245/45R18 — one diameter, three wheel sizes

Base sporting fitment on a long list of rear and all-wheel-drive saloons and coupes — Mustang, Charger, the mid-size Audi and BMW ranges, Camaro — and a common front size in staggered arrangements where the rear runs wider. The reason it appears so consistently is that it was designed as the middle rung of something.

The dimensions

26.68 in overall, 678 mm. Section width 9.65 in. Sidewall 4.34 in. Circumference 83.8 in and 755.9 revolutions per mile.

The 245 mm stack: 18, 19 and 20 inches, same height

This is the part worth reading carefully.

Size	Overall diameter	Sidewall	Difference from 245/45R18
245/45R18	26.68 in	4.34 in	—
245/40R19	26.72 in	3.86 in	+0.04 in
245/35R20	26.75 in	3.38 in	+0.07 in

Three wheel diameters, an inch apart each time. Three aspect ratios, five points apart each time. Section width identical at 245 mm throughout. And the overall diameter moves by seven hundredths of an inch across the whole range — 0.27% from bottom to top.

That is not luck. It is what the aspect-ratio system exists to make possible: the wheel grows by one inch, the sidewall shrinks by half an inch at each end, and the sum stays put. Speedometer, odometer, ABS calibration and final drive all see the same tire. What changes is the sidewall — 4.34 in in the first row of that table, 3.38 in in the last — and everything that follows from having it or not having it.

The plus sizing reference works through the general method. This stack is the cleanest instance of it in the whole size catalogue here.

The 18-inch alternative that is not the same height

255/40R18 shares the wheel and is the other size these cars are commonly seen on. It measures 26.03 in — 0.65 in shorter than this size, 2.43% down — while being 0.39 in wider at the section.

So on one wheel there are two widely-fitted options, and they are not interchangeable in the way the three sizes in the table above are. Fitting 255/40R18 where the vehicle was calibrated for 245/45R18 puts an indicated 70 mph at an actual 68.3 mph and 100 indicated miles at 97.6.

Where the ladder sits inside the 45-series family on an 18

At the same aspect ratio and wheel, the widths available here run 225, 235, 245 and 255 mm, giving 25.97, 26.33, 26.68 and 27.04 in. The step is 0.35 in per 10 mm — the same constant that governs every 45-series family, because the arithmetic does not care which wheel it is on.

245 mm, read across the whole catalogue

Twelve other sizes here share this section width, and they span most of the passenger and light vehicle market. The shortest is 245/45R17 at 25.68 in; the tallest is 245/60R20 at 31.57 in. In between sit crossover fitments at 245/65R17 (29.54 in) and 245/60R18 (29.57 in) — two sizes on different wheels that land within three hundredths of an inch of each other — and light-truck fitments at 245/75R16 (30.47 in) and 245/75R17 (31.47 in).

Nearly six inches of diameter range, one tread stance. Whatever else 245 mm tells you, it does not tell you how tall the tire is.

Sidewall, speed symbols and the ZR question

4.34 in of sidewall is generous by the standards of the 19 and 20-inch sizes above, and slim by any other standard. It is also the depth at which the size field itself starts carrying extra letters: a proportion of tires here are marked ZR rather than R, which is a survival of an older rating scheme and describes construction rather than any dimension on this page. The tire construction codes reference covers the letters that can appear there, and the speed rating table covers the symbol that does the actual work now.

Staggered sets

Because this size is so often a front size, a lot of the vehicles wearing it carry a different size at the rear. When the two axles run different rolling diameters the arithmetic on this page describes only half the car, and the consequences — for the differential, for four-wheel-drive systems and for anything that compares front and rear wheel speeds — belong to the pair, not to either size alone. The staggered fitments reference treats that case.

To put this size against a specific alternative, in either notation, use the comparison tool.