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Tire Size Calculator

Compare tire dimensions and speedometer error between sizes.

Original tire

Compare to

Speedometer would read
-1.02%
low (taller tire = speedo reads slower)

Original 225/45R17

  • Sidewall: 3.99 in
  • Width: 8.86 in
  • Diameter: 24.97 in
  • Circumference: 78.5 in
  • Revs/mile: 808

Compare 245/40R17

  • Sidewall: 3.86 in
  • Width: 9.65 in
  • Diameter: 24.72 in
  • Diameter diff: -0.26 in
  • Revs/mile: 816

Most carmakers consider a 3 percent diameter change acceptable. Going taller raises the car (and skews speedometer slow). Stay within 0.5 inch diameter on AWD vehicles to keep the differential happy.

About

Type two tire sizes (e.g., 225/45R17 and 245/40R17). The tool shows diameter, sidewall, width, and revs per mile for each, plus how off your speedometer would read after swapping.

How to use

  1. Enter the original tire size.
  2. Enter the new tire size.
  3. Read the difference.

FAQ

How much can I deviate?+

Most carmakers consider 3 percent diameter change acceptable. Beyond that, AWD differentials, ABS, and traction control begin misreading speeds.