Tire Size Calculator
Compare two tire sizes before you switch. Enter your current size and the new one, as printed on the sidewall, to see the diameter of each, the difference in percent, and your real speed when the speedometer reads 60 mph.
Diameter difference
+5.42%
Check the highlighted fields to see your result.
- Current diameter
- 24.97 in (634 mm)
- New diameter
- 26.33 in (669 mm)
- New sidewall height
- 105.8 mm
- At 60 mph on the speedometer
- 63.3 mph
Most tire shops recommend staying within about 3% of the original diameter.
Results are estimates for information only and do not constitute financial, medical, or legal advice.
How it works
A tire's overall diameter is the wheel diameter plus two sidewalls. The calculator converts everything to millimeters, compares the two diameters, and uses the difference to correct the speedometer reading.
The formulas
Sidewall = width × aspect ratio ÷ 100
Diameter = wheel (in) × 25.4 + 2 × sidewall
Worked example
Compare 225/45R17 with 235/45R18.
- 225/45R17: sidewall 225 × 0.45 = 101.25 mm; diameter 17 × 25.4 + 2 × 101.25 = 634.3 mm (24.97 in).
- 235/45R18: sidewall 105.75 mm; diameter 668.7 mm (26.33 in).
- The new tire is 5.42% taller, so at 60 mph on the speedometer you'd really be going about 63.3 mph.
Frequently asked questions
What do the numbers in a tire size mean?
In 225/45R17, 225 is the tread width in millimeters, 45 is the aspect ratio (the sidewall height is 45% of the width), R means radial construction, and 17 is the wheel diameter in inches. A "P" in front means a passenger tire and "LT" a light-truck tire.
Why does a bigger tire change my speedometer?
The speedometer counts wheel turns. A taller tire covers more ground in each turn, so you actually go faster than the speedometer shows. A smaller tire has the opposite effect, and your odometer will show more miles than you really drove.
Can I change my tire size?
Often, within limits. Most shops suggest staying within about 3% of the original diameter and keeping a load index and speed rating at least as high as the original. Check your owner's manual and the placard inside the driver's door, and ask a tire professional.