WhatSizeTire

Tire sizes, decoded

What size tire do I have?

Type the size off your sidewall and see what every part means, how tall the tire really is, and how a new size compares: diameter, speedometer difference and plus-size options. Every rule links to its source.

Decode my tire sizeHow to read a size

  • Inches and millimeters
  • Sources on every rule
  • No sign-up
A couple on a desert road trip crouch by the front tire of their dusty green SUV at a scenic overlook, a winding highway behind them (AI-generated image)

Tire size decoder and comparison

Results update as you type. Add a second size to compare.

As printed on the sidewall or door label.
A size you're thinking of switching to.
mph
Try an example:

Type a tire size such as 225/65R17 to see the results.

Read it like a pro

One size code, six facts

P225/65R17 102H

  1. P: tire typePassenger. LT is light truck, ST special trailer, T temporary spare; no letter is Euro-metric.[1 · Oct 29, 2026]
  2. 225: widthSection width in millimeters, sidewall to sidewall.[1 · Oct 29, 2026]
  3. 65: aspect ratioSidewall height as a percentage of width: here about 146 mm.[1 · Oct 29, 2026]
  4. R and 17: build and wheelRadial construction, for a 17 inch wheel.[1 · Oct 29, 2026]
  5. 102: load indexUp to 1,874 lb (850 kg) per tire. Load index chart.[2 · Oct 29, 2026]
  6. H: speed symbolSpeed category up to 130 mph (210 km/h). Speed rating chart.[3 · Oct 29, 2026]
A dad kneels by a silver sedan in the driveway and points at the tire sidewall while his daughter holds a notebook (AI-generated image)

Before you shop

Find the size your vehicle needs

  • Check the Tire and Loading Information label on the driver's door edge or post, or the owner's manual.[4 · Oct 29, 2026]
  • Buy that size, or another size the vehicle maker recommends.[4 · Oct 29, 2026]
  • Set pressure from the vehicle label, measured cold, not from the number on the tire.[4 · Oct 29, 2026]

How to read a tire size · Tire size by vehicle type

Changing size?

The four checks tire makers give

±3%Keep overall diameter within about 3% of the original.[5 · Oct 29, 2026][6 · Oct 29, 2026]
≥ loadLoad capacity at least the original tire's.[5 · Oct 29, 2026][6 · Oct 29, 2026]
≥ speedSpeed rating at least the original's (winter tires can be an exception).[6 · Oct 29, 2026]
FitsApproved rim width and clearance in every condition.[6 · Oct 29, 2026]

Tire size comparison · Tire size chart in inches

A young man and his grandfather crouch beside a red hatchback on a coastal pull-off, comparing a loose wheel and tire with the one on the car (AI-generated image)

Bigger wheels

Plus sizing without the speedometer surprise

Per inch of wheel, Tire Rack's rule of thumb is about 10 mm more width and 5 to 10 points less aspect ratio, so a 205/55R16 becomes 215/45R17, then 225/40R18.[7 · Oct 29, 2026] The decoder suggests calculated plus-one and plus-two sizes for whatever you type.

Plus sizing guide

A smiling tire shop technician and a customer in a flannel shirt hold up two tires side by side in front of a full tire rack (AI-generated image)

Trucks and SUVs

P-metric or LT?

LT tires aren't automatically tougher. Size for size, they need more air to carry the same load: to match a P245/75R16 109 at 35 psi, Toyo says an LT245/75R16 Load Range C needs 50 psi.[8 · Oct 29, 2026]

LT vs P-metric tires · Load index chart

A woman checks the tire on a small camping trailer hitched to a pickup at a pine-forest campground at sunset while her teenage son watches (AI-generated image)

Towing this weekend?

Trailers need ST tires

Special trailer tires have stiffer sidewalls to reduce sway. Carlisle says most are rated to 65 mph, recommends towing at 60 mph or less, and says to replace trailer tires after five years in all cases.[9 · Oct 29, 2026]

Trailer tire basics · How old is my tire?

Guides

Everything about tire sizes

Sources: 1. Tire Industry Association, Reading a tire sidewall. 2. Tire Rack, What is load index?. 3. Tire Rack, How do I check a tire's speed rating?. 4. NHTSA, Tires. 5. BFGoodrich, Changing tire size. 6. Continental Tire, PSIB 06-03: Tire plus-sizing for passenger and light truck vehicles (PDF, 2006). 7. Tire Rack, What are plus size wheels and tires?. 8. Toyo Tires, Technical bulletin TSD-12-011: Replacing tires on light trucks (PDF, Aug 2012). 9. Carlisle Transportation Products, Trailer tires: tips and best practices (PDF). Checked Oct 29, 2026. Size dimensions, revolutions per mile and speedometer figures are our own arithmetic from the size code. Trademarks belong to their owners; figures are restated in our own words and tables, not reproduced.