Tire Size Calculator

This page predicts the shift from the size codes. To measure what your dashboard actually does, drive at a steady speed against GPS and put the two readings into the speedometer error calculator — if prediction and measurement disagree, believe the measurement.

How to read a tire size: what 225/65R17 98H means

Every code on the sidewall is standardised, so once you can read one tire you can read any tire. Taking 225/65R17 98H from left to right:

• 225 — section width in millimetres, measured across the widest part of the inflated tire.
• 65 — the aspect ratio: sidewall height as a percentage of that width. Here 225 × 0.65 = 146.25 mm of sidewall, and there are two of them, one above the wheel and one below.
• R — radial construction, which is almost every modern tire. A D or B here means diagonal or belted bias, mostly trailers and classics.
• 17 — wheel diameter in inches. Yes, the code mixes millimetres and inches; that is historical, not a typo.
• 98 — the load index, a coded maximum weight per tire. 91 is 615 kg, 95 is 690 kg, 98 is 750 kg, 100 is 800 kg, 105 is 925 kg.
• H — the speed rating, the maximum sustained speed the tire is certified for.

Overall diameter is therefore the wheel plus two sidewalls: 17 inches (431.8 mm) + 2 × 146.25 mm = 724.3 mm, or about 28.5 inches. That number is the one your speedometer cares about, and it is what the calculator above compares.

Speed rating letters

Fitting a lower rating than the manufacturer specified can affect insurance and roadworthiness in some countries, so match or exceed what is on the door placard.

The other markings worth knowing

• A letter before the width — P means P-metric (US passenger), LT means light truck with stiffer construction and higher pressures. No letter is Euro-metric.
• Flotation sizes like 35×12.50R17 state the overall diameter in inches directly (35 inches tall, 12.5 inches wide, 17-inch wheel). Common on trucks and off-road builds.
• DOT code — the last four digits are the build date. 3624 means week 36 of 2024. Rubber ages whether or not you drive on it.
• M+S is a tread-pattern claim; the three-peak mountain snowflake symbol is the one that means the tire passed a snow traction test.

Only the first four values change your speedometer. Load index, speed rating and the date code matter for safety and legality, but a 98H and a 100V of the same 225/65R17 roll the same distance per turn.

How tire size affects speed reading

Your car does not measure speed directly. The transmission and ABS sensors count how fast each wheel rotates. The computer multiplies that by a stored tire circumference (set at the factory for the original tire size) to produce a number it sends to the dashboard. Change the tire size and the rotation-to-distance ratio changes, but the stored constant does not — unless you recalibrate. The speedometer ends up off by exactly the percentage that the new tire diameter differs from the original.

Most off-road and overland tire upsizing pushes diameter up 5 to 12 percent. A 6 percent change at 70 mph means the dashboard reads 70 but you are actually doing 74. That is enough to matter on a highway, and enough to attract a police radar that sees true speed regardless of what your dial says.

The fix is either an ECU recalibration through a dealer or an aftermarket programmer (most modern cars), a transmission drive gear swap (older vehicles with mechanical speedometer cables), or an inline correction module. We cover what each option costs in our speedometer recalibration guide.