Decoding Tires and Wheels

You hit a big pothole and hear that distinctive “womp” of a flat tire. You get out to check—the rim is cracked, too. You call a few shops. Your dealership. You get a few price quotes—all outrageous. You want to look online to find better prices, but you have no idea where to start. You put in your make and model and are faced with a full page of choices and it all looks like gibberish. Ugh!

Don’t worry, decoding tire and wheel size is easy once you have a reference for what you’re looking at.

Tires

The size of your tire, assuming your vehicle is stock, is going to be on a sticker on the driver-side door frame. It will also be in the owner’s manual. The sticker should give you information on the front, rear, and spare tire sizes. More than likely the front and rear tires will be the same; spares are all over the place.

All of this info is also molded onto the tire. If you have modified your vehicle and you know the tires are different, look on the sidewall.

The size will read something like this: P245/50R17 98V. None of that makes sense, right? Here’s the breakdown.

  • P” — stands for passenger. As in passenger car. Sometimes it will say “LT” for light truck. Unless you are driving a large commercial truck, it will be “P” or “LT.”
      
  • 245” — the width of the tire in millimeters. Why millimeters? Because Michelin invented modern tires and they’re a French company.
     
  • 50” — the percentage ratio of sidewall to tire width measured from tread to bead (e.g. 50% of 245mm is 122.5mm). The bead is the part that will touch the rim.
      
  • R”—radial. This refers to the construction of the tire. Unless your car is over 40 years old, it has radial tires. 
      
  • 17” — the diameter of your rim in inches. It will be anywhere from 14 to 22 inches.
      
  • 98” — load index. The higher the number, the more load the tire can carry. This number will range from 70 to 126. The heavier your vehicle and intended load, the bigger the number. Some vehicles may have “XL” tires, meaning extreme load.
      
  • V” — the speed rating. “V” is the highest you’d find on a normal car and has a rating of 149mph. Most vehicles will have “S” or “T” or around 112mph or 118mph. Mind you, this is sustained speed. Most tires are going to be going well under these ratings. Look for letters from L to V.

Other markings you will sometimes see are “M+S” or “M/S” for mud and snow, or “AT” for all terrain and “AS” for all season. Most vehicles will come with “AS” tires.

If you have run-flat tires on the other wheels, it's unsafe to replace one with a standard tire. Run-flats are marked with ZP, RFT, SEAL, or sometimes a flat tire with an arrow pointing away.

Wheels

Finding this is going to be a little more difficult. But there is less information to decode, and the tire already gives you some of it. Once you find it, most likely on the backside of the rim itself, it will look something like this: 7.5Jx17H2 ET44.

  • 7.5” — the width of the rim from bead lip to bead lip—the widest part of the inside of the rim.
      
  • J” — refers to the bead profile. It’s rare that you’d find something besides “J” on passenger cars. Larger trucks and vans may have “G”, “H”, or “JJ.” Make sure it matches.
      
  • 17” — same as the tire: the diameter of the rim.
      
  • H2” — refers to the “hump” of the rim. Wheel humps add strength to the rim. This isn’t as important as the other measurements, but it won’t be hard to find the right hump.
      
  • ET44” — the offset of the wheel mount from the center of the rim. This can be positive or negative. Negative is closer to the suspension. Positive is further from the suspension of the vehicle. “ET44” means a 44-millimeter positive offset or 44mm off center on the roadside of the rim. “ET” comes from the German word “Einpresstiefe” which means “insertion depth.”

The hole pattern is also important. It's self-explanatory—four, five, six, or eight holes for most commuter vehicles. The pitch circle diameter (PCD) is the measurement from the center of one bolt hole to the one across from it. This is pretty standard for wheel size and hole pattern.

The center bore and axle spigot need to match too. However, most will fit if the wheel size and bolt pattern match.

So go save a few dollars. Use the power of the internet and this new knowledge to get the best deal on your new wheels.