To figure out your tire size, read the sidewall code or the driver-door placard that lists the manufacturer’s recommended size.
Your tires already tell you their size. The code molded on each sidewall—something like P225/60R16 98H—packs width, aspect ratio, construction, rim size, load index, and speed rating in one line. You can also confirm the vehicle’s specified size on the Tire and Loading Information label on the driver’s door jamb. Matching those two spots gives you the right answer with no guesswork.
Where To Find Tire Size On Your Car
Start with the driver-door label and the tire sidewalls. The label shows the size your automaker selected for safe load and handling, while the sidewall shows what’s actually installed. If they match, you’re set. If they differ, use the door label as the baseline for replacements.
| Marking | What It Tells You | Notes |
|---|---|---|
| P / LT / ST / T | Vehicle class (Passenger, Light Truck, Special Trailer, Temporary) | Absence of letter often means Euro-metric |
| 225 | Nominal section width in millimeters | Wider number = wider tire |
| 60 | Aspect ratio: sidewall height as % of width | Lower number = shorter sidewall |
| R | Radial construction | Most modern tires are radial |
| 16 | Wheel (rim) diameter in inches | Must match the wheel |
| 98 | Load index | Lookup number to find max load per tire |
| H | Speed rating | Letter corresponds to a tested top speed |
How To Figure Out Your Tire Size — Step-By-Step
This section uses a real code so you can do the math when you need to. Take 225/60R16 as the example. The 225 is width in millimeters. The 60 means the sidewall is 60 percent of that width. Multiply 225 by 0.60 to get 135 millimeters of sidewall height. Convert to inches by dividing by 25.4. That’s 5.31 inches.
Overall diameter equals rim diameter plus two sidewalls. So 16 + (2 × 5.31) = 26.62 inches. Circumference is diameter times π. That’s 83.6 inches. You now know how one size affects ride height, gearing, and speedometer reading.
Close Variant: Figuring Out Tire Size For Trucks And SUVs
Light-truck sizing sometimes uses formats like 37×12.50R17LT. The first number is overall diameter in inches. The second is section width in inches. The R17 is the wheel size, and LT flags a light-truck tire. Many trucks also carry P-metric or Euro-metric sizes, so always check the door label before buying.
Door Label Versus Sidewall: Which One Wins?
Use the door label as the primary guide. It bakes in axle loads, handling, and braking needs picked by the vehicle maker. If you run a different size, make sure load index and speed rating meet or exceed the door label. Staying in that range helps keep the chassis balanced and avoids rubbing.
When You’re Changing Sizes, Do The Right Checks
Small changes are common, like moving from 225/60R16 to 225/65R16 for a taller profile. Before you commit, compare overall diameter, section width, and wheel width range on the tire spec sheet. Make sure the new diameter clears struts, fenders, and liners through the full steering and suspension travel. Check speedometer error too—bigger tires read slow, smaller read fast.
Simple Calculator Method
Here’s a quick recipe you can repeat any time:
- Write down the current size from the door label and the mounted tires.
- Decode width and aspect to get sidewall height.
- Add twice the sidewall to the rim to get diameter.
- Compare the new size’s diameter to stock to estimate speedometer difference.
- Confirm load index and speed rating meet or exceed the label.
Load Index And Speed Rating Made Plain
The two or three digits after the rim size form the load index. It maps to a weight per tire on a standard chart. The letter that follows is the speed rating based on a lab test. Match or go higher when replacing. That keeps braking distances, ABS tuning, and stability aids in the zone the automaker expected.
Where The Tire Placard Lives
On modern cars and light trucks, the Tire and Loading Information label sits on the driver’s door or door jamb. Some older models used the glovebox door. That sticker lists the recommended size and cold inflation pressures for front, rear, and spare. If a shop ever replaced the sticker under a recall, it belongs in that same door-jamb spot.
How To Figure Out Your Tire Size For Winter Or All-Terrain Swaps
Seasonal sets sometimes run a narrower width for snow grip and slush clearing or a taller sidewall for off-pavement. Start with the label size and keep the same overall diameter and load index. A one-step width change paired with the right aspect ratio usually keeps ABS and traction systems happy.
Reading Extra Codes That Affect Fit
Beyond size, watch for XL/Extra Load, RF/Run-Flat, and directional or asymmetric tread arrows. XL tires carry more weight at the same pressure. RF tires may need specific wheels and TPMS procedures. Directional or asymmetric designs must be mounted as marked. All of these details live on the sidewall near the main size line.
Wheel And Rim Fit Basics
Tires list a wheel diameter, but every size also has an approved wheel-width range. Mounting a tire on a wheel that’s too narrow pinches the tread; too wide can stretch the sidewall. Stick to the maker’s range and match load rating to the vehicle’s needs. If your wheels change offset, recheck inside and outside clearance before driving.
Sample Size Comparisons
Use these quick comparisons to see how a small change affects height and readings. The stock line uses 225/65R17 as a common baseline.
| Size | Diameter | Speedo Error |
|---|---|---|
| 225/65R17 (stock) | 28.5 in | — |
| 225/60R17 | 27.6 in | Reads ~3% fast |
| 235/60R17 | 28.1 in | Reads ~1% fast |
| 245/65R17 | 29.5 in | Reads ~3% slow |
| 255/60R17 | 29.0 in | Reads ~2% slow |
| 215/65R17 | 28.0 in | Reads ~2% fast |
| 235/65R17 | 29.0 in | Reads ~2% slow |
Tips That Save Time And Money
Use The Exact Phrase When You Search
Typing “how to figure out your tire size” into a search box brings up guides and calculators. Cross-check results with your door label so you don’t pick a size based only on looks.
Keep Two Records
Take a clear photo of each sidewall and the door label. Keep both in your phone notes. That snapshot saves a trip outside when you’re ordering tires at night.
Mind Load And Speed
Match or exceed the label’s load index and speed letter. Going lower can reduce safety margins during a panic stop or a hot-day highway run.
Ask About Wheel Width
If a shop is mounting to existing wheels, ask for the approved wheel-width range for the new size. That check helps avoid a vague steering feel or premature shoulder wear.
Recheck Pressure After Install
New tires can settle slightly on day one. Check pressures cold the next morning and set them to the values printed on the door label.
Common Mistakes To Avoid
Don’t mix P-metric and LT-metric on the same axle. Don’t fit a wheel diameter that doesn’t match the tire. Don’t ignore rubbing at full lock in a parking lot; that rub can cut a sidewall during a hard corner. And don’t skip a speedometer check after a size change.
Quick Reference: Decoding A Full Line
Take P225/60R16 98H:
- P = passenger tire type
- 225 = width in millimeters
- 60 = aspect ratio
- R = radial construction
- 16 = wheel in inches
- 98 = load index
- H = speed rating
Why This Method Works
It uses the standard coding molded into every modern tire plus the official vehicle label. Both are defined by safety rules and tested procedures. When you match the label with a tire that meets those codes, fit and function line up as designed. If you want the rulebook, see FMVSS No. 110, which sets the placard and sizing requirements.
Final Checks Before You Buy
Scroll back to the first table and confirm you can read each part on your sidewall. Then compare the two tables against the size you plan to buy. If you’re still unsure, share a photo of your label and sidewall with a tire shop so they can match the right spec, mount the set, and torque the lugs to spec.