Reading Tire Size Codes: Overall Diameter, Sidewall Height, and Speedometer Error
October 21, 2026
The code on a tire sidewall, such as 225/45R17, packs the size into three numbers. The first, 225, is the section width in millimeters. The second, 45, is the aspect ratio: the sidewall height as a percentage of the width. The letter R means radial construction, and the last number, 17, is the wheel (rim) diameter in inches. With these, you can compute the overall diameter, which determines how far the car travels per wheel revolution and therefore how accurate the speedometer is.
For a 225/45R17: the sidewall height is 225 × 0.45 = 101.25 mm. The rim diameter is 17 × 25.4 = 431.8 mm. The overall diameter is 431.8 + 2 × 101.25 = 634.3 mm, or 24.97 inches.
Skip the math and get your answer instantly:
Open the Length Converter →The Diameter Formula
Overall diameter (mm) = rim diameter in inches × 25.4 + 2 × width × aspect ratio ÷ 100. Divide by 25.4 for inches. The circumference is π × diameter: for 634.3 mm, 1.993 meters, so the tire makes about 807.6 revolutions per mile (1,609,344 mm ÷ 1,993 mm) or 502 per kilometer. Manufacturers publish these numbers as revolutions per mile in tire data books; real tires deflect under load, so the effective rolling radius is slightly smaller than the unloaded value.
The speedometer reads wheel rotation speed, so it assumes the original tire's circumference.
| Tire size | Sidewall (mm) | Overall diameter (in) | Diameter vs. 225/45R17 |
|---|---|---|---|
| 225/45R17 (reference) | 101.3 | 24.97 | 0.0% |
| 235/45R17 | 105.8 | 25.33 | +1.4% |
| 225/50R17 | 112.5 | 25.86 | +3.5% |
| 245/40R18 | 98.0 | 25.72 | +3.0% |
| 215/50R17 | 107.5 | 25.46 | +2.0% |
Speedometer Error After a Size Change
If the new tire is larger in diameter, the car travels farther per revolution, so the true speed is higher than the speedometer shows. True speed = indicated speed × (new diameter ÷ old diameter). Changing from 225/45R17 to 235/45R17 increases the diameter by 1.4%, so at an indicated 60 mph the true speed is 60 × 1.0142 = 60.85 mph, and at 70 mph it is 70.99 mph. The odometer under-reports distance by the same percent: after 10,000 indicated miles you have actually driven about 10,142. A change to 225/50R17 (+3.5%) causes an indicated 70 mph to be a true 72.5 mph. Many speedometers read a few percent high from the factory (by design, and by regulation in some regions), which partly offsets this.
Rules for Replacement Sizes
Keep the overall diameter within about 3% of the original to avoid problems with the speedometer, anti-lock braking, stability control, and gear ratios, and check fender clearance. A 'plus-size' wheel upgrade increases the rim diameter by one inch and reduces the aspect ratio to keep the overall diameter about the same, such as from 225/45R17 to 225/40R18 (25.09 inches, within 0.5% of the original). Wider tires increase grip but may add noise, cost, and rolling resistance, and the wheel width must support the tire width within the manufacturer's range. Always confirm the load index and speed rating meet or exceed the original specification.
Reading the Rest of the Sidewall
After the size is a load index (such as 94, corresponding to 1,477 pounds per tire) and a speed rating letter (such as W for 168 mph or V for 149 mph). The DOT code ends with four digits for the week and year of manufacture, such as 2524 for the 25th week of 2024. Tires age even when unused; many manufacturers advise replacing tires after six to ten years. Tread wear, treadwear grade, traction, and temperature ratings appear on the UTQG label. Check pressure monthly using the car's placard, not the maximum pressure on the tire.
Practical Tips
Before changing sizes, compare the overall diameters of the old and new tire using an online comparison or the formula above, and use your GPS speed to check your speedometer. Recalibrate if needed. If you use different front and rear sizes on all-wheel-drive vehicles, keep the diameters close to prevent drivetrain stress. Consult the vehicle manual or a tire professional for the right choice.
Frequently Asked Questions
What do the numbers in 225/45R17 mean?
Section width of 225 mm, sidewall height of 45% of the width, radial construction, and a 17-inch wheel.
How do I calculate tire diameter?
Rim diameter in mm plus twice the sidewall height, where the sidewall is the width times the aspect ratio.
How much speedometer error does a larger tire cause?
The true speed is higher than indicated by the percentage increase in diameter, such as 1.4% for the example.
How much can I change the diameter safely?
Generally within about 3% of the original.
How old are my tires?
Check the four-digit DOT date code: the first two digits are the week and the last two the year.