What the comparison shows
- Diameter: the overall height. This drives the speedometer error, the gearing change and the ground clearance.
- Width: the section width. A wider tire sits closer to the suspension on the inside and closer to the fender on the outside, split evenly if the wheel stays the same.
- Sidewall: the height of the rubber between the rim and the tread.
- Ground clearance change: the axle rises by half the diameter change, so a tire 1 inch taller lifts the axle by about 0.5 inch.
How the speedometer error is calculated
The speedometer counts wheel revolutions. A taller tire covers more ground per revolution, so you go faster than the dial shows:
real speed = speedometer reading × new diameter ÷ old diameterExample: 265/70R17 (31.61 in) to 285/70R17 (32.71 in). The ratio is 32.71 ÷ 31.61 = 1.0349, so at an indicated 60 mph you are doing 62.1 mph. The odometer under-reads by the same 3.5%.
How much bigger can I go?
A common rule of thumb is to stay within about 3% of the original diameter. Past that, the speedometer error grows, the engine turns slower at the same road speed, and a heavy truck can feel sluggish until the axle gears are changed. Whether a tire physically fits depends on your vehicle, wheels and lift: check fender, frame and suspension clearance at full steering lock.
Questions
How do I compare two tire sizes?
Enter both sizes above. The table shows the difference in diameter, width, sidewall and revolutions per mile, and the speedometer section shows your real speed at each speedometer reading.
Will bigger tires make my speedometer wrong?
Yes. A taller tire makes the speedometer read low. The real speed is the reading multiplied by the new diameter divided by the old one: 3.5% taller means 62.1 mph when the dial says 60.
How much ground clearance do bigger tires add?
Half the diameter difference, measured at the axle. Going from a 31.6-inch tire to a 32.7-inch tire raises the axle by about 0.55 inch.
Can I compare metric and inch sizes?
Yes. Mix sizes like 275/60R20 and 35x12.50R20; both are converted to inches and millimetres before comparing.