Tire size calculator and comparison
To compare two tire sizes, work out each one's overall diameter: the wheel size plus two sidewalls, where the sidewall is the width in millimeters times the aspect ratio. A 225/65R17 stands about 28.5 inches tall and a 265/70R17 about 31.6. A taller tire makes the speedometer read low by the same percentage the diameter grows. Enter both sizes below.
Updated
Metric (225/65R17), light truck (LT245/75R16) or flotation (33x12.50R15). Anything after the size, like a load index and speed rating, is ignored.
225/65R17 → 235/65R17
The new tire is 1.8% taller (+0.51 in)
That is inside the 3% upper limit of Dunlop Indonesia's plus-sizing guidance. Your speedometer will read low: you are going faster than it shows.
| Current | New | Change | |
|---|---|---|---|
| Section width | 225 mm | 235 mm | +0.39 in |
| Sidewall height | 5.76 in | 6.01 in | +0.26 in |
| Wheel diameter | 17 in | 17 in | ±0.0 in |
| Overall diameter | 28.52 in | 29.03 in | +1.8% |
| Circumference | 89.6 in | 91.2 in | +1.6 in |
| Revs per mile | 707 | 695 | −12 |
| Other notation | 28.5x8.9R17 | 29.0x9.3R17 |
Speedometer error on the new size
| Speedometer shows | Actual speed |
|---|---|
| 30 mph | 30.5 mph |
| 60 mph | 61.1 mph |
| 75 mph | 76.3 mph |
Ground clearance change
+0.26 in
Half the diameter change, at the axle. Body and fender clearance is a separate check: a wider or taller tire can rub even when the math looks small.
These are nominal figures from the size code. Real tires of the same size differ slightly by brand, tread depth and wheel width, so check the maker's published spec for the exact tire. The size is only one part of a swap: also match the load index and speed rating — see how to read a tire sidewall.
How the math works
- Sidewall height = section width × aspect ratio ÷ 100. For 225/65R17: 225 × 0.65 = 146.25 mm.
- Overall diameter = wheel diameter + 2 × sidewall ÷ 25.4. For 225/65R17: 17 + 2 × 146.25 ÷ 25.4 = 28.5 in. Flotation sizes state it directly: 33x12.50R15 is 33 in tall and 12.5 in wide, for a 15 in wheel.
- Circumference = π × diameter, and revolutions per mile = 63,360 ÷ circumference in inches.
- Actual speed = speedometer reading × new diameter ÷ old diameter, because the speedometer counts wheel turns calibrated to the original size.
- Ground clearance change = (new diameter − old diameter) ÷ 2.
Before you change sizes
NHTSA advises buying tires the same size as the originals or another size the vehicle manufacturer recommends. Whatever the size, don't go below the original load index or speed rating — our guide to reading a tire sidewall shows where both are printed. Then check what pressure to run and ask the tire shop whether the new size needs a different figure.
Related
Browse every tire guide, check how old a tire is with the tire age decoder, and keep a tire inflator on hand to set the pressure once new tires go on.