To accurately measure your spare tire size, you can either directly measure its diameter across the center or refer to the tire's sidewall markings, often used in conjunction with a tire cover chart. Understanding these measurements ensures you select the correct replacement or accessories for your vehicle.
How Do You Measure Spare Tire Size?
Measuring your spare tire size involves either a physical measurement of its diameter or decoding the tire size information printed on its sidewall. Both methods provide the necessary dimensions for proper fitment.
Method 1: Direct Measurement
The most straightforward way to determine your spare tire's size is to measure its overall diameter. This method is particularly useful if the tire's sidewall markings are worn or unclear.
- Preparation: Place the spare tire on a flat surface.
- Locate the Center: Identify the approximate center of the tire.
- Measure Across: Using a tape measure, extend it across the tire from one edge to the opposite edge, passing directly through the center. Ensure the tape measure is taut and straight.
- Record Diameter: The measurement you obtain is the tire's overall diameter. For example, a common diameter might be 32-1/4 inches.
Method 2: Decoding Your Tire's Sidewall
Every tire has a series of numbers and letters on its sidewall that denote its size, type, and performance characteristics. Understanding this code allows you to determine its dimensions without physical measurement.
A typical tire size code looks something like P205/55R16 91V. Here's a breakdown of the key elements related to size:
- Tire Type (P):
- P: Passenger car tire.
- LT: Light truck tire.
- ST: Special trailer tire.
- Section Width (205): This is the width of the tire in millimeters from sidewall to sidewall.
- Aspect Ratio (55): This number represents the tire's height as a percentage of its width. In this example, the tire's height is 55% of 205mm.
- Construction Type (R):
- R: Radial construction (most common).
- D: Diagonal ply (bias ply) construction.
- B: Belted bias construction.
- Rim Diameter (16): This indicates the diameter of the wheel (rim) in inches that the tire is designed to fit.
To calculate the overall diameter from these numbers, you would:
- Convert the section width to inches (e.g., 205mm / 25.4 mm/inch).
- Calculate the sidewall height (e.g., 55% of the section width).
- Multiply the sidewall height by two (for top and bottom).
- Add the rim diameter.
For a detailed explanation of tire codes, you can refer to resources like Goodyear's Tire Size Explained.
Method 3: Consulting a Tire Cover Chart
Many manufacturers and accessory providers offer charts that list common tire sizes and their corresponding diameters, often for fitting spare tire covers. If your tire's sidewall markings are clear, you can use these charts as a quick reference.
Here's an example from a common chart format:
| Spare Tire Cover Size | Diameter (Approx.) | Fits Tire Codes (Examples) |
|---|---|---|
| Size B | 32-1/4" | 7.50 x 16, 315 70R 15 |
| Size C | 30-1/2" | P235/75R15, 235/75R15 |
| Size D | 28-3/4" | P215/75R15, 215/75R15 |
- 7.50 x 16: This older notation indicates a tire width of 7.50 inches and a rim diameter of 16 inches.
- 315 70R 15: This is a metric tire size, similar to the P-metric system, indicating a 315mm section width, 70% aspect ratio, radial construction, and a 15-inch rim diameter.
By finding your tire's specific code in such a chart, you can quickly identify its diameter and ensure compatibility with accessories like spare tire covers.
Why Accurate Measurement Matters
Knowing your spare tire's exact size is crucial for several reasons:
- Proper Fitment: Ensures any spare tire cover or mounting bracket fits securely.
- Vehicle Compatibility: If replacing the spare, it must match the overall diameter of your vehicle's other tires to maintain proper vehicle dynamics and safety, especially for all-wheel-drive (AWD) vehicles.
- Safety: An improperly sized spare can affect handling and braking.
Whether you measure directly or decode the sidewall, having accurate spare tire dimensions is a fundamental aspect of vehicle maintenance.
