How to Choose a Spare Tire Cover Size Without Guessing

A cover that is too small may not seat over the tire; one that is too large can look loose or move more than intended. The good news is that you do not need to guess from a photo or memorize vehicle specifications. The tire itself contains most of the information you need.

This guide covers common metric tire codes such as 245/75R17 and 315/70R17. It also explains when a tape measurement is more useful and why the final product listing—not a generic chart—must decide the available size.

Why vehicle year and model are not enough

A vehicle model may be sold with several wheel-and-tire packages. Previous owners can install aftermarket wheels, off-road tires, or a different spare. Even tires carrying the same nominal code can vary slightly by manufacturer and tread design. That makes “fits this Jeep” or “fits this SUV” an incomplete sizing method.

Use the vehicle model only as context. Use the code and physical dimensions of the actual rear spare as evidence.

Step 1: find and read the sidewall code

How the code is verified: Continental’s tire-marking guide identifies the first number as nominal tire width in millimeters, the number after the slash as sidewall height expressed as a percentage of width, R as radial construction, and the following number as rim diameter. The calculator below applies those labeled dimensions; the mounted tire and exact cover listing still control the purchase decision.

Look for a sequence similar to 245/75R17 on the tire’s sidewall:

• 245 is the nominal section width in millimeters.
• 75 is the aspect ratio. The sidewall height is 75% of the section width.
• R indicates radial construction.
• 17 is the wheel, or rim, diameter in inches.

Do not confuse this sequence with load, speed, pressure, or manufacturing-date markings elsewhere on the tire. Photograph the full code so you can recheck it during checkout.

Step 2: calculate the approximate overall diameter

For a common metric code, the estimated diameter is:

The division by 25.4 converts millimeters to inches. The sidewall is counted twice because there is one sidewall above and one below the wheel.

Use the Spare Tire Cover Size Calculator to check the arithmetic. A calculated number is still an estimate: load, inflation, tread, wear, and manufacturer specifications can affect the tire’s real outside diameter.

Step 3: measure the mounted spare

A tape measure is especially useful when the code is unreadable, the tire is heavily worn, the setup is modified, or the listing asks for physical dimensions.

1. Park safely on level ground and do not work around moving traffic.
2. Measure vertically from the tire’s outermost edge to the opposite outermost edge, passing through the center. That is the approximate overall diameter.
3. Measure the widest visible section from front to back if you can do so safely. Use the mounted tire’s measured width rather than treating the section width in the code as the final mounted width.
4. Repeat the diameter measurement horizontally. A large disagreement suggests the tape did not cross the center.
5. Record the result and the sidewall code together.

Do not add arbitrary “extra room” to the number. The seller’s size ranges and construction determine how a cover is meant to fit.

Step 4: inspect cameras, brackets, and clearance

Diameter is not the only constraint. Look at everything near the spare:

• a rear-view camera mounted through or above the wheel;
• a license-plate relocation bracket;
• lights, ladders, bike racks, trash carriers, or recovery equipment;
• an offset carrier or unusually deep wheel;
• tight clearance between the tire and bumper.

If hardware crosses the face or edge of the tire, a standard cover may be blocked even when the diameter is correct. A camera opening must align with the actual camera position; similar vehicle names do not guarantee identical hardware.

Step 5: match the exact listing—not a universal chart

Open the product you intend to buy and compare your measurements with its current variant names and fit instructions. The Flower Fetch Dog Spare Tire Cover, for example, turns affection for a dog into something other drivers can see on the road. It is also the page that must be checked for its own current sizes, options, and ordering details.

A 31.5-inch calculation does not automatically mean “round up” or select a generically named 32-inch cover. Some listings specify ranges; others use a nominal size. Follow the exact listing. If your result sits near a boundary or your hardware is unusual, ask before ordering.

Common sizing mistakes and how to avoid them

Using the driver-door label only

The placard describes the vehicle’s intended tire specification, but the mounted spare may have been changed. Read both, then let the actual spare control.

Reading the wheel diameter as the tire diameter

In 245/75R17, 17 inches describes the wheel—not the outside of the tire. The complete tire is much larger because it includes two sidewalls.

Measuring only across the wheel

Measure rubber edge to rubber edge through the center. A rim-only measurement cannot size the cover.

Assuming every “35-inch tire” measures exactly 35 inches

Nominal flotation sizes and real mounted dimensions can differ. Check the manufacturer specification or measure when the fit tolerance matters.

Ignoring tread or an accessory mounted over the spare

Deep off-road tread, chains, covers, or attached equipment can change the outside envelope. Size the setup the new cover must actually go over.

Your checkout-ready fit record

This record gives you something concrete to compare and something useful to share if you need help. It also prevents the most common last-minute error: choosing a variant from memory instead of rechecking the tire.

Last reviewed: September 10, 2026. Tire dimensions, product sizes, and availability can change. Calculations are estimates; verify the mounted tire and the exact product listing before ordering.