Formula
Sidewall height is section width × aspect ratio, converted from millimetres. Overall diameter is the rim plus two sidewalls. Section width in inches is the millimetre width divided by 25.4. Revolutions per mile are 63,360 ÷ (π × diameter). If the speedometer was calibrated to the old tire, actual speed is indicated speed × new diameter ÷ old diameter.
sidewall in = width mm × (aspect ÷ 100) ÷ 25.4
diameter = rim + 2 × sidewall
revs/mile = 63,360 ÷ (π × diameter)
actual = indicated × new ÷ old
The diameter is nominal and unloaded. It will not match a manufacturer’s revolutions-per-mile table, which is measured loaded. The gear ratio calculator uses this same diameter. The wheel offset calculator moves the wheel, which is a separate question from the tire’s section width. The drawing scales the two circles to overall diameter.
Worked examples
265/70R17 against 285/70R17. Diameters 31.61 in and 32.71 in. Widths 10.43 in and 11.22 in. The old tire turns 638.1 times a mile. An indicated 60 mph is actually 62.09 mph on the taller tire.
The same size on both sides. The speedometer error is zero. The table reads the indicated speed straight through.
A shorter replacement. Actual speed falls below the indicated speed, because each revolution covers less ground.
Reference
Indicated speeds used in the table: 20, 40, 60, 80. The headline figure is 60.
Questions
How much taller is a 285/70R17 than a 265/70R17?
The 265 is 31.61 inches tall. The 285 is 32.71 inches tall, 1.10 inches more, and about 0.79 inches wider. The 265 turns 638 times a mile. At an indicated 60 mph on a speedometer calibrated to the 265, the 285 is actually doing 62.1 mph.
Which way does the speedometer err?
A taller tire travels farther each revolution, so the actual speed is higher than the indicated speed. A shorter tire does the opposite. The table is indicated 20, 40, 60, and 80.
Is the diameter the loaded diameter?
No. It is the nominal, unloaded diameter from the molded size: rim plus two sidewalls. A loaded tire is shorter. Manufacturer revolutions-per-mile tables are shorter still. This page does not use those tables.
Sources
- Nominal tire geometry: diameter = rim + 2 × width × aspect / 25.4. Unloaded. Not a loaded revolutions-per-mile table.
- Revolutions per mile = 63,360 / (π × diameter in inches).
- Speedometer: actual = indicated × new diameter / old diameter, when the old tire is what the speedometer was calibrated to.