Deciphering tire sizes.

Zephyrman · 03-31-2010, 05:50 PM

Ever wondered what all those numbers on the sidewall of your tires mean? Here's a readers digest version to help you on your quest for buying the correct rubber for your ride.

Take this size as a case in point: LT275/60/R17. The LT means it is meant for a light truck or SUV. All Radial tires come in metric sizes, so the first number, 275 is the tread width in millimeters. The second number in the line, 60 is the sidewall height, which is a percentage of the tread width, in this case, the sidewall is 60% of the first number 275. The last one, R17, the R means radial tire the 17 means the tire fits a 17 inch wheel. I realize most of the more experienced guys already know this stuff, but there are always people new to the hobby that don't, so this is for them. Oh, there are a couple other tire designations I should add as well. There is also P for Passenger tire, and T for Trailer. I hope this helps anyone who was wondering.

militarydave · 03-31-2010, 09:14 PM

Not to steal Zypherman's (and Readers Digest's) thunder but here is some additional info:

I've taken the liberty (and lunch hour) to put in layman's terms what the numbers/letter on the sidewall of your tires mean. this has been re-worded and taken from various sources to not violate any copyright laws and/or infringements!

I shall use my stock tires from my Crown Victoria, which are P215/70R/15's.

common tires are P and LT, which P is for passenger tire, and LT for light truck, and T for "temporary" spare tires.

the next number in the group is 215, which is the aspect ratio for the width of the tire that is measured from sidewall to sidewall in (mm).

The following number gives the sidewall height as a percentage of the width. so for mine it is 215mm for the width, and height of 70 as a % of the 215

Next comes the Letter designated in R, B, or D.
R=Radial
B=Belted Bias
D=Diagonal Bias

the next number after the letter (R 15) is the total amount of inches of you wheel size. my case: i have 15 inch wheels.

there is also a load weight max that the tire supports when cold along with a Load Index.

a letter also indicated the "Speed Rating". such as V, Z, S, which all have their maximum speed rating (if you can drive that fast)

The type of tire is also along the sidewalls, such as All Season, Mud and Snow, etc.

Some brands have tire wear indicators, that pop up automatically once the tire tread starts to go below as certain height. the tire pressure is also listed on the tire itself, if not found on the car rear door or service manual.

it is stated that "cracked" looking tires or "weather checking" is normal and not a defect of the tire. check for uneven treadwear, which is a clear indication of a much needed alignment, ball joint or other suspension notice.

Nitrogen inflation helps when in extremely hot/cold temps, but can cost upwards of $30-$50 for all 4 or 5 tires.

remember to always balance rotate your tires for maximum tire life, and constant gas mileage. hope this helps!

Zephyrman · 04-01-2010, 08:45 AM

Thanks for the extra info, I was falling asleep when I posted, and I knew I left some stuff out. And I could have sworn The T designation was for trailer specific tires. My bad!

militarydave · 04-01-2010, 09:21 PM

no problem.... thats why were here. and if anyone has any other tips feel free to post

thanks,

-dave

cjones · 04-03-2010, 02:06 AM

Originally Posted by militarydave

Some brands have tire wear indicators, that pop up automatically once the tire tread starts to go below as certain height. the tire pressure is also listed on the tire itself, if not found on the car rear door or service manual.

just to clarify...the tire pressure on the tire sidewall is the MAXimum tire pressure, NOT necessarily the recommended tire pressure for the vehicle.

also, i thought all DOT approved passenger vehicle tires had to have wear indicators. thought that's what they said when i worked for Bridgestone/Firestone 20yrs ago. oh well

militarydave · 04-03-2010, 08:40 PM

good point cjones!

Justaguy · 04-19-2010, 09:48 PM

...and not ALL radial tires come in metric sizing. Take for instance my '73 Land Cruiser it wears 31x11/15. In this case, the overall height of the tire measuring from the ground to the top of the tire as sit will be on your truck is 31 inches. The tires are 11 inches wide from flex point across the tread patch to flex point. and it rides on a 15 inch wheel.

I used to run a 91 f150 on 40x18/15. I had a 6 inch lift and lots of fender trimming. Even these tires were radials. I wont run bias if I can help it, they wander WAY too much, and follow ruts and cracks in the roads.

Zephyrman · 04-20-2010, 03:07 AM

You are absolutely right, justaguy. I was just trying to cover the more common sizing, as the metric tires are by far more common.

