Can a Spare Tire Be a Different Size From the Other Tires?

Can a spare tire be a different size from the other tires?

Yes, a spare tire can be a different size from the other tires, but only within limits set by the vehicle manufacturer. A temporary spare (space-saver spare) is usually narrower than the road tires and often has a different aspect ratio. It may also be mounted on a wheel with a different rim diameter. What it is designed to keep close to the road tires is the overall diameter, meaning the total height of the inflated tire.

A spare that is not the same size is acceptable when it is the vehicle-specified spare size. A spare taken from another car, or one whose overall diameter is noticeably smaller or larger, may not fit the hub or clear the brakes. It can also rotate at a different speed from the other tires and strain the drivetrain. To judge any spare, start with the numbers on its sidewall.

What do the numbers on a spare tire sidewall mean?

A spare tire sidewall carries the same type of size code as a road tire, sometimes with a prefix that marks it as temporary. Take the example marking T125/80R17 99M:

• T is the service-type prefix and means temporary-use tire. Road tires may carry P (passenger, P-metric), LT (light truck), or no prefix (European metric).
• 125 is the section width in millimetres, measured from sidewall to sidewall on a specified rim. This is a dimension.
• 80 is the aspect ratio. It means the sidewall height is 80% of the section width. This is a ratio, not a measurement in millimetres.
• R is the construction type. R means radial. Some older temporary spares show D for diagonal (bias-ply).
• 17 is the rim diameter in inches. It is the diameter of the wheel the tire fits, not the diameter of the tire.
• 99 is the load index, a coded number for maximum load per tire at the specified pressure. Load index 99 corresponds to 775 kg (1,709 lb). This is a performance rating.
• M is the speed symbol. M corresponds to 130 km/h (81 mph) under test conditions. This is a performance rating.

Dimensions, ratios, units and ratings: how they differ

The tire size code mixes three kinds of values, and only some of them describe physical size:

• Dimensions: section width (millimetres) and rim diameter (inches). The two different units in one code are normal for metric tire sizes.
• Ratio: the aspect ratio is a percentage of the width. Two tires with the same aspect ratio but different widths have different sidewall heights.
• Ratings: load index and speed symbol describe capability, not size. A narrow temporary spare can carry a similar or higher load index than a wider road tire, because it is designed for high inflation pressure.

The T prefix versus a T speed rating

The letter T at the start of a size, as in T125/80R17, means a temporary-use tire. The letter T at the end of the service description, as in 205/55R16 91T, is a speed symbol meaning 190 km/h (118 mph). The position of the letter tells you which meaning applies.

Pressure and speed-limit markings on temporary spares

Temporary spares usually carry extra sidewall or wheel labels for inflation pressure and use limits. Many T-type spares specify a much higher pressure than road tires, often around 420 kPa (60 psi). Many also carry a speed limit, commonly 80 km/h (50 mph), which is lower than the speed symbol. The speed symbol describes a test rating. The printed use limit is the restriction the driver should follow. The label on the tire or wheel and the owner's manual govern the exact values.

Does the size of a spare tire matter?

Yes, the size of a spare tire matters. The value that matters most for compatibility is overall diameter and the rolling circumference that follows from it. Width alone matters much less.

A tire with a smaller overall diameter must turn more times to cover the same distance. When one wheel turns faster than the others, the differential on that axle works constantly. Stability and anti-lock braking systems may also detect the speed difference. A narrower width reduces grip and is the reason temporary spares carry speed and distance limits. A small difference in width does not, by itself, create a rotation mismatch.

How do you calculate spare tire diameter and rolling circumference?

You can calculate a tire's nominal overall diameter from its size code in four steps:

1. Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100.
2. Rim diameter (mm) = rim diameter (inches) × 25.4.
3. Overall diameter (mm) = rim diameter (mm) + 2 × sidewall height. The sidewall counts twice, once above the rim and once below it.
4. Circumference (mm) = overall diameter × π (about 3.1416). Revolutions per kilometre = 1,000,000 ÷ circumference in mm.

Example 1: a different-size spare with a matching overall diameter

This example compares a road tire of 205/55R16 with a temporary spare of T125/80R17. The pairing is for illustration only and is not a statement about any particular vehicle.

The spare is 80 mm narrower and fits a 17-inch wheel instead of a 16-inch wheel. Its calculated overall diameter, however, is almost identical. A taller aspect ratio on a narrower tire, combined with a larger rim, compensates for the lost sidewall height. This is the design principle behind many space-saver spares.

Example 2: a spare tire that is not the same size and does not match in diameter

This example compares a road tire of 225/45R18 with a temporary spare of T125/70R16. Both are widely used sizes, shown here together only to illustrate a mismatch.

The spare is 78.3 mm smaller in overall diameter, about 11.9% less than the road tire. It would turn roughly 13.5% more times per kilometre. A difference of this size is far beyond the close match that vehicle-specified spares are designed to provide.

