Space-Saver and Collapsible Spare Tire Sizes Explained

A space-saver or collapsible spare tire size is read the same way as a regular metric tire size, with a few extra codes. In a marking such as T135/80D16 101M, the parts mean the following:

• T marks a temporary-use spare.
• 135 is the nominal section width in millimetres.
• 80 is the aspect ratio, meaning sidewall height as a percentage of width.
• D is the construction code (diagonal/bias; R would mean radial).
• 16 is the rim diameter in inches.
• 101M is the load index and speed rating.

From the first three numbers you can calculate the sidewall height, overall diameter and rolling circumference. That lets you compare the compact spare with your regular tire on paper. The calculation does not confirm actual measured size or whether the spare fits your vehicle.

What is a compact spare tire?

A compact spare tire is a temporary spare that is narrower, and often lighter, than the vehicle's regular tires. It is designed to get the car to a repair point, not for normal driving. It is also called a space-saver spare, mini spare tire or temporary spare. In North American usage it is often called a T-type spare because its size designation starts with the letter T. Compact spares usually run at a much higher inflation pressure than regular tires and carry restrictions on speed and distance. These limits are printed on the sidewall or in the vehicle's owner's manual.

The terms overlap by region. In the UK and Europe, "space saver tyre" is the common phrase. In the US, "compact spare" or "mini spare" is more usual. The size markings follow the same logic whatever the name.

How does a collapsible spare tire differ?

A collapsible spare tire is a temporary spare stored deflated and folded on its wheel to save boot or trunk space. Before use, it is inflated, typically with a compressor carried in the vehicle.

The size marking on a collapsible spare describes the tire in its inflated state. The calculated diameter and circumference apply only once it is inflated to the specified pressure. A collapsible spare tire size uses the same width, aspect ratio, construction and rim diameter structure as other temporary spares. It is still a temporary-use tire with its own speed and pressure instructions.

How to read temporary spare tire size markings

Temporary spare tire size markings combine three kinds of information:

• Physical dimensions: width and rim diameter.
• A ratio: the aspect ratio.
• Codes: the T prefix, construction letter, load index and speed rating.

Keeping these categories separate is the key to reading the sidewall correctly. The table uses the example marking T135/80D16 101M.

What does the T prefix mean?

The T prefix is a temporary-use designation. A T-prefix size designation, as in T125/70R16 or T135/80D16, identifies the tire as a temporary spare rather than a tire for everyday driving. T-type spare tire sizes are built and rated for short-term use. They usually require a high inflation pressure, commonly 420 kPa (60 psi), as stated on the sidewall. The T is not a dimension and does not change any size calculation.

Section width: the first number

Section width is the nominal width of the inflated tire, in millimetres, measured across the sidewalls on a specified measuring rim. Compact spares are typically much narrower than regular tires; widths such as 115, 125, 135, 145 or 155 mm are common examples. A width of 125 mm is about 4.9 inches. Section width does not include raised lettering or rim protectors. The actual width also depends on the width of the rim the tire is mounted on.

Aspect ratio: the number after the slash

The aspect ratio is the sidewall height expressed as a percentage of the section width. It is not a measurement in millimetres. An aspect ratio of 80 on a 135 mm tire means a sidewall of about 0.80 × 135 = 108 mm. The same aspect ratio on different widths gives different sidewall heights. This is why the aspect ratio must always be read together with the width.

Construction code: R or D

The letter before the rim diameter describes the internal construction. R means radial construction. D means diagonal, also called bias-ply construction, which is common on temporary spares. The construction code does not enter the size calculation. It tells you how the tire is built, not how big it is.

Rim diameter: the number after the construction code

Rim diameter is the diameter of the wheel the tire is designed to fit, given in inches even though the width is in millimetres. A 16-inch rim diameter equals 16 × 25.4 = 406.4 mm. A tire must match the rim diameter exactly; a T125/70R17 will not mount on a 16-inch wheel.

Load index and speed rating

The load index and speed rating are performance ratings, not dimensions. The load index is a number that refers to a standard table of maximum loads per tire at the specified pressure. Some common values:

• 96 = 710 kg
• 98 = 750 kg
• 101 = 825 kg
• 112 = 1,120 kg

A compact spare may show a higher load index than the regular tire despite being narrower. This is because it is rated at its much higher inflation pressure.

The speed rating letter gives the tire's speed category. M stands for 130 km/h (81 mph). However, temporary spares typically carry a separate, lower operating limit printed on the sidewall or in the owner's manual, often 80 km/h (50 mph). That printed limit is the one to follow.

Other sidewall text on a temporary spare

Besides the size, a temporary spare usually carries other sidewall size markings and instructions. These may include "Temporary Use Only", the required or maximum inflation pressure, and a maximum speed. These lines are operating instructions rather than dimensions. They apply regardless of how closely the spare's calculated size matches the regular tire.

