Here’s how to read a tire’s sidewall: it’s covered in numbers and letters that look cryptic at first glance but actually contain everything you need to know about a tire’s size, rating, and capabilities.
Tire size (e.g., 245/40R18):
- 245 is the tire’s width in millimeters.
- 40 is the aspect ratio — the sidewall height as a percentage of the width. Lower numbers mean a shorter, stiffer sidewall, common on performance tires.
- R indicates radial construction, the standard for virtually all modern tires.
- 18 is the wheel diameter in inches the tire is designed to fit.
Load index and speed rating (e.g., 94Y):
- The number (94 in this example) is the load index, referring to a standardized chart of maximum weight capacity per tire — higher numbers mean higher load capacity.
- The letter (Y) is the speed rating, indicating the maximum sustained speed the tire is rated for under its specified load. Common ratings run from S (112 mph) up through Y (186 mph) for high-performance tires.
Treadwear, traction, and temperature (the UTQG rating):
- Found printed separately on the sidewall, this rating includes a treadwear number (higher means longer-lasting, though it’s only comparable within the same manufacturer’s lineup), a traction letter grade, and a temperature resistance letter grade.
DOT date code:
- A four-digit code (e.g., 2324) indicates the week and year of manufacture — in this case, the 23rd week of 2024. Tires more than six years old, even with usable tread, are generally considered past their safe service life due to rubber degradation.
Why this matters for performance driving: Matching the correct load index and speed rating to your vehicle and driving style isn’t optional — an underrated tire can fail under sustained high-speed or high-load conditions. Always replace tires with an equivalent or higher rating than what came stock on your vehicle, and remember to properly break in any new set before pushing it hard. For more detail on decoding these specs, Tire Rack is a solid reference.
Recommended Gear
Once you know how to read a sidewall, it’s worth checking those numbers regularly. A few tools make that easy:
- Tire Tread Depth Gauge — quickly checks remaining tread against the wear bars.
- Digital Tire Pressure Gauge — confirms you’re running the PSI your sidewall and door jamb specs call for.
- Tire Sidewall Protectant — helps keep sidewall lettering and rubber from cracking prematurely.
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