Tire size comparison

Compare two labeled sizes by theoretical diameter, circumference and revolutions per mile.

Formuladifference % = comparison diameter ÷ reference diameter − 1

A geometric comparison only

The percentage compares theoretical overall diameters. A positive result means the comparison label produces a larger diameter in the formula.

A percentage is not an approval

Fitment also depends on wheel width and offset, clearance, load and speed ratings, manufacturer tolerances and vehicle systems.

What plus sizing is trying to achieve

Going up a rim size while dropping the aspect ratio is meant to keep the overall diameter roughly where it was. From 225/45R17 to 235/40R18: the first is 634.3 mm across, the second 645.2 mm, a difference of 1.7%. The wheel is an inch bigger and the sidewall about 7 mm shorter, and the car still rolls almost the same distance per revolution.

Get that balance wrong and everything calibrated against wheel rotation drifts with it. The speedometer, the odometer, the trip computer’s fuel figures and, in a modern car, the traction and stability systems all assume the distance per revolution they were programmed with.

Frequently asked questions

How much diameter difference is acceptable?

Three percent is the figure the industry works to, and it is a convention rather than a law. Inside that band the speedometer error stays small and most systems cope. Beyond it you are into rubbing at full lock, a speedometer that reads visibly wrong and, on some cars, warning lights. Your vehicle manufacturer’s approved list beats any rule of thumb.

What does a 2% larger tire do to my speedometer?

It makes it read low by roughly the same 2%, because each revolution now covers more ground than the calibration assumes. At an indicated 70 mph you would actually be doing about 71.4. The odometer under-counts by the same proportion, so mileage, service intervals and any per-mile figures all shift.

Do both tires have to have the same rim size to compare?

No. The calculation converts each label into an overall diameter, so a 17-inch and an 18-inch size compare directly. That is exactly what makes it useful for plus sizing, where the point is to change the rim and keep the diameter.

Does a wider tire always mean a taller one?

It does if the aspect ratio stays the same, because the sidewall is a percentage of the width. Going from 225/45R17 to 245/45R17 adds 18 mm of diameter, about 2.8%. Keeping the height while gaining width means dropping the aspect ratio at the same time, which is where 245/40R17 comes from.

What else changes besides the size on the label?

Effective gearing, for one: a taller tire lowers engine RPM at a given speed and blunts acceleration slightly, while a shorter one does the reverse. Load and speed ratings change with the tire, ground clearance changes with the diameter, and a wider tire may need a different rim width to sit correctly.