Reading the code on the sidewall
A tire size like 225/65R17 is a compressed specification hiding in three different unit systems at once, which is why comparing two sizes by eye fails so reliably. The first number is the tread width in millimetres: 225 mm across. The second is the aspect ratio: the sidewall's height as a percentage of that width, so a 65-series tire's sidewall is 65% of 225 mm, or 146.25 mm — which is why a wider tire on the same series number is also a taller tire, the trap this calculator exists to disarm. The letter R simply says radial construction, and the last number is the rim diameter in inches: 17. Add the rim to two sidewalls (top and bottom), converting millimetres to inches at 25.4 to the inch, and you have the tire's true overall diameter — the single figure that decides clearance, gearing and speedometer truth.
The formulas, and what changes when diameter changes
Everything a driver feels follows from the diameter. A larger tire travels further per revolution, so at any given speedometer reading — which counts revolutions, not road — the car is genuinely going faster, in the exact ratio of the diameters. The odometer under-records distance by the same percentage, gearing effectively lengthens, and ride height rises by half the diameter change. The defaults show a typical plus-width swap done carefully: 225/65R17 stands 28.52 inches tall (sidewall 5.76 in, circumference 89.59 in), while 235/65R17 stands 29.03 inches — the extra 10 mm of width, at the same 65 ratio, adds half an inch of diameter, +1.79%, so an indicated 60 mph is a true 61.08 mph. Same ratio, same rim, noticeably different tire.
The 3% rule and its limits
Fitters commonly treat an overall diameter change within about ±3% as the acceptable window for a swap on an otherwise stock vehicle: inside it, speedometer error stays near a mile per hour or two, gearing and clearance behave, and safety systems calibrated to wheel speed are not badly misled. This page flags any change beyond that range rather than blessing it. Two cautions the arithmetic cannot settle: diameter is only one dimension — a wider tire also needs width clearance at full steering lock and suspension travel, which no formula here measures — and load index and speed rating, the numbers after the size on the sidewall, must match or exceed the original fitment regardless of size games. Treat a result inside 3% as “worth discussing with a fitter”, not as permission, and never mix diameters across an axle.
Worked example
The default comparison, step by step. 225/65R17: sidewall = 225 × 65% = 146.25 mm = 5.76 in; diameter = 17 + 2 × 5.76 = 28.52 in; circumference = π × 28.52 = 89.59 in, about 707 revolutions per mile. 235/65R17: sidewall 6.01 in, diameter 29.03 in. Difference +1.79%: at an indicated 60 mph the car truly travels 61.08 mph, and the odometer under-records by the same 1.79%.
How to use this calculator
- Read the three numbers off the current tire's sidewall: width, aspect ratio, rim.
- Enter them as Tire 1.
- Enter the candidate size as Tire 2.
- Read the diameter difference in the headline — inside ±3% is the usual comfort zone.
- Check the true speed at an indicated 60 mph, and both circumferences, in the table.
Plus-sizing: bigger wheels, same diameter
The deliberate version of a size swap is plus-sizing: fitting a larger rim while choosing a lower aspect ratio so the overall diameter stays nearly constant. Moving from 225/65R17 to a 235/55R18, for example, trades sidewall height for wheel diameter — the calculator shows the two landing within a fraction of a percent of each other — keeping the speedometer honest while changing steering feel and appearance. The trade is real in both directions: less sidewall means sharper response and more vulnerability to pothole damage, and the tire itself usually costs more. Run any plus-size candidate through this page before buying: the ratio that “looks equivalent” is often a full percent or two off, and a percent of diameter is a mile per hour at motorway speeds that your speedometer will never mention.