How the math works
Take 265/70R17. The tire is 265 mm wide. The sidewall is 70% of that, so 185.5 mm, or 7.3 inches. The wheel is 17 inches. Stack a sidewall above and below the wheel and you get the total height: 17 + 7.3 + 7.3 = 31.6 inches.
Your speedometer counts how many times the tires turn. A taller tire travels further each turn, so you go faster than the speedometer says. A shorter tire does the opposite.
The 3% rule
Tire shops generally suggest staying within about 3% of your original height. That keeps the speedometer close and usually avoids rubbing and problems with ABS or stability control. Trucks often go bigger, but that usually means a leveling kit or lift and recalibrating the speedometer.
What this can't tell you
- Real tires vary by a few tenths of an inch between brands, even in the same size.
- Whether it rubs also depends on your wheels. The wheel offset calculator covers that part.
- The load and speed rating on the new tire should meet or beat the sticker in your door frame.