Tire Sizes: What Do All the Numbers and Letters Mean?

You’ve been told that your vehicle needs tires, and you’re on your way to the tire store. Or maybe not. Perhaps you’re online at any number of web-based tire suppliers. Of course, you know the year, make and model of your vehicle, but you have no idea what size tires are on there now. The current tires might be the ones installed by the factory. Or, they could be the set you bought 5 years ago which are now shot. Maybe you bought your car used and have no idea if what’s on there is correct.

What size tires are you going to buy?

You could go outside and look at the lettering on the sidewall. But it’s (pick one or more) dark/cold/rainy/late. Besides, you can’t find your flashlight. Or, you can’t do this right now because the car’s not here. Your wife/son/unemployed friend borrowed it for the evening. But, the online sites know your tire size, except, when you punch in the vehicle specifics, the screen invites you to “select your tire size from these choices”.

Argghh!

Not to worry. We’re here to help. Really. We are going to walk you through an understanding of all the alpha-numeric characters on a tire’s sidewall so that you can be certain that you’re getting the correct size tires for your car or truck.

First things first: unless you (or a previous owner) have modified your ride with larger wheels or significantly upgraded suspension, we will presume that you need to purchase tires in the same size as the factory used. If you are certain that the current tires are the same size, get that info from the sidewall. If you have doubts, you have other options. Look for the mandated tire pressure decal (it may be in the door jamb, fuel door, or glovebox lid), or look in the vehicle owner’s manual. You can also call the vehicle maker’s Customer Service Department. (They may ask you for the vehicle’s VIN, or Vehicle Identification Number, which you’ll find on a tag on top of the dash, or on your registration papers.)

You should have something very close to a string of alpha-numeric characters like this (the ones in parentheses may not be part of your particular tire’s size):

(A) NNN / NN (A) NN (NN) (A)

Here’s an example of an actual tire size:

P 195 / 65 R 16 95 V

We’ll decipher this coding very shorty. Before we do, here is the most important bit of information in our article: STAY WITH THE ORIGINAL, RECOMMENDED TIRE SIZE FOR YOUR CAR OR TRUCK. THIS SERIES OF LETTERS AND NUMBERS DETERMINES THE OVERALL DIAMETER OF YOUR WHEEL & TIRE COMBINATION. VARYING FROM THIS SIZE CAN AFFECT THE CAR’S HANDLING, BRAKING, SPEEDOMETER ACCURACY, AND OVERALL SAFETY.

You may find that the tire salesperson “recommends” a different size tire, for any number of reasons (price, brand, availability, perceived performance enhancements). Does everyone in fact always use the same size as originally installed? Of course not. But those who venture toward a different size usually have some deeper automotive technical knowledge, and this article is NOT intended for those folks. By giving you the decoding info, we’re arming you with the data to help you intelligently communicate with the tire seller, whether in person, on the phone, or online, during the sales transaction.

Let’s take the tire size above, P 195 / 65 R 15 95V, and assign a position to each character:

1^st character = P

2^nd character = 195

3^rd character = 65

4^th character = R

5^th character = 15

6^th character = 95

7^th character = V

Each character is named and deciphered as follows:

1^ST character (letter): SERVICE TYPE

• P = P-metric (also referred to as “passenger”); intended for use on passenger vehicles, including cars, SUVs, minivans, and light trucks.
• LT = Light Truck; intended for vehicles with heavier cargo or heavier towing capabilities, including heavier-duty pickups, SUVs, and vans.
• T = Temporary; almost exclusively found on spare tires (so-called mini spares) intended for short term only use.

2^nd character (number): TREAD WIDTH

• Measurement across the tire’s tread from sidewall to sidewall
• Always expressed in millimeters
• Common examples: 195, 225
• To convert to inches, divide by 25.4
• 195mm = 7.7”; 225mm = 8.9”

3^rd character (number): ASPECT RATIO

• The sidewall height (from wheel opening to outer edge of sidewall) expressed as a percentage of tread width
• Always indicated as “percent”
• Common examples: 65, 50
• Example #1: if the tread width is 195mm, and the aspect ratio is 65, the sidewall height = (195 x 65%), or (195 x .65), which equals 126.75mm
• Example #2: if the tread width is 225mm, and the aspect ratio is 50, the sidewall height = (225 x 50%), or (225 x .50), which equals 112.5mm
• To convert to inches, divide by 25.4
• 75mm = 5”; 112.5mm = 4.4”

4^th character (letter): TIRE CONSTRUCTION

• Indication of the way the tire’s internal plies and belts are arranged
• Most commonly found: “R” for “radial” tire (likely the case with 99.5% of new tires sold)
• Also found (but rare):
  • “D” for “diagonal” (bias ply);
  • “B” for “belted” (additional belts under the tread)

5^th character (number): WHEEL SIZE

• Indicates the diameter of the wheel, or rim
• Always expressed in inches
• Common examples: 15, 16, 18
• Also indicates the size of the hole in the middle of the tire
• The tire’s “wheel size” and the diameter of the wheel MUST MATCH

6^th character (number): LOAD INDEX

• Indicates the weight that a car is rated to carry
• Always expressed as a 2-digit number
• Conversion chart must be referenced to equate the load index to pounds
• Examples: 91; 95
• A load index of “91” equates to 1356 lb. per tire (at maximum allowed pressure)
• A load index of “95” equates to 1521 lb. per tire (at maximum allowed pressure)
• Never purchase tires with a lower load index than originally equipped!

7^th character (letter): SPEED RATING

• Indicates the maximum speed that the vehicle is rated to travel on these tires
• Always expressed as a single letter
• Conversion chart must be referenced to equate the speed rating to MPH
• Examples: S; V
• A speed rating of “S” equates to 112 MPH / 180 KPH (at maximum allowed pressure)
• A speed rating of “V” equates to 149 MPH / 240 KPH (at maximum allowed pressure)
• Never purchase tires with a lower speed rating than originally equipped!

What should you do with all this information? Print this out (or access it on your mobile device) as a reference guide. If you’re offered tires in the EXACT same size, load index, and speed rating as your original equipment (OE) tires, you’re there. (Information on selecting a brand, quality level, and tread design are for a future article.) If you are offered a tire that’s “almost the same size” (perhaps the aspect ratio is a larger number) and you’re told “it’s close enough”, STOP. That is not true. Either insist on the correct size tire, or find another retailer who has the sizes you need. Any change in tread width, aspect ratio, load index or speed rating will affect the way your vehicle drives, and could seriously affect the safety of everyone who rides with you. There are enough reputable dealers out there to allow you to find someone who understands, as you now do, that the only correct tires for your car are the right-sized tires.