Changing tire size changes how tall the tire is, and that changes your speedometer reading and how much room the tire needs. Here is how to compare two sizes before you buy.
What the numbers mean
Take 275/65R18:
- 275 is the tire width in millimetres.
- 65 is the sidewall height as a percentage of the width.
- 18 is the rim diameter in inches.
Working out the height
The sidewall is 65% of 275 mm, which is about 179 mm. A tire has a sidewall above and below the rim, and an 18 inch rim is 457 mm across. Add them up and a 275/65R18 works out to about 815 mm tall, or 32.1 inches.
A worked comparison
| Size | Calculated height |
|---|---|
| 275/65R18 | 815 mm (32.1") |
| 275/70R18 | 842 mm (33.2") |
The 275/70R18 is about 3.4% taller.
What that does to your speedometer
A taller tire covers more ground on each turn, so the truck is moving faster than the speedometer shows. With the 3.4% taller tire in the example, a speedometer reading of 100 km/h is about 103 km/h on the road.
What else changes
A taller or wider tire needs more room. Whether it clears depends on your vehicle, the wheel's offset and width, and any lift or levelling kit. These heights are calculated from the size, and real tires vary a little by brand and model.
Compare your own sizes
Use our tire size calculator to compare any two sizes, and the offset calculator if you are changing wheels as well. Then text 306-361-4412 with your vehicle and the size you want, and we check that it fits before you pay.