Tire Size Comparison
Two sizes, every number that changes, and what it means on the road.
Comparing tire sizes means comparing overall diameter first, because diameter drives speedometer error, gearing, and clearance. Going from 265/70R17 (31.6 in) to 285/70R17 (32.7 in) is a 3.5 percent diameter increase: the speedometer reads 60 mph while true speed is 62.1 mph, and revs per mile drop from about 638. Stay within 3 percent of stock diameter to keep everything calibrated.
Diameter is the master number
Sidewall, circumference, and revs per mile all derive from diameter. A 3.5 percent diameter increase, like 265/70R17 to 285/70R17, is a 3.5 percent circumference increase and a 3.5 percent speedometer error.
Width changes grip and looks; diameter changes the vehicle's calibration. When comparing, watch diameter first and width second.
What the error does to you
The speedometer reads low with larger tires, so you drive faster than indicated. The odometer under-counts miles, which flatters fuel economy math and can affect lease mileage and service intervals.
Gearing effectively gets taller: slower acceleration, lower cruising RPM, and a small fuel economy change that cuts both ways depending on driving.
How to compare well
Enter stock first and the candidate second so the percent change reads correctly. Check the 3 percent rule, then check real-world clearance at full lock and full compression.
Also compare load index and speed rating, and confirm the wheel width suits the new tire width. The calculator covers the geometry; a tire shop confirms the fitment.
Skip the arithmetic
Compare your two sizes on the calculator.
Tire comparison questions
Which number matters most when comparing tires?
Overall diameter. It determines circumference and revs per mile, which set speedometer and odometer error, effective gearing, and whether the tire clears the bodywork.
How much speedometer error is acceptable?
Within 3 percent of stock diameter, the error stays small enough that speedometer, ABS, and gearing behave normally. Beyond that, verify fitment and consider recalibration.
Do wider tires change the speedometer?
Width alone does not change revolutions per mile. But wider tires often come with a different aspect ratio or rim size, and if the overall diameter changes, the speedometer error follows the diameter.