Choosing a spare tire cover should be simple: measure the tire, match the measurements to a size chart, and order a cover that will stay secure on the road. In practice, many fit problems happen because only one measurement is taken or because the wheel is measured instead of the tire. This guide explains exactly what to measure, how to double-check your numbers, and what to consider when your spare tire has a backup camera or a tight carrier. What You Will Need A flexible tape measure (a tailor's tape works best) A straight ruler or measuring stick A pen and paper or your phone A calculator Measure the tire while it is mounted on the vehicle and properly inflated. A mounted tire gives you the dimensions the cover will actually need to fit. The Three Measurements That Matter 1. Measure the tire diameter The diameter is the distance from the outside edge of the tread at the top to the outside edge at the bottom. Keep the tape straight through the center of the wheel, rather than measuring from the rim. If the spare is mounted on the back of a Jeep, SUV, trailer, or RV, place the tape vertically across the tire. Read the measurement at the widest outside points, including the tread, but do not include the carrier or bumper. Record the result in inches. Round to the nearest half inch only after measuring twice. 2. Measure the tire width Width is the distance from one sidewall to the other, across the tread. Hold the ruler or tape parallel to the axle and measure the widest point of the inflated tire. Do not measure the wheel face, rim lip, or the space between the tire and the carrier. A tire can have the right diameter but still be too wide for a cover, so do not skip this step. 3. Check the circumference (optional, but useful) For an extra check, wrap the flexible tape around the center of the tire and note the circumference. If you cannot wrap a tape around the tire, measure the diameter and multiply it by 3.1416: Circumference = diameter x 3.1416 For example, a tire with a 30-inch diameter has a circumference of about 94.2 inches. If your wrapped measurement is dramatically different, repeat the diameter and width measurements before ordering. Use the Tire Sidewall as a Second Reference Most tires include a size code on the sidewall, such as 225/75R15 . The numbers describe the tire's nominal width, aspect ratio, and wheel diameter, but they do not always equal the tire's exact outside dimensions. Tread design, inflation pressure, and the manufacturer can change the final size. Use the sidewall code to identify your tire, then use your physical measurements to choose the cover. When the two sources disagree, the measured outside diameter and width should guide your decision. Allow for a Secure, Not Baggy, Fit A tire cover needs enough room to slide over the tire without stretching the fabric or straining the elastic hem. It should not hang loosely, flap in the wind, or cover the license plate and lights. When your measurements fall between two available sizes, check the seller's size chart and fit notes. A cover made specifically for your measured range is usually better than sizing up by several inches. If the cover uses a rear strap, buckle, or drawstring, leave access to that adjustment point during installation. Check Your Spare-Tire Carrier and Backup Camera Before ordering, look at everything that sits around the tire: A raised center hub or wheel lock A factory or aftermarket tire carrier A backup camera mounted through the wheel A third brake light or license-plate bracket A rear ladder, rack, or other accessory These parts can change how a cover sits. If your vehicle uses a backup camera, choose a cover with a camera opening or a compatible camera option. For example, you can review a custom tire cover with a backup-camera option when a standard solid cover would block the camera view. Do not cut a camera opening yourself unless the cover manufacturer specifically allows it. An uneven opening can fray, let water in, or interfere with the camera image. A Quick Measurement Example Imagine you measure: Diameter: 31 inches Width: 10.5 inches Circumference: about 97.4 inches Write the size as 31 x 10.5 inches and compare both numbers with the product's size chart. The 31-inch diameter is not a substitute for the 10.5-inch width; the cover must accommodate both. Common Measuring Mistakes Measuring the rim instead of the tire: The wheel diameter printed in the tire code (such as R15) is not the cover size. A 15-inch wheel can be fitted with a tire that is 27, 29, or 31 inches tall. Measuring an uninflated tire: Low pressure changes the tire's shape and can produce an undersized result. Inflate the spare to the vehicle manufacturer's recommended pressure first. Ignoring accessories: A carrier, camera, or bracket may need a special opening or a slightly different design even when the tire measurements are correct. Relying on a guess: Tire names such as “30-inch” are rounded descriptions. Always verify with a tape measure and the seller's chart. Find a Cover That Fits Your Setup Once you have the diameter, width, and any carrier requirements, you are ready to shop. Browse the Customization collection for personalized designs and camera-compatible options. For more patterns and vehicle-ready styles, explore Shop All Products . If you use your vehicle for road trips, camping, or off-road travel, the Adventure & Camping collection is another useful place to compare designs built for an active lifestyle. Final Checklist Before You Order Measure the outside diameter of the inflated tire. Measure the widest sidewall-to-sidewall width. Confirm the measurements twice and note the units in inches. Compare both dimensions with the product's size chart. Check for a backup camera, carrier, lock, or other obstruction. Choose the correct opening or accessory option if your vehicle needs one. With those measurements in hand, you can order a spare tire cover that looks clean, stays in place, and protects the spare from sun, dirt, and weather between adventures.