Tire Size Box

Speedometer calculator

Changed tire size? Enter the original size, the new size and what the speedometer shows to see how fast you are really going.

With 33x12.50R17 tires, when your speedometer says 65 mph you are really going

67.9 mph

33x12.50R17 is +1.39 inches (+4.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

265/70R1733x12.50R17Difference%
Overall diameter31.61"33.00"+1.39"+4.4%
Section width10.43"12.50"+2.07"+19.8%
Sidewall height7.30"8.00"+0.70"+9.5%
Circumference99.29"103.67"+4.38"+4.4%
Revolutions per mile638.1611.2−26.9−4.2%

Questions

Why is my speedometer wrong after new tires?
The speedometer counts wheel turns. A taller tire travels farther each turn, so your real speed is higher than the reading. A shorter tire does the opposite.
How do I fix the speedometer after bigger tires?
Most modern vehicles can be recalibrated with a programmer or by a dealer. Older vehicles use a speedometer gear in the transmission that can be swapped.
Is it illegal for my speedometer to be off?
Speedometers are allowed some error, but you are responsible for your real speed. A ticket for doing 66 in a 60 is still a ticket even if the speedometer said 60.