Tire Size Comparison

Every tire carries its own specification molded into the rubber, and almost nobody reads past the first three numbers. That is reasonable. Those three numbers, 225/45R17 on a typical sedan, are what the person at the counter asks for. But the quantity that decides whether a different size will work on your car is not among them. Overall diameter is never printed on a metric sidewall. It has to be worked out, and nearly every consequence of changing sizes follows from that one derived figure.

So this is an argument for doing one small sum before buying anything, and for knowing exactly what the sum leaves out.

Reading the code

Take 225/45R17 91V apart. The 225 is the nominal section width in millimeters, measured across the inflated tire from sidewall to sidewall rather than across the tread. The 45 is the aspect ratio: the sidewall stands 45 percent as tall as the tire is wide. R means radial construction, and 17 is the diameter in inches of the rim the tire fits. The U.S. Tire Manufacturers Association, the industry’s trade body, lays the format out the same way Michelin does in its guide for drivers.

Letters in front name the sizing system. P marks a P-metric passenger tire and LT a light truck tire, while a size with no prefix is a plain metric size, often called Euro-metric. Trucks add a second dialect. In a flotation size such as 33x12.50R15LT the first number is the nominal overall diameter in inches, the second is the section width in inches, and the third is the rim. It is the only common format that states diameter outright.

The 91V that follows is not part of the size at all. It is the service description, a load index and a speed symbol. Index 91 corresponds to 615 kg per tire, and V to a speed category of 240 km/h, or 149 mph. A Z tucked inside the size, as in 225/45ZR17, signals capability above that. Two tires can share every dimension and still differ here, which matters later.

None of this is decoration. Federal Motor Vehicle Safety Standard 139 requires the size designation, the maximum load rating and the maximum inflation pressure to be marked on every new radial tire for light vehicles. It ties those designations to the published tables of standards bodies, among them the Tire and Rim Association in the United States and the European Tyre and Rim Technical Organisation. A separate federal rule requires the owner’s manual to explain each marking.

The arithmetic nobody prints

Diameter comes from three steps. Multiply width by aspect ratio to get the sidewall height. Double it, because there is a sidewall above the rim and another below. Add the rim diameter, converted at 25.4 mm to the inch. The trade association’s own definition says the same thing in fewer words: twice the section height, unloaded, plus the nominal rim diameter.

For 225/45R17 the sidewall is 225 times 0.45, or 101.25 mm. Twice that plus a 431.8 mm rim gives 634.3 mm, a shade under 25 inches. Now try a common upgrade, 235/40R18. The sidewall shrinks to 94 mm, the rim grows to 457.2 mm, and the diameter lands at 645.2 mm. The new tire is wider, sits on a bigger wheel, has a visibly shorter sidewall, and stands only 10.9 mm taller. That is a change of 1.7 percent.

This is plus sizing, and the arithmetic explains why the practice works at all. The industry’s replacement guidelines describe it as four moves made together: keep the overall diameter of the original tire, increase the section width, lower the aspect ratio, and raise the rim diameter. One inch more wheel is Plus 1, two inches is Plus 2. The rim grows by about as much as the two sidewalls give up, so the outside of the tire stays where the car expects it.

The trade is real, though. A shorter, stiffer sidewall flexes less and sharpens steering. BFGoodrich lists the bill: a firmer ride, more road noise, more unsprung weight, and wheels more easily hurt by potholes.

What a diameter change touches

Circumference is diameter times pi, and a tire turns once per circumference. Divide the 63,360 inches in a mile by the circumference and you have revolutions per mile: about 808 for the 225/45R17 and 794 for the 235/40R18. Everything the car believes about its own motion is counted in those revolutions.

The speedometer comes first. It was calibrated for the original tire, so a taller replacement covers more road per turn than the gauge assumes, and the car travels faster than the needle says. The error is simply the ratio of the two diameters. In the example, an indicated 60 mph is a true 61.0 mph, and the odometer falls behind by the same 1.7 percent, logging about 98 miles for every 100 driven. Fit a smaller tire and both errors reverse.

Direction matters more than it seems. Britain’s inspection manual for Individual Vehicle Approval requires that a speedometer never indicate less than the true speed, while allowing it to read high by a tenth of true speed plus 2.5 mph, so an indicated 60 may be a true 52. A gauge built to that rule can only err on the high side, and a taller tire spends the margin it was given. In the United States, federal motor carrier rules require the speedometers of buses and trucks to be accurate within 5 mph at 50 mph, in either direction.

