Tyre Size Calculator

Overall diameter = rim diameter (inches × 25.4) + 2 × sidewall height, where sidewall height = width × profile ÷ 100. A 265/65R17 is 431.8 mm + 2 × 172.3 mm = 776.3 mm. Under load a tyre flattens slightly, so the ETRTO standard uses a rolling circumference of 3.05 × the diameter. That figure is what manufacturers use for revs per kilometre and what the speedo sees.

By the same percentage as the diameter change. Going from 265/65R17 to 285/70R17 adds 7%, so when the speedo shows 100 km/h you are actually doing about 107 km/h. Keep changes within about 3% unless the speedo is recalibrated.

Under the national guidelines (VSB 14, Section LS), passenger cars can generally go up to 15 mm larger in overall diameter than the largest tyre the manufacturer lists for the model. Off-road 4WDs and utes can generally go up to 50 mm larger without an engineering certificate. The load index and speed rating must also meet your tyre placard. States add their own conditions, so check with your state transport authority before modifying.

The number after the size is the load index: how much weight each tyre can carry (91 = 615 kg, 112 = 1,120 kg). The letter is the speed rating: the maximum sustained speed (T = 190 km/h, H = 210 km/h, V = 240 km/h). Replacement tyres must be at least what your vehicle’s tyre placard shows.

Look for “DOT” on the sidewall. The last four digits are the week and year of manufacture: 2319 means week 23 of 2019. Most manufacturers recommend yearly inspections from 5 years old and replacement by 10 years, whatever the tread depth.

Fitting a larger wheel with a lower-profile tyre so the overall diameter stays about the same. For example, 205/55R16 to 225/45R17 changes diameter by only 0.4%. The plus sizing tool lists sizes that stay within your chosen tolerance and shows which are commonly stocked.