Key takeaways
- The number that matters most is the overall diameter difference as a percent.
- Stay within ±3% of the old diameter to keep the speedometer and odometer honest.
- A larger new tire makes the speedometer read slow; a smaller one reads fast.
- True speed = indicated speed × new diameter ÷ old diameter.
How to compare two tire sizes
A tire size comparison is just a side-by-side of two sizes reduced to the one figure that drives everything else: the percent change in overall diameter. Each size — like 225/45R17 — gives you a width in millimeters, an aspect ratio (sidewall as a percent of width), and a rim diameter in inches. Convert both to a diameter and the rest follows.
If the percent change is positive the new tire is taller; negative means it's shorter. The further it strays from zero, the more your speedometer, odometer, gearing, and clearance shift.
Worked example: 225/45R17 vs 245/40R18
The old 225/45R17 works out to 24.97 in (sidewall 3.99 in, twice that plus a 17-in rim). The new 245/40R18 is 25.72 in (sidewall 3.86 in on an 18-in rim). The difference is (25.72 − 24.97) ÷ 24.97 × 100 ≈ +3.0% — right at the limit. At an indicated 60 mph the true speed is 60 × 25.72 ÷ 24.97 ≈ 61.8 mph, so the speedometer reads slightly slow.
Old vs new, side by side
| Dimension | Old 225/45R17 | New 245/40R18 |
|---|---|---|
| Overall diameter | 24.97 in | 25.72 in |
| Sidewall | 3.99 in | 3.86 in |
| Section width | 8.86 in | 9.65 in |
| Circumference | 78.4 in | 80.8 in |
| Revs/mile | 808 | 784 |
Reading the result the right way
Treat ±3% as a soft ceiling: inside it, your speedometer error stays small and most stock fitments still clear. Outside it, expect a noticeable speedometer offset and a real chance of rubbing. To decode a single size first, use the tire size calculator; to see the exact speed offset across a range of speeds, use the speedometer error calculator. Always confirm clearance and load rating with a professional before committing.
Frequently asked questions
Is a 3% difference okay?
Yes — keeping the new diameter within about 3% of the old one keeps the speedometer, odometer, and ABS accurate and usually preserves clearance. Beyond 3% is where issues start.
Will bigger tires rub or fit?
A larger diameter or wider section can rub the fender or suspension at full lock or compression. This tool compares dimensions only — confirm real clearance with a pro.
How does it affect the speedometer?
A taller new tire reads slow, a shorter one reads fast. True speed = indicated × new diameter ÷ old diameter.
Does it change my gearing?
Effectively yes — a larger diameter raises effective gearing (lower RPM, softer acceleration); smaller does the reverse. The transmission ratios don't change.
What is plus/minus sizing?
Plus sizing fits a bigger rim with a lower profile to hold diameter near stock; minus sizing does the opposite with a taller sidewall.
What's the effect on the odometer?
The odometer counts revolutions, so the same percent error skews recorded distance — a larger tire under-counts miles, a smaller one over-counts.
Tire size notation follows the standard P-metric system used across the industry — see Tire Rack's tire-size reference. The 63,360 inches-per-mile and 25.4 mm-per-inch conversions are exact.
Last reviewed June 2026