Swapping tires is rarely a like-for-like change. A new set may be wider for grip, shorter in the sidewall for looks, mounted on a larger wheel, or taller for ground clearance. What you really want to know is how different the new tire is from the old one in the ways that affect the car: overall diameter, width, sidewall height and what that does to ride height and engine speed. This calculator puts two tire sizes next to each other and reports every difference.
How to use the tire size comparison calculator
- Enter the original size — width, aspect ratio and rim — from your current tires' sidewall.
- Enter the new size you are considering.
- Read the diameter change first. Keeping it within about ±3% is the usual rule for fitting without speedometer or traction-control problems.
- Check the width, sidewall and ride height changes for fitment, and the engine rpm change for the effect on gearing.
Need the dimensions of just one size? Use the tire size calculator. To see the speedometer effect in mph, use the speedometer error calculator.
Tire size comparison formula
Diameter change = new ÷ original − 1
Ride height change = (new − original) ÷ 2
Engine rpm change = original ÷ new − 1
- Rim is in inches; the sidewall comes out of millimetres, so divide by 25.4
- Ride height changes by half the diameter difference because only the radius lifts the axle
- Engine rpm falls as the tire grows, since a larger tire needs fewer turns per mile
Worked example: 225/45R17 to 235/45R18
- Original: 17 + 2 × 3.986 = 24.97 in.
- New: 18 + 2 × 4.163 = 26.33 in.
- Diameter change: 26.33 ÷ 24.97 − 1 = +5.4% — more than the usual 3% guideline.
- Ride height: (26.33 − 24.97) ÷ 2 = +0.68 in (17 mm).
- Width: 235 − 225 = +10 mm; sidewall +4.5 mm.
- Engine rpm at the same speed: 24.97 ÷ 26.33 − 1 = −5.1%.
Comparison of common upgrades
These pairs were calculated with the formula above.
| Original | New size | Original diameter | New diameter | Diameter change | Width change |
|---|---|---|---|---|---|
| 225/45R17 | 235/45R18 | 24.97 in | 26.33 in | +5.42% | 10 mm |
| 205/55R16 | 225/50R16 | 24.88 in | 24.86 in | -0.08% | 20 mm |
| 235/55R18 | 245/50R18 | 28.18 in | 27.65 in | -1.89% | 10 mm |
| 265/70R17 | 285/70R17 | 31.61 in | 32.71 in | +3.49% | 20 mm |
| 225/65R17 | 245/60R18 | 28.52 in | 29.57 in | +3.71% | 20 mm |
| 215/60R16 | 225/55R17 | 26.16 in | 26.74 in | +2.24% | 10 mm |
Interpreting the differences
Diameter differences are the first thing to look at. A change under 3% usually causes no practical problem; above that, expect the speedometer and odometer to read low when the new tire is bigger, and high when it is smaller, by the same percentage. The engine also turns at a different speed at highway cruise: bigger tires mean slightly lower revs and a taller effective gear, which can slow acceleration a little and improve cruising economy.
Width and sidewall differences matter for fitment. A wider tire needs more room inside the wheel well and on the rim, and a lower-profile sidewall gives a firmer ride and less protection for the wheel. If a new size changes the ride height by more than a few millimetres, check that the suspension, fender and steering lock still clear.
Finally, a size that matches in dimensions must still match in capability: the load index and speed rating on a replacement should be at least those specified for the vehicle. For gearing effects of larger tires see the gear ratio calculator; for exact diameters use the tire diameter calculator.
Sources
Size markings follow the ISO metric tire designation. The diameter of 225/45R17 (24.97 in) agrees with tiresize.com reference data collected 2026-10-09. The ±3% diameter guideline is the common tire-industry rule of thumb for keeping speedometer error small; table values are computed from the formulas shown.
Frequently asked questions
How much diameter difference is acceptable?
A common guideline is to stay within about 3% of the original overall diameter. Beyond that the speedometer, odometer, ABS and traction control can be noticeably affected.
What does the ride height change mean?
The axle sits half the diameter change higher (or lower) than before, so the car rises by half the difference in overall diameter. A 1.35 in larger diameter lifts the body about 0.68 in (17 mm).
Why does engine rpm change with a bigger tire?
A larger tire travels farther per revolution, so at the same road speed the wheels — and the engine — turn slower. The percentage shown is how much lower (negative) or higher (positive) the engine speed will be.
Does a wider tire always fit?
Not necessarily. Width must suit the rim width and fit inside the wheel well and clear the suspension at full lock. Check the wheel offset with the wheel-fitment tools as well.
Does this account for the speedometer?
It shows the diameter change, which is the speedometer error as a percentage: a tire 5% larger makes the true speed about 5% higher than the speedometer reads. See the speedometer error calculator for exact figures.