on the sidewall, such as P225/45R17 91V, LT265/70R17 or 33x12.50R15. The load index and speed rating after the rim size are ignored.
When your speedometer reads 60 mph, you will actually be going 61.0 mph.
Ground clearance changes by +0.21 in. Within the 3% difference many tire shops use as a rule of thumb.
| 225/45R17 | 235/40R18 | Difference | |
|---|---|---|---|
| Overall diameter | 24.97 in (634.3 mm) | 25.40 in (645.2 mm) | +0.43 in |
| Section width | 8.86 in (225.0 mm) | 9.25 in (235.0 mm) | +0.39 in |
| Sidewall height | 3.99 in (101.3 mm) | 3.70 in (94.0 mm) | -0.29 in |
| Wheel (rim) diameter | 17 in | 18 in | +1.0 in |
| Circumference | 78.45 in (1992.7 mm) | 79.80 in (2027.0 mm) | +1.35 in |
| Revolutions per mile | 808 | 794 | -14 |
| Revolutions per km | 502 | 493 | -8 |
Speedometer error table
| Speedometer reads | Actual speed |
|---|---|
| 20 mph | 20.3 mph |
| 30 mph | 30.5 mph |
| 40 mph | 40.7 mph |
| 50 mph | 50.9 mph |
| 60 mph | 61.0 mph |
| 70 mph | 71.2 mph |
| 80 mph | 81.4 mph |
What Tire Size Calculator Does
This tire size calculator decodes any size printed on a sidewall — metric sizes like P225/45R17 91V or LT265/70R17, and flotation sizes like 33x12.50R15 — into overall diameter, section width, sidewall height, circumference and revolutions per mile. Enter a second size to compare them side by side with the difference for each measurement.
The comparison answers the questions that matter when changing sizes: how far off your speedometer will be, how much ground clearance changes, and whether the new tire stays within the roughly 3% diameter difference that many tire shops use as a rule of thumb. A table shows the true speed at common speedometer readings.
How to Use Tire Size Calculator
- Enter your current tire size from the sidewall
- Enter the new size you are considering
- Compare diameter, sidewall, width and revolutions per mile
- Read the speedometer error and ground-clearance change
- Check the speed table before deciding
Formula Used by Tire Size Calculator
Overall diameter (metric size)
Diameter = rim (in) × 25.4 + 2 × width (mm) × aspect ratio ÷ 100
Worked example
225/45R17
- Sidewall = 225 × 0.45 = 101.25 mm
- Rim = 17 × 25.4 = 431.8 mm
- 431.8 + 2 × 101.25 = 634.3 mm
Result: 634.3 mm = 24.97 inches.
Speedometer error
Actual speed = indicated speed × new diameter ÷ old diameter
Worked example
225/45R17 (24.97 in) replaced with 235/40R18 (25.40 in), speedometer shows 60 mph.
- 25.40 ÷ 24.97 = 1.0172
- 60 × 1.0172
Result: About 61.0 mph — 1.72% faster than displayed.
Reading a Tire Size: P225/45R17 91V
| Part | Meaning |
|---|---|
| P | Passenger tire (LT = light truck, ST = special trailer) |
| 225 | Section width in millimeters |
| 45 | Aspect ratio: sidewall height is 45% of the width |
| R | Radial construction |
| 17 | Wheel diameter in inches |
| 91 | Load index (maximum load per tire, from a standard table) |
| V | Speed rating |
How to Read Your Result
Plus sizing
Going up in wheel size while lowering the aspect ratio (225/45R17 → 225/40R18) can keep the overall diameter almost unchanged, so the speedometer stays accurate. The trade is a shorter, stiffer sidewall: sharper steering but a firmer ride and more risk of wheel damage from potholes.
Before you buy
Diameter is only one check. The new tire must physically fit — width against the strut and fender, clearance at full steering lock and full suspension compression — and its load index and speed rating should meet or exceed the vehicle placard on the driver’s door jamb.
Limitations & Accuracy Notes
- Dimensions are nominal from the size code; real tires vary by brand, tread depth and the rim width they are mounted on.
- Speedometer calculations assume the original speedometer was accurate with the original tire.
- Fitment, load and speed ratings must be checked separately with a tire professional.