Changing tire size: what the numbers mean and what really changes

Reading the size on the sidewall

A size such as 275/65R18 packs three measurements into one code. The first number is the tread width in millimeters. The second is the aspect ratio — how tall the sidewall is, expressed as a share of the tread width. “R” tells you the carcass is radial, and the final figure is the rim size the tire mounts on, in inches. Mixed units are normal — the code grew up on both sides of the Atlantic.

Stack one sidewall above the rim and one below it and you get the height of the whole tire: diameter = wheel + 2 × (width × aspect ratio), with the width converted to inches first. That single number decides most of what follows.

What a different diameter changes

• Speedometer and odometer. They count wheel revolutions. Each turn of a taller tire rolls further down the road, so the needle sits below your true speed and the trip counter falls behind the distance really driven. A shorter tire does the opposite.
• Gearing. A taller tire acts like a taller final drive: a little less pull when towing or climbing, slightly lower engine speed on the highway.
• Ground clearance. The axle rises by half the diameter gain — not the whole of it.
• Stability and braking systems. ABS and stability control compare wheel speeds; a large mismatch against what the car expects can upset them.

Wider is a different question

Width barely touches the speedometer, but it moves the tire toward the suspension on one side and the fender on the other. Whether it fits depends on the wheel’s width and offset, which is why a wider tire is often paired with a new wheel.

Before you buy

Check the load index and speed rating against the door-jamb placard, check the turning clearance at full lock and with the suspension compressed, and check what your manufacturer and your insurer say about non-standard sizes. The calculator shows what changes; the fitter confirms it fits.