How to calculate compact spare tire dimensions

You can calculate a compact spare's nominal sidewall height, overall diameter and rolling circumference from the size marking in four steps. This is the same method a space saver wheel size calculator in metric units uses.

1. Calculate sidewall height: section width (mm) × aspect ratio ÷ 100.
2. Convert rim diameter to millimetres: rim diameter (in) × 25.4.
3. Calculate overall diameter: (2 × sidewall height) + rim diameter in mm. The sidewall appears twice because the tire sits above and below the wheel.
4. Calculate rolling circumference: overall diameter × π (about 3.1416). To find revolutions per kilometre, divide 1,000,000 mm by the circumference in mm.

Worked example: T135/80D16

• Sidewall height: 135 × 80 ÷ 100 = 108.0 mm
• Rim diameter: 16 × 25.4 = 406.4 mm
• Overall diameter: (2 × 108.0) + 406.4 = 622.4 mm (about 24.5 in)
• Rolling circumference: 622.4 × 3.1416 ≈ 1,955.3 mm
• Revolutions per kilometre: 1,000,000 ÷ 1,955.3 ≈ 511

How does a space-saver compare with the regular tire?

To compare a space-saver with a regular tire, calculate the overall diameter and circumference of both with the same method, then compare the results. Width and aspect ratio together set the sidewall height. Rim diameter sets the core size. All three together determine overall diameter and circumference. A narrow spare can still have a similar overall diameter to a wide regular tire if it has a taller aspect ratio.

What the comparison shows

The two 16-inch compact spares in the table differ by about 41 mm in calculated diameter, even though they share the same rim diameter:

• T135/80D16: its taller 80 aspect ratio on a 135 mm width brings it within about 1.5% of a 205/55R16.
• T125/70R16: its narrower width and lower aspect ratio leave it about 8% smaller.

A smaller overall diameter means a shorter rolling circumference. The tire must therefore turn more times to cover the same distance. In the example, the T135/80D16 makes about 511 revolutions per kilometre, against about 504 for the 205/55R16. The 17-inch pair shows that a 145 mm spare can match the calculated diameter of a 225 mm regular tire almost exactly.

Space saver tyre size chart: calculated geometry

This compact spare tire size chart shows the nominal geometry calculated from example temporary-spare size designations. It covers the kinds of sizes found on space-saver spares from small cars to larger vehicles.

The values are arithmetic results from the size marking. They are not manufacturer-measured dimensions or a list of which spare suits which vehicle. Use it to understand how width, aspect ratio and rim diameter combine, not as a fitment guide.

The chart shows three patterns:

• Same width and aspect ratio, larger rim: T125/70R15 through T125/70R18 have the same sidewall height. Each inch of rim diameter adds 25.4 mm to the overall diameter.
• Same rim, different width or aspect ratio: the overall diameter changes by twice the change in sidewall height.

Space saver wheel sizes: what the wheel markings mean

A space saver wheel is usually a narrow wheel made specifically for the temporary spare. Its size is often stamped in a form such as 16x4T:

• 16 is the rim diameter in inches.
• 4 is the rim width in inches.
• The letter refers to the rim flange contour; T is associated with temporary spare wheels.

The rim diameter on the wheel must match the rim diameter in the tire size.

The tire size and the wheel diameter and width do not describe everything that determines whether a spare wheel fits a vehicle. Bolt pattern, centre bore, offset, brake clearance and the vehicle's rated load all matter too, and none of them appears in the tire size marking. A space saver wheel size calculator can compare diameters and circumferences, but it cannot confirm these fitment factors.

Calculated geometry vs actual tire dimensions

Calculated dimensions are nominal values derived from the size marking. Actual inflated tire dimensions can differ from them for several reasons:

• Tire standards define section width and overall diameter on a specified measuring rim at a specified pressure.
• Real tires of the same nominal size vary between manufacturers and models.
• The tire sits lower under load: the loaded radius is smaller than half the unloaded overall diameter.
• Tread depth changes the diameter as the tire wears.
• Inflation pressure affects the inflated shape. A collapsible spare in particular has no meaningful running size until it is fully inflated.

For exact figures, the tire manufacturer's published dimensions for that specific tire are the reference. The calculation is best used to understand and compare sizes, not to replace measured data.

What dimensional differences can and cannot tell you

A size comparison tells you how a compact spare's nominal height, overall diameter and rolling circumference relate to the regular tire. It shows whether the spare is calculated to be larger or smaller, by how many millimetres, and by what percentage. It also shows how many more or fewer revolutions it makes per kilometre.

A size comparison does not tell you whether a given spare is approved for your vehicle. It also does not tell you whether it fits over the brakes or bolts to the hub, or whether its load index is adequate. A close diameter match does not change the spare's temporary-use status, its required inflation pressure, or the speed limit printed on its sidewall.

For the correct temporary spare tire size for a particular vehicle, the vehicle manufacturer's specification is the authority. This is typically shown in the owner's manual or on a placard on the vehicle.