Diameter, sidewall, revolutions per mile and what a new size does to your speedometer
Updated October 2026
In 225/45R17, 225 is the section width in millimeters from sidewall to sidewall; 45 is the aspect ratio, the sidewall height as a percentage of the width; R means radial construction; and 17 is the wheel diameter in inches. A leading P marks a P-metric passenger tire, LT a light truck tire, and no letter a Euro-metric size. The load index and speed rating that follow (for example 94W) do not change the dimensions. Your current size is molded on the sidewall, and the original-equipment size is on the tire placard on the driver’s door jamb.
Type sizes in the usual format; the calculator reads width, aspect ratio and rim size and ignores the letters. Results give each tire’s height, width, sidewall and revolutions per mile, then the percentage difference, the real speed at the speedometer reading you enter, and the change in ground clearance.
Sidewall height = width × aspect ratio ÷ 100. For 225/45R17 that is 225 × 0.45 = 101.25 mm, or 3.99 inches. Overall diameter = rim + 2 × sidewall = 17 + 2 × 3.99 = 24.97 inches. A 235/40R18 has a 94 mm (3.70-inch) sidewall and stands 25.40 inches tall, 1.72% taller. Revolutions per mile = 63,360 ÷ (π × diameter): 807.6 for the original and 794.0 for the new size. Because the taller tire turns fewer times per mile, a speedometer that reads 60 mph means you are actually doing 60 × 25.40 ÷ 24.97 = 61.03 mph.
| Original | Diameter | Swap | Diameter | Difference |
|---|---|---|---|---|
| 205/55R16 | 24.88 in | 225/45R17 | 24.97 in | +0.38% |
| 225/45R17 | 24.97 in | 225/40R18 | 25.09 in | +0.46% |
| 225/45R17 | 24.97 in | 235/40R18 | 25.40 in | +1.72% |
| 235/65R17 | 29.03 in | 245/65R17 | 29.54 in | +1.76% |
| 265/70R17 | 31.61 in | 285/70R17 | 32.71 in | +3.49% |
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 mph |
| 45 mph | 45.8 mph | +0.8 mph |
| 60 mph | 61.0 mph | +1.0 mph |
| 70 mph | 71.2 mph | +1.2 mph |
| 80 mph | 81.4 mph | +1.4 mph |
The error is a fixed percentage, so it grows with speed. The odometer under-counts miles by the same percentage.
A taller tire covers more ground per revolution, so the speedometer and odometer read low: you travel farther and faster than displayed. It also lowers effective gearing slightly, may rub on suspension or fenders, and can affect anti-lock brake and stability-control calibration. A shorter tire does the opposite. Measured dimensions vary a little by brand and wheel width, so check the manufacturer’s specifications for exact clearance.
Matching the diameter but not the load index. A low-profile plus size often carries a lower load index; the replacement must meet the load the vehicle’s placard calls for, and a speed rating at least as high. Mixing LT and P-metric sizes with the same numbers: they are built and inflated differently. Assuming a wider tire fits the same wheel. Each size has an approved rim-width range in the tire maker’s data. Trusting nominal numbers for tight clearances: actual height and width vary by brand and model. Replacing only some tires on an all-wheel-drive vehicle: many owner’s manuals require closely matched diameters on all four corners, so check yours before mixing sizes or tread depths.