Tire guides
Short guides to the numbers and codes on a tire. Each guide uses official sources: US federal regulations, NHTSA, the UN tire regulations and the tire and vehicle makers' own publications.
- How to read a tire size
How to read a tire size such as LT285/70R17 121/118S: width, aspect ratio, construction, rim, load index and speed rating, with official sources. - How to find your vehicle's tire size and pressure
Where to find your vehicle's original tire size and recommended pressure: the door placard required by FMVSS No. 110, and what each line means. - How to read the tire date code
How to read the DOT date code on a tire: the last four digits of the TIN show the week and year it was made. With a decoder and official sources. - Load range and ply rating
What the load range letter on an LT tire means, how it matches the old ply rating, and why a 10-ply rating does not mean 10 plies. Official sources. - UTQG ratings: treadwear, traction and temperature
What the UTQG treadwear, traction and temperature grades on a passenger tire mean, how NHTSA defines them, and which tires have no UTQG grades. - Changing tire size: what changes and how to calculate it
What changes when you fit a different tire size: speedometer error, ground clearance, width and load capacity, with worked numbers and official sources. - Inch tire sizes: what 33x12.50R15 means
What an inch tire size such as 33x12.50R15LT means, the official UN R54 dimensions for flotation sizes, and which metric sizes are near 33 inches. - Tire pressure light: what it means and what to do
What the tire pressure (TPMS) warning light means, steady versus flashing, the pressure at which it must come on, and what to do, with official sources. - How to check tire tread depth
How to check tire tread depth with the wear bars, the penny test or a gauge, and why 2/32 in is the point to replace a tire. With official NHTSA sources.
Printed from https://tiresizecalc.net/guides/. Figures as published on 2026-11-01. The sources are listed below.
Sources
These are the sources that the guides use. Each guide lists its own sources with the section numbers.
- 2025 MX-5 Owner's Manual - Tire Information (U.S.A.). Mazda North American Operations. mazdausa.com manual 70180200. 2025 model year. Retrieved 2026-11-01.
- Reading a Sidewall. Toyo Tire U.S.A. Corp. Undated web page. Retrieved 2026-11-01.
- How to Read Your Tire Sidewall. Continental Tire the Americas. Undated web page. Retrieved 2026-11-01.
- 49 CFR 571.139 - Standard No. 139; New pneumatic radial tires for light vehicles. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations (eCFR). 49 CFR 571.139. eCFR up to date as of 2026-10-15. Retrieved 2026-11-01.
- UN Regulation No. 30 - Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Revision 3). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.29/Rev.3. 2007-03-29. Retrieved 2026-11-01.
- Commercial Tire Safety Tips. Federal Motor Carrier Safety Administration (FMCSA), US DOT. FMCSA-ADO-16-003. 2016-06. Retrieved 2026-11-01.
- UN Regulation No. 54 - Uniform provisions concerning the approval of pneumatic tyres for commercial vehicles and their trailers (Revision 2). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.53/Rev.2. 2004-04-16. Retrieved 2026-11-01.
- Tire Identification and Recordkeeping - Final rule (49 CFR Part 574). National Highway Traffic Safety Administration (NHTSA), Federal Register. FR Doc. 2015-08418. 2015-04-13. Retrieved 2026-11-01.
- Tires (Tire Safety). National Highway Traffic Safety Administration (NHTSA). Undated web page. Retrieved 2026-11-01.
- 49 CFR 575.104 - Uniform tire quality grading standards. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations (eCFR). 49 CFR 575.104. eCFR up to date as of 2026-10-21. Retrieved 2026-11-01.
- 49 CFR 571.110 - Standard No. 110; Tire selection and rims for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations, annual edition (GovInfo). 2006-10-01. Retrieved 2026-11-01.
- Interpretation letter: ply rating and load range (to Federazione Italiana Fuoristrada). National Highway Traffic Safety Administration (NHTSA), Office of Chief Counsel. Undated on the web page. Retrieved 2026-11-01.
- Toyo Tires 2024 Commercial Tires - Dimensions and Load Limits. Toyo Tire U.S.A. Corp. 2024-04-04. Retrieved 2026-11-01.
- Tire Upgrade (plus sizing). BFGoodrich Tires (Michelin North America (Canada) Inc.). Undated web page. Retrieved 2026-11-01.
- UN Regulation No. 39 - Uniform provisions concerning the approval of vehicles with regard to the speedometer equipment including its installation (Revision 1). UNECE; Official Journal of the European Union. OJ L 120, 13.5.2010. 2010-05-13. Retrieved 2026-11-01.
- 49 CFR 571.138 - Standard No. 138; Tire pressure monitoring systems. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations, annual edition (GovInfo). 2022-10-01. Retrieved 2026-11-01.
Last reviewed 2026-11-01. The full source list, with dates and file checksums, is in How the numbers are calculated. To report an error, email contactus@tiresizecalc.net.