Sidewall Check

Tire Size Calculator

Enter the size on your tires and the size you are thinking about. You get the dimensions side by side, your real speed at an indicated speedometer reading, and a plain answer on whether the swap is sensible.

Current tire
New tire

Metric sizes such as 225/45R17 and flotation sizes such as 31x10.50R15 both work. Load index and speed rating are optional; fill them in and the check includes them.

What the numbers on your tire mean

A marking such as 225/45R17 94W is five separate pieces of information, and each one changes a different thing about how the tire behaves.

  • 225 — section widthThe width of the tire in millimetres, measured across the widest part of the inflated sidewall, not the tread.
  • 45 — aspect ratioSidewall height as a percentage of section width. Here 45% of 225 mm, so 101 mm of sidewall. A lower number means a shorter sidewall.
  • R — constructionRadial, which is effectively every modern car tire. A size with no letter before the width is a European metric size.
  • 17 — rim diameterThe wheel diameter the tire fits, in inches. This is the one number in the size that must match your wheel exactly.
  • 94 — load indexMaximum load per tire. 94 means 670 kg, or 1,477 lb. Never go below the figure on your door placard.
  • W — speed ratingMaximum sustained speed. W is 270 km/h, about 168 mph. T is 190 km/h and H is 210 km/h.

Overall diameter, the figure that actually drives your speedometer, is not written on the tire. It is calculated: rim diameter plus two sidewalls. For 225/45R17 that is 17 inches plus twice 3.99 inches, or 24.97 inches overall.

How much change is too much

The working rule is to keep overall diameter within 3% of the original. Inside that band your speedometer, odometer, ABS, traction control and transmission shift points all stay close enough to their calibration to behave normally. Past it, the error becomes visible and, on modern cars, other systems start reading it too.

Width and sidewall are separate questions. A wider tire needs a rim wide enough to support it and enough room in the wheel well at full steering lock. A shorter sidewall firms up the ride and leaves less rubber protecting the rim. Neither shows up in the diameter figure, which is why this calculator reports all three.

Load rating is the one that is not negotiable. Diameter and width are judgement calls about ride, looks and clearance. Fitting a tire with a lower load index than your vehicle requires is a safety defect, and it will not pass inspection in most jurisdictions. Enter your load index above and the check will flag it.

Common size comparisons

The swaps people ask about most, each worked through with its own numbers.

About these results

Every figure on this page is calculated from the size markings you enter, using the standard geometry: sidewall height is section width times aspect ratio, overall diameter is rim diameter plus two sidewalls, and speedometer error is the ratio of new diameter to old. Load index and speed rating come from the standardised ETRTO tables.

Results are estimates calculated from the size markings you enter. They do not account for your vehicle’s suspension, brake clearance, fender shape, ride height or alignment, and tires of the same marked size vary between makers. Always confirm the load rating and inflation pressure on the placard in your driver’s door jamb, and have a qualified tire fitter check any size change before you drive on it.