Comparing Two Tire Sizes: The Six Numbers That Change

A tire size has three numbers and changing any of them moves six measurable things. Here is each one, and how to get it.

1. Overall diameter

diameter (in) = rim + 2 × (width_mm × aspect ÷ 100) ÷ 25.4

225/45R17 → 17 + 2 × (225 × 0.45) ÷ 25.4 = 24.97 in

This is the number everything else follows from, and it is why the aspect ratio matters more than people expect: it is a percentage of the width, so widening a tire while holding the aspect ratio also makes it taller.

2. Section width

The first number, in millimeters, measured at the widest point of the sidewall — not the tread.

This is the clearance-critical dimension, and it shifts about 5 mm per half-inch of rim width away from the tire's measuring rim.

3. Sidewall height

width_mm × aspect ÷ 100

225/45 → 101 mm. 225/55 → 124 mm.

Sidewall height is the ride-comfort variable. It is also the pothole-survival variable, and the two go together.

4. Revolutions per mile

63,360 ÷ (π × diameter × 0.97)

The 0.97 is the loaded-rolling correction. This is what the speedometer, odometer and transmission actually count.

5. Speedometer reading

error % = (new diameter − old) ÷ old × 100

Taller tire, speedometer under-reads. Shorter tire, over-reads. Stay within ±3% and nothing needs recalibrating.

6. Ground clearance

Half the diameter change. A tire 1 inch larger in diameter raises the axle center — and everything attached to it — by 0.5 inch.

This is the number that surprises people going to a much taller tire for off-road use: a 2-inch diameter increase is a 1-inch lift, not a 2-inch one.

A worked comparison

That +5.2% is outside the 3% window. It is a size worth recalibrating for, and a size where the odometer will quietly under-record by one mile in twenty for as long as the tires are on the car.

Run any two sizes side by side on the tire comparison calculator — it produces all six figures rather than just the diameter.