Tire Size Comparer
Compare two tire sizes side by side — diameter, sidewall, circumference, revs per mile and speedometer error.
Codes like 205/55R16 or 195 65 15. Updates as you type.
225/45R17 is +0.38% in diameter versus 205/55R16 — within the ±3% rule of thumb.
| Measurement | 205/55R16 | 225/45R17 | Difference |
|---|---|---|---|
| Section width | 205 mm | 225 mm | +20 mm |
| Sidewall height | 112.8 mm | 101.3 mm | -11.5 mm |
| Overall diameter | 631.9 mm (24.88 in) | 634.3 mm (24.97 in) | +2.4 mm |
| Diameter change | Inside the usual ±3% fitment window | +0.38% | |
| Circumference | 1985.2 mm | 1992.7 mm | +7.5 mm |
| Revolutions per mile | 811 | 808 | -3 |
| Revolutions per km | 503.7 | 501.8 | -1.9 |
| Ride height (axle) | Half the diameter change | +1.2 mm | |
Closest alternatives to 205/55R16
Sizes within ±3% of the original diameter, same rim ±2 in and section width ±30 mm.
Figures are the nominal dimensions from the tire code. Real tires vary by a few millimetres between brands, and load index, speed rating, rim width and wheel-arch clearance still have to match your vehicle. Check the placard in your door jamb before changing sizes.
How it works
This comparer uses the standard ISO metric tire code. In 205/55R16, 205 is the
section width in millimetres, 55 is the aspect ratio (the sidewall height as a percentage of
the width) and 16 is the rim diameter in inches. So the sidewall is
205 × 55 ÷ 100 = 112.75 mm and the overall diameter is
16 × 25.4 + 2 × 112.75 = 631.9 mm. Circumference is π × diameter,
revolutions per mile is 63,360 ÷ circumference in inches, and revolutions per
kilometre is 1,000,000 ÷ circumference in mm.
Because the speedometer counts wheel revolutions, a larger tire makes the needle read low:
your real speed is indicated × new diameter ÷ old diameter. The same ratio
applies to the odometer. Fitters usually keep a replacement within ±3% of the original rolling
diameter, so the tool flags anything outside that band and lists the closest alternative sizes
it can find. Everything is plain arithmetic done in your browser — nothing is uploaded, there
is no sign-up and there are no ads.
Frequently asked questions
What do the numbers in a tire size like 205/55R16 mean?
205 is the section width in millimetres, 55 is the aspect ratio (the sidewall height as a percentage of that width) and R16 means radial construction on a 16-inch rim. So the sidewall is 205 × 55 ÷ 100 = 112.75 mm and the overall diameter is 16 × 25.4 + 2 × 112.75 = 631.9 mm.
How much can a new tire size differ from the original?
The usual rule of thumb is to stay within ±3% of the original rolling diameter, which keeps the speedometer, ABS and traction control close to their calibration. This tool shows the exact percentage and flags anything outside that band, but you still need a matching load index, speed rating and rim width.
Will bigger tires make my speedometer wrong?
Yes. The speedometer counts wheel revolutions, so a larger diameter means you travel further per turn and the needle reads low. Real speed = indicated speed × new diameter ÷ old diameter. Going from 205/55R16 to 225/45R17, 100 km/h on the dial is really about 100.4 km/h.