How to express the overall diameter comparison

To compare two tires, divide the difference in overall diameter by the road tire's diameter and multiply by 100. In Example 1 the result is effectively 0%. In Example 2 it is about 11.9%. Rules of thumb such as staying within 3% are often quoted. These are general guidelines, not a fitment standard. Some vehicles, especially those with all-wheel drive, specify tighter matching in the owner's manual.

Wheel diameter versus tire diameter: what is the difference?

Wheel diameter is the size of the metal rim, given in inches as the last number of the tire size. Tire diameter, or overall tire diameter, is the full height of the mounted and inflated tire, including both sidewalls.

A 17-inch spare is therefore not larger than a 16-inch road tire just because of the rim number. Example 1 shows a 17-inch spare and a 16-inch road tire with the same overall diameter. Comparing only rim sizes, or only widths, cannot tell you whether two tires roll at the same speed.

Do space saver wheels have to be the same size?

No, a space-saver wheel does not have to have the same rim diameter or width as the road wheels. It does have to fit the vehicle's hub and brakes, and the tire on it must provide a compatible overall diameter. Temporary spare wheels are often narrow, for example 4 inches wide. Some are marked with a code such as 17x4T, meaning 17-inch diameter, 4-inch width and a T-type flange intended for temporary tires.

Several fitment dimensions are not shown in the tire size at all:

• Bolt pattern: the number of studs and the diameter of the circle they sit on.
• Centre bore: the hole that centres the wheel on the hub.
• Offset: the position of the mounting face relative to the wheel's centreline.
• Brake clearance: the space between the wheel's inner shape and the brake caliper.

A spare tire can have a perfect calculated diameter and still be unusable if its wheel does not match these points. This is why the safest reference is the vehicle-specified spare size, not a calculation alone. A rim diameter that is smaller than the road wheels can be fine on one car and fail to clear the brakes on another.

Why calculated diameter differs from actual tire dimensions

The overall diameter you calculate from the size code is a nominal figure. Real tires differ from it for several reasons:

• Manufacturer variation: tires of the same nominal size from different brands or models can measure slightly differently.
• Tread wear: a worn road tire is smaller than a new one. Every millimetre of tread lost reduces the overall diameter by about 2 mm. For example, 5 mm of wear reduces the diameter by about 10 mm, roughly 1.6% of a 632 mm tire.
• Load and inflation: under the car's weight, the tire flattens where it touches the road. Its effective rolling radius is smaller than half the unloaded diameter. Underinflation increases this effect, which matters for high-pressure temporary spares.
• Rim width: the section width is defined on a specified measuring rim. A narrower or wider rim changes the actual width and slightly changes the profile.

For these reasons, calculation is useful for comparing sizes and spotting obvious mismatches. It does not confirm fitment. Manufacturers' published dimensions and the vehicle's own specifications are more reliable than nominal figures.

Spare tire size compatibility and drivetrain compatibility

Drivetrain compatibility means whether the drivetrain tolerates one wheel turning at a different speed from the others. Diameter differences matter most on driven axles.

• Two-wheel drive: a spare with a slightly different diameter on the driven axle keeps the differential working constantly. That is acceptable for the short, slow use temporary spares are designed for.
• All-wheel drive and four-wheel drive: centre differentials, couplings and clutch packs can be sensitive to sustained differences in rolling circumference between axles. Some manufacturers give specific instructions, such as limiting distance and speed or placing the spare on a particular axle.
• Worn full-size spares: a full-size spare that is the same nominal size but has much more tread than the worn road tires also has a larger overall diameter. On some all-wheel-drive vehicles, that difference can matter as well.

Follow the owner's manual for the specific vehicle. General rules cannot replace the manufacturer's instructions for a given drivetrain.

How to check whether a spare tire is the right size for your vehicle

1. Find the vehicle-specified spare size. Look at the tire information placard, usually on the driver's door jamb, and in the owner's manual. Many vehicles list the spare size and its inflation pressure there.
2. Read the spare's sidewall. Note the prefix, width, aspect ratio, construction, rim diameter, load index and speed symbol. Also note any printed pressure or speed limit.
3. Compare the size code with the specification. An exact match to the vehicle-specified size is the clearest sign of compatibility.
4. If the code differs, calculate both overall diameters. Use the sidewall-height and overall-diameter formulas above, then work out the percentage difference.
5. Confirm the wheel fits. The bolt pattern, centre bore, offset and brake clearance must suit the vehicle. A diameter calculation cannot confirm these.
6. Check pressure and limits before driving. Inflate to the pressure marked for the spare and observe its speed and distance limits.

Full-size spare vs space-saver spare

In short, a spare tire can be a different size from the other tires. The difference should be in width, aspect ratio or rim diameter, not in overall diameter. The spare should also be the size the vehicle manufacturer specifies. The sidewall numbers let you understand and compare sizes. The vehicle's specification and the wheel's fitment decide whether a particular spare belongs on the car.