Tire Size Calculator
Enter your current tire size and the size you are considering to compare diameter, width, sidewall height and revolutions per mile, and see how much your speedometer will be off.
Formulas shown on this page · Last reviewed October 2026
| 225/65R17 | 235/65R17 | Difference | |
|---|---|---|---|
| Overall diameter | 28.52 in | 29.03 in | +0.51 |
| Section width | 8.86 in | 9.25 in | +0.39 |
| Sidewall height | 5.76 in | 6.01 in | +0.26 |
| Circumference | 89.6 in | 91.2 in | +1.61 |
| Revolutions per mile | 707 | 695 | −12 |
Tire size formula
sidewall height = width (mm) × aspect ratio ÷ 100
overall diameter = rim (in) × 25.4 + 2 × sidewall (mm)
revolutions per mile = 1,609,344 mm ÷ (π × diameter)
actual speed = speedometer reading × new diameter ÷ old diameter
These are nominal dimensions from the size code. Real tires vary by brand and model and change with tread wear and inflation, so treat differences under a few tenths of an inch as approximate.
Popular tire size comparisons
| Comparison | Diameter | Difference | 60 mph reads as |
|---|---|---|---|
| 225/65R17 vs 235/65R17 | 28.5″ → 29″ | +1.79% | 61.1 mph actual |
| 225/65R17 vs 225/60R17 | 28.5″ → 27.6″ | −3.11% | 58.1 mph actual |
| 235/65R17 vs 235/60R18 | 29″ → 29.1″ | +0.26% | 60.2 mph actual |
| 265/70R17 vs 285/70R17 | 31.6″ → 32.7″ | +3.49% | 62.1 mph actual |
| 265/70R17 vs 275/70R17 | 31.6″ → 32.2″ | +1.74% | 61 mph actual |
| 275/55R20 vs 275/60R20 | 31.9″ → 33″ | +3.39% | 62 mph actual |
| 275/60R20 vs 275/65R20 | 33″ → 34.1″ | +3.28% | 62 mph actual |
| 265/70R16 vs 265/75R16 | 30.6″ → 31.6″ | +3.41% | 62 mph actual |
| 245/75R16 vs 265/75R16 | 30.5″ → 31.6″ | +3.88% | 62.3 mph actual |
| 205/55R16 vs 215/55R16 | 24.9″ → 25.3″ | +1.74% | 61 mph actual |
| 205/55R16 vs 205/60R16 | 24.9″ → 25.7″ | +3.24% | 61.9 mph actual |
| 215/55R17 vs 225/55R17 | 26.3″ → 26.7″ | +1.65% | 61 mph actual |
| 225/45R17 vs 225/50R17 | 25″ → 25.9″ | +3.55% | 62.1 mph actual |
| 225/40R18 vs 225/45R18 | 25.1″ → 26″ | +3.53% | 62.1 mph actual |
| 235/45R18 vs 245/45R18 | 26.3″ → 26.7″ | +1.35% | 60.8 mph actual |
| 235/55R18 vs 235/60R18 | 28.2″ → 29.1″ | +3.28% | 62 mph actual |
| 215/60R16 vs 215/55R17 | 26.2″ → 26.3″ | +0.59% | 60.4 mph actual |
| 265/65R18 vs 275/65R18 | 31.6″ → 32.1″ | +1.62% | 61 mph actual |
| 275/65R18 vs 285/65R18 | 32.1″ → 32.6″ | +1.6% | 61 mph actual |
| 255/70R18 vs 265/70R18 | 32.1″ → 32.6″ | +1.72% | 61 mph actual |
Before you change tire size
- Check the size, load index and speed rating on your door-jamb placard; a replacement should meet or exceed them.
- Wider or taller tires can rub on suspension parts or fender liners at full steering lock – check clearance.
- Use the same size on all four wheels of AWD and 4WD vehicles unless the maker says otherwise.
- A large diameter change may need speedometer recalibration.
Frequently asked questions
How do you read a tire size like 225/65R17?
225 is the section width in millimeters, 65 is the aspect ratio (sidewall height is 65% of the width), R means radial construction and 17 is the wheel diameter in inches. That tire has a 5.76-inch sidewall and an overall diameter of about 28.52 inches.
How much can tire diameter change safely?
A common rule of thumb is to stay within about 3% of the original overall diameter. Bigger changes throw off the speedometer and odometer and can affect ABS, traction control and clearance. Always check your vehicle placard and owner manual.
Does a bigger tire make the speedometer read slower?
Yes. A taller tire covers more ground per turn, so you travel faster than the speedometer shows. Going from 225/65R17 to 235/65R17 (+1.79% diameter), an indicated 60 mph is about 61.1 mph.
Can I put a wider tire on the same rim?
Usually by one size step (10 mm) if the rim width is in the tire maker’s approved range. Check the approved rim width for the exact tire, and make sure the load index and speed rating meet or exceed the original.