Spare tire sizes: the temporary turns 195 more times a mile

Spares come in three kinds, and the differences between them are dimensional before they are anything else.

The temporary spare

Marked with a T before the size — T125/80D16 is typical — and built to a completely different brief from the tires it stands in for. Narrow, high-pressure, and dramatically smaller.

T125/80D16 computes to 23.87 in of overall diameter on a 4.92 in section. Against a 255/70R17 at 31.06 in, that is 7.18 in shorter: a 23.12% reduction in rolling diameter.

The consequences follow directly from that figure:

• The spare turns 844.8 times per mile where the tire it replaced turned 649.4 — an extra 195 rotations for every mile driven.
• With it fitted to one corner, the vehicle has one wheel rotating 30% faster than the other three at any road speed.
• The speedometer, which is calibrated for the original circumference, reads high while the spare is driving the sensor it derives from.

That rotational mismatch is the real reason for the speed and distance restrictions printed on the sidewall, and it is why those restrictions exist even on a spare in perfect condition. Differentials and, on four-wheel-drive vehicles, transfer cases absorb the difference continuously, which is something they are built to do intermittently.

Note the D in the size code: diagonal bias construction, not radial. Temporary spares are commonly built that way, and the inflation pressure moulded on the sidewall — frequently 60 psi — is part of how a narrow casing carries a full corner's load.

The full-size non-matching spare

A steel wheel carrying a tire of the same nominal size as the road tires, but often a different model, a different tread pattern and, occasionally, a different size entirely.

The dimensional question here is subtler and easier to get wrong. Even a spare of the identical size code differs in rolling circumference from the four on the vehicle, because those four are worn and it is not. A tire that has lost 8/32 in of tread has lost half an inch of overall diameter — around 1.5% on a typical light-truck size — and that difference is present from the moment the spare goes on.

The matching full-size spare

Same size, same model, same wheel, and on some vehicles included in the rotation so that all five wear together. This is the only arrangement in which the spare is dimensionally identical to what it replaces, and it is the arrangement that costs the most in money, weight and load space.

Rotating five tires keeps the circumferences matched across the whole set, which is exactly the property that matters on a permanently driven axle set.

What changes after you resize

Fit larger tires and the spare becomes the odd one out by whatever the change was. A 9% upgrade leaves a 9% mismatch waiting under the vehicle, in addition to whatever mismatch the original spare already had.

Three practical points, in the order they usually bite:

• Physical fit. A spare carrier, well or under-bed cradle is sized for the original tire. Larger tires frequently do not fit the factory location, which is discovered after the other four are mounted.
• Wheel compatibility. A new wheel package means the spare's wheel may no longer bolt on, or may no longer clear the brakes.
• The mismatch itself. On a vehicle with a locking or limited-slip differential, or with permanent four-wheel drive, a spare several percent smaller than the other three is a driveline input rather than an inconvenience.

Reading the markings

A temporary spare's sidewall carries its own maximum speed and, usually, a maximum distance — figures that are properties of that tire's construction rather than general rules. They are printed on the tire because they are specific to it.

The tire also carries a date code, and it is frequently the oldest tire anyone owns: a spare that has never touched the road has aged for the vehicle's entire life without wearing at all. Reading a DOT date code explains where to find it and how to decode the four digits.

Working out your own figures

Every spare has a size code, and every size code goes through the same arithmetic as any other. Put yours and your road tire into the comparison tool and you get the diameter difference, the percentage, and the revolutions-per-mile gap directly — which is a more useful number to know before you need it than after.