Gearing shifts too. Fewer wheel revolutions per mile means fewer engine revolutions per mile in every gear, exactly as if a taller final drive had been fitted. The trade association’s list of systems that a diameter change may affect is longer: speedometer, odometer, anti-lock brakes, tire pressure monitoring, traction control and four-wheel drive, any of which may need recalibrating. It adds that on 4WD and AWD vehicles even small differences in outside diameter between tires may cause drivetrain damage.

Then there is the body of the car. The axle rides at the tire’s radius, so ride height changes by half the diameter change: 5.45 mm in the example, about a fifth of an inch. Ground clearance gains that much. The wheel arch loses it, plus whatever the extra width takes, and the guidelines insist on checking every wheel position at full suspension travel and full steering lock rather than trusting the numbers.

How much change is too much? No standard gives a number, and the people who write the standards are careful not to invent one.

The trade association says only that the overall diameter of all four tires must stay within the tolerance accepted by the vehicle manufacturer. The figure most often quoted is 3 percent. BFGoodrich advises keeping overall diameter within roughly 3 percent of original to preserve speedometer accuracy and drivetrain calibration, and the retailer Les Schwab offers the same general rule. Treat it as a rule of thumb, not a limit that anyone certifies.

The label on the door

Diameter is necessary, not sufficient. A federal rule published in 2002 put a tire placard on the driver’s side B-pillar of every light vehicle, or on the door edge where there is no pillar. It lists the original tire size and the recommended cold inflation pressure, along with seating capacity and the combined weight of occupants and cargo. Before that rule the label could sit on the glove compartment door. The regulators’ reason for moving it was plain: one standard location would make people more aware of inflation pressures and load limits. The tire makers’ guidelines begin their replacement chapter the same way, by saying to read the placard instead of guessing.

The placard is the reference because carrying load is what a tire is for. A replacement must carry at least what the original carried at the placard pressure, and its speed rating must match or exceed the original if the car’s speed capability is to be kept. Toyo’s technical bulletin calls this the golden rule and adds a trap. A passenger-type tire fitted to a pickup, SUV or minivan has its capacity divided by 1.1, so a tire marked for 3,086 pounds is good for 2,805.

Nor is the load index a safe shortcut. The trade association warns that an equal or greater index does not always mean equal or greater capacity at every pressure. Its own bulletin shows how slippery the number is: the same LT265/70R17 in the same load range appears under a superseded standard at index 121, rated for 3,195 pounds, and under the updated one at index 123, rated for 3,415.

Where geometry stops

Everything above treats the tire as a rigid drawing. Real tires are neither rigid nor identical. Toyo’s published table for one all-terrain model lists the LT265/70R17 at 31.7 inches where the arithmetic gives 31.6, and shows the LT285/70R17 at 32.8 or 33.0 inches depending on the version. Flotation sizes are looser still, since its 35X12.50R17LT measures 34.5 inches. The federal standard itself lets a tire’s measured section width exceed the listed figure by as much as 7 percent.

Load changes the picture again. Continental’s data book gives the 225/45 R 17 a static radius of 295 mm under maximum load, some 22 mm less than half its nominal diameter. It also lists a rolling circumference of 1,934 mm, defined as the distance covered in one turn at 60 km/h, where pi times 634.3 mm is 1,993. That gap of about 3 percent is why makers publish more revolutions per mile than geometry predicts. Toyo lists 729 for a 225/65R17 that calculates to 707.

Wear matters as well. That same 225/65R17 starts life with 12.9/32 inch of tread and is worn out at 2/32 inch, the depth at which the wear bars come flush. Losing the difference at both the top and the bottom takes about 0.68 inch off a 28.5 inch tire, roughly 2.4 percent. A worn set of the correct size can therefore sit further from its new-tire figure than a fresh set of a carefully chosen alternative. Inflation pressure moves the rolling circumference too, which is one reason four-wheel-drive systems want matched tires at matched pressures.

So a calculated comparison is a first filter, and a good one. It tells you which candidate sizes preserve diameter, how the sidewall and width will change, and roughly what the gauge will do. It cannot tell you whether the tire clears the strut, carries the load at your placard pressure, or measures what its label implies. For those answers go, in order, to the placard, the owner’s manual, the tire maker’s own table, and finally the car itself with the wheels turned to full lock.