Tire size comparison
Diameter and speedometer change between tire sizes.
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How it's calculated
A tire size like 225/45R17 is the width in millimetres, the sidewall height as a percentage of that width, and the rim diameter in inches. Sidewall height is width times aspect ratio, and the overall diameter is the rim plus two sidewalls, converted at 25.4 mm to the inch. The speedometer counts wheel turns, so a tire that is 3% bigger makes the car go 3% faster than indicated and the odometer under-count by the same share.
How much can I change tire size safely?
The usual rule is to keep the overall diameter within 3% of the original, so the speedometer, ABS, traction control and automatic gearbox see the numbers they were calibrated for. Plus-sizing (bigger rim, lower profile) is normally done to keep the diameter the same.
What is plus sizing?
Going up one inch of rim while dropping the aspect ratio and widening the tire by about 10 mm, so the diameter stays put: 225/45R17 to 245/40R18 is a classic plus-one. Lower sidewalls sharpen steering but ride harder and are easier to pothole-damage.
Will bigger tires change my fuel economy or MPG reading?
Slightly. A larger diameter lowers engine revs at a given speed, which can help on the highway, but wider tires add drag and weight. More importantly the odometer under-counts miles by the diameter change, so a trip-computer MPG reading will look worse than reality by the same percentage.
Results are estimates for informational purposes only and are not financial, medical or legal advice.