Tire Size Calculator & Offset

Tire Size Calculator compares two tire sizes and shows what changes: overall diameter, sidewall, circumference, ride height, speedometer error and, new in this version, wheel offset and rim width.

▶ TIRE SIZE COMPARISON

• Current vs new size, e.g. 225/45R17 → 235/40R18 or 265/70R17 → 33x12.50R17

• Overall diameter, sidewall height, circumference, ride height, revs per km or mile

• Both tires drawn to scale on one picture

• Metric and flotation (inch) sizes, mm or inch display

▶ SPEEDOMETER ERROR

• Real speed at an indicated 30, 50, 60, 80, 100, 120 km/h

• In inch mode: 20, 30, 40, 50, 60, 70, 80 mph

• Odometer error in percent

▶ PLUS SIZE FINDER

• Metric sizes (widths 135–355, rims 13–24) within ±3% of the base diameter

• Grouped by rim size with a Plus 1 / Plus 2 badge

• Tap a size to compare it with the base size

▶ WHEEL OFFSET (ET) AND RIM WIDTH

• Rim width 4 to 13 J, offset −50 to +70 mm, for the old and the new wheel

• How far the outer edge moves out (poke) and the inner edge moves toward the suspension

• Tire edge change and approximate backspacing

• To-scale cross-section from the hub face

▶ SIDEWALL CODES

• P, LT, XL and RF explained

• Speed rating table L to (Y), km/h and mph

• Load index 60 to 130, kg and lb

▶ MY GARAGE

• Save the tire sizes of your cars, load one with a tap

Calculations run on the phone, no sign-in.

The numbers are geometric estimates from the size markings: the mounted tire width depends on the rim, and backspacing assumes a 12.7 mm flange. The app does not decide whether a wheel fits. Check fitment and clearance with the vehicle manual or a tire shop.