How to read tire size

The stamp on the sidewall, left to right, with a worked example.

Recommended Tire Care & Emergency Gear

Protect your tire investment, prevent uneven tread wear, and stay prepared for highway roadside emergencies.

Driver Equipment
Roadside Power Roadside Must

NOCO Boost Plus GB40 1000A 12V Lithium Jump Starter

Ultra-compact 1000-amp lithium jump starter rated for gasoline engines up to 6 liters and diesels up to 3 liters. Delivers up to 20 jump starts per charge with spark-proof technology and reverse polarity safety protection.

Revives dead vehicle batteries instantly without needing a second jump car
Check Price on Amazon
Emergency Air Pro Compressor

Heavy-Duty 12V Metal Cylinder Portable Tire Inflator

High-output direct battery clamp air compressor delivering up to 120 PSI. Inflates standard passenger tires and oversized SUV/truck tires (up to 33 inches) in minutes with thermal overload cutoff protection.

Eliminates roadside strandings and avoids coin-operated gas station air pumps
Check Price on Amazon
Wheel Service Torque Spec

LEXIVON 1/2-Inch Drive Click Torque Wrench (10-150 ft-lb)

Pre-calibrated reversible click-stop torque wrench with electro-black finish and dual-range highlighted scale. Ensures wheel lug nuts are tightened to exact manufacturer torque specifications.

Prevents warped brake rotors, broken wheel studs, and loose lug nuts
Check Price on Amazon
Pressure Calibration Driver Essential

AstroAI Heavy-Duty Digital Tire Inflator Gauge (250 PSI)

Commercial pistol-grip inflator gauge with backlit LCD, heavy-duty brass air chuck, and quick-disconnect bleeder valve. Delivers precise 0.1 PSI resolution for exact cold-pressure balancing.

Maximizes tire contact patch, fuel efficiency, and tread longevity
Check Price on Amazon
Puncture Repair Flat Defense

Boulder Tools Heavy-Duty All-Metal Tire Plug Repair Kit

Heavy solid steel T-handle spiral reamer and split-eye insertion needle with self-vulcanizing rubber repair strings. Plugs nail punctures and screw holes in steel-belted radial tires without demounting wheels.

Permanently seals tread punctures on the shoulder in under 10 minutes
Check Price on Amazon
Safety Inspection Wear Inspector

Digital Tire Tread Depth Gauge with Backlit LCD Screen

High-precision hardened probe gauge measuring remaining rubber depth in 32nds of an inch and millimeters. Features one-touch zero tare to detect early suspension camber wear.

Alerts you before tread wears past the critical 4/32" wet hydroplane threshold
Check Price on Amazon

Tire Maintenance & Wheel Service Matrix

Standard vehicle maintenance intervals to maximize tread lifespan, fuel economy, and highway braking stability.

Service Task Recommended Equipment Service Interval Vehicle Safety & Handling Impact
Cold Pressure Calibration Digital Inflator Gauge (0.1 PSI resolution) Monthly or before highway road trips Maintains uniform footprint, avoids edge-shoulder wear, and saves fuel.
Lug Nut Torque Verification 1/2-Inch Drive Click Torque Wrench Every tire rotation or 50 miles after wheel installation Eliminates wheel wobble, sheared wheel studs, and warped brake discs.
Tread Depth Inspection Digital Tread Depth Gauge Every 3,000 miles across all 4 tires Catches toe or camber alignment issues before tires fail wet braking tests.
Roadside Puncture Sealing T-Handle Vulcanizing Plug Kit On-demand when encountering road debris Seals nail punctures in the tread crown without waiting for roadside tow trucks.
Mobile Air Delivery 12V Battery-Clamp Air Compressor Pre-trip safety and roadside emergency inflation Re-inflates low tires after slow bead leaks or temperature drops.
Emergency Powertrain Start 1000A Lithium Jump Pack Vehicle emergency glovebox standby Provides standalone cranking amps during cold morning battery drains.
Mechanic Tip
Cold Inflation Calibration Rule: Tire pressure drops roughly 1 PSI for every 10 degrees Fahrenheit drop in ambient temperature. Furthermore, driving even 1 mile generates internal friction that raises tire pressure by 3 to 5 PSI above true ambient resting level. Never set your tire pressure to the "Max PSI" molded on the tire sidewall, which indicates the maximum mechanical burst rating of the casing. Always calibrate tire pressure when the vehicle has rested for at least 3 hours, matching the cold PSI specified on the yellow-and-white driver-side door jamb placard.

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How to read tire size is a left-to-right walk of the sidewall stamp. On a 275/65R20 116T, the first four tokens are geometry and the last two are load and speed. The calculator on the homepage uses only the geometry. This page names every token so the door-jamb sticker and the tire agree.

The metric stamp, token by token

  1. 275. Section width in millimetres. Divide by 25.4 for inches: 10.8 in on this size.
  2. 65. Aspect ratio. The sidewall is 65% of the section width, so 178.75 mm or 7.0 in.
  3. R. Radial construction. Almost every passenger and light-truck tire sold today is an R. You will still see D (diagonal / bias) on some trailers and farm tires.
  4. 20. Rim diameter in inches. The wheel the tire is built for, not the overall diameter of the tire.
  5. 116. Load index. 116 is 2,756 lb per tire. The load index page has the table.
  6. T. Speed rating. T is 118 mph. That is a service limit, not a suggestion.

A prefix sometimes sits in front. P is P-metric (passenger). LT is light truck, usually a higher load range and a different inflation table. If there is no prefix, treat it as metric. An LT275/65R20 is not interchangeable with a P275/65R20 just because the geometry matches.

Flotation sizes read differently

Off-road and some truck tires use a flotation stamp: 33x12.50R15. The first number is already the overall diameter in inches. The second is section width in inches. The third is the rim. There is no aspect-ratio step. The converter will take either stamp.

Same information, two stamps.
What you wantMetric exampleFlotation example
Overall diameter34.1 in (calculated)33 in (stamped)
Section width275 mm / 10.8 in12.50 in
Rim20 in15 in

Where to find the size on the vehicle

The tire has the size it was built to. The vehicle has the size it was designed for. Those can disagree after a previous owner "plus sized" the truck. Read both.

  • Sidewall. Outboard, near the rim. Stamped, not painted.
  • Driver door jamb. The placard lists the OEM size and the cold inflation pressure.
  • Owner's manual. Same placard data, plus optional sizes the manufacturer approved.
The door-jamb size wins if you want the speedometer honest

If the tires on the truck and the placard disagree, the placard is what the speedometer was calibrated for. Put that size in the calculator as the current size, and the size you are considering as the new one.

What the calculator ignores, on purpose

Tread depth, brand, load range and whether the tire is new or worn all change the real-world diameter by a few tenths of an inch. A "33" that has been worn to 5/32 is no longer 33. This site reports the geometry of the stamp. Measure a standing tire if you need the height that is actually on the truck today.

The tire size meaning page is a shorter version of this one, aimed at the search that stops at "what does 275/65R20 mean." Come back here when you want the load index too.

Frequently asked questions

Where is the tire size written?

On the outboard sidewall, near the rim, as a stamp. The same size should also appear on the driver door-jamb placard for the vehicle.

What does the R in 275/65R20 mean?

Radial construction. The cords run across the tire. Almost every modern car and light-truck tire is an R.

Is LT275/65R20 the same as 275/65R20?

Same geometry, different service description. LT is a light-truck tire with a different load and inflation table. Do not treat them as interchangeable.