Tire Size Comparison Calculator and Speedometer Error
Compare two tire sizes side by side: the difference in diameter in mm, inches and percent, and your true speed when the speedometer reads 60 mph or 100 km/h.
- Diameter difference
- +22.5 mm (+0.89 in, +3.55 %)
- True speed
- Enter the indicated speed to see the true speed.
- Diameter A
- 634.3 mm (24.97 in)
- Diameter B
- 656.8 mm (25.86 in)
Dimensions are nominal values from the size code. Real tires vary by brand, rim width, pressure and wear.
About the Tire Size Comparison Calculator
The Tire Size Comparison Calculator puts two tire sizes side by side. Type size A, the size your speedometer was calibrated for, usually the size the car left the factory on, and size B, the size you want to fit. The calculator shows how much the overall diameter changes in millimetres, inches and percent. Type the speed your speedometer shows, in mph or km/h, and it tells you how fast you are really going on the new tires.
The page opens in comparison mode. Switch to One size to see the full dimensions of a single size: width, sidewall, diameter, circumference and revolutions per mile and per kilometre.
All figures are nominal values from the size codes. Real tires vary by brand, rim width, pressure and wear, so a speedometer check against GPS is still worthwhile after a change.
How the speedometer error works
A speedometer counts how fast the wheels turn, not how fast the car moves. A taller tire covers more ground per turn, so at the same wheel speed the car goes faster than the dial says. A shorter tire does the opposite. The true speed is
The odometer is off by the same percentage, so a 3 percent taller tire also under-records mileage by about 3 percent.
Worked examples
Taller sidewall
225/45R17 is 634.3 mm across and 225/50R17 is 656.8 mm, a difference of +22.5 mm (+0.89 in, +3.55 %). When the speedometer reads 60 mph, the car is really doing 60 × 656.8 ÷ 634.3 = 62.13 mph.
Wider tire
Going from 205/55R16 (631.9 mm) to 215/55R16 (642.9 mm) adds 11.0 mm, or 1.74 %, because the same aspect ratio of a wider tire gives a taller sidewall. At an indicated 100 km/h the true speed is 101.74 km/h.
Plus sizing
Fitting a larger rim with a lower-profile tire, such as 205/55R16 to 225/45R17, is called plus sizing. Done well, the overall diameter barely changes: those two sizes differ by only 2.4 mm. A common rule of thumb is to keep the difference within 3 percent, and to check the load index and the clearance to the suspension and wheel arches.
How to use it
- Type size A exactly as printed on the original tires or on the sticker inside the driver's door.
- Type size B, the size you are thinking of buying.
- Type the speed your speedometer shows and pick mph or km/h. Leave it empty if you only want the size difference.
A positive difference means size B is taller than size A, so your true speed is higher than the speedometer reading. A negative difference means size B is shorter, so you are going slower than the dial shows and the odometer counts extra miles.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
See the full dimensions of one size on the Tire Size Calculator, or work out what a trip costs with the Fuel Cost Calculator.
Frequently asked questions
How much can tire diameter change safely?
A common rule of thumb is to stay within about 3 percent of the original overall diameter. Larger changes affect the speedometer, the gearing and how the ABS and stability systems read wheel speed.
Will bigger tires make my speedometer read low?
Yes. A taller tire covers more ground per turn, so the car travels faster than the speedometer shows. At an indicated 60 mph, a 3.55 percent taller tire means a true 62.13 mph.
Which size is size A?
Size A is the size the speedometer was calibrated for, usually the original equipment size on the door sticker. Size B is the size you plan to fit instead.
Why does a wider tire with the same aspect ratio change the diameter?
The aspect ratio is a percentage of the width, so a wider tire with the same ratio has a taller sidewall. Going from 205/55R16 to 215/55R16 adds 11 mm of diameter.
Does the comparison account for tire wear?
No. It compares nominal sizes from the size codes. A worn tire is a few millimetres smaller in diameter than a new one, which changes the speedometer reading slightly.