Jonzo77 · 04-20-2010, 03:34 PM

here's one for you to figure out. the sticker on the passenger door of my fairmont says my tire size is...CR78-14 C78-14 AND DR78-14. what the hell did that mean? lets see if any of you can figure out the metric size for them. zeph, you have the ability to cheat...

Justaguy · 04-20-2010, 06:29 PM

ALPHA-NUMERIC
SIZE ON VEHICLE
PLACARD
(32 PSI MAX. PRESSURE)

IF VEHICLE TIRE PLACARD SPECIFIES AN
ALPHA-NUMERIC TIRE SIZE, THE FOLLOWING ARE
ACCEPTABLE SUBSTITUE SIZES.

IMPORTANT: Do not exceed maximum tire pressure imprinted on tire sidewall.
13 INCH TIRE SIZE AR78-13
P165/80Rl3, P175/75Rl3, P185/70RI3, P195/60Rl3, P215/50Rl3, 185/70Rl3

BR78-13

P175/80Rl3, P185/75Rl3, P195/70Rl3, P195/65Rl3, P205/60Rl3

CR78-13

P185/80Rl3, P195/70Rl3, P215/60Rl3, P235/50Rl3
14 INCH TIRE SIZE

BR78-14

P175/75Rl4, P185/70Rl4, 185/70Rl4

CR78-14

P185/75Rl4, P195/70Rl4, P205/65Rl4, P215/60Rl4, 195/70Rl4

DR78-14

P185/80Rl4, P195/75Rl4, P205/70Rl4, P205/65Rl4, P215/60Rl4, P245/50Rl4

ER78-14

P195/75Rl4, P205/70Rl4, P225/60Rl4, P245/50Rl4

HR78-14

P225/75Rl4, P235/70Rl4
15 INCH TIRE SIZE
BR78-15 P165/80Rl5, P175/75Rl5, P185/70Rl5
ER78-15 P195/75Rl5, P215/65Rl5, P245/50Rl5
FR78-15 P205/75Rl5, P215/70Rl5, P215/65Rl5, P235/60Rl5, P245/50Rl5
GR78-15 P215/75Rl5, P215/70RI5, P235/60Rl5, P255/55Rl5, P265/50Rl5
HR78-15 P225/75Rl5, P235/70Rl5, P255/60Rl5, P275/50R15
JR78-15 P225/75Rl5, P235/70Rl5, P255/60Rl5
LR78-15 P235/75Rl5, P245/70Rl5, P255/65Rl5, P265/60Rl5, P295/50Rl5

Here, try this chart... gives you a nice tire size grouping that keeps your speedometer accurate.

Jonzo77 · 04-20-2010, 08:40 PM

it works fine, but what do the letters mean?

Zephyrman · 04-20-2010, 08:51 PM

LOL your right, Jonzo, I could cheat, but I'm too lazy to go out to the car.

Justaguy · 04-20-2010, 11:14 PM

Originally Posted by Jonzo77

it works fine, but what do the letters mean?

Oh you didnt ask what they mean, just what the metric equivelant was.

MissInformation · 04-21-2010, 03:15 AM

Originally Posted by Jonzo77

what do the letters mean?

Hummm, I'm not too bright and it's early here, but let me have a try at this anyway.

The C/D means the size of the tire.
The R means it is radial ply.
The 78 means the height to width ratio.
The 14 means the rim diameter in inches.

Therefore, CR78-14 and DR78-14 would both be radial ply tires and I am going to hazard a guess that C78-14 would be a bias ply, maybe even a belted bias ply.

Jonzo77 · 04-21-2010, 10:31 AM

ok. thay makes sense to me. the whole system was a little before my time...

thank you

EscortTard · 12-21-2010, 01:45 AM

One more thing to add.. for each PSI, "LT" tires actually handle less of a load than "P" tires do. They more than make up for this by running at higher pressures.

If you 'upgrade' from a P tire to an LT tire, typically you want to add 15psi to the recommended tire pressure (but not exceeding the tire's maximum rated pressure). Example, if your door jamb says 30psi, you'd want to put 45psi in the LT tire.

The benefits are that you have a heavier duty tire, often with more aggressive tread. The drawback is a rougher ride, decreased fuel economy, and often noisier tires.

LT's also have a load rating. C, D, and E. (There are A and B also but these typically aren't used for normal vehicles as they don't support enough weight, although I HAVE seen B rated tires on Jeep Wranglers before).

C rated tires usually have a max pressure of 50psi.
D rated tires usually have a max pressure of 65psi.
E rated tires usually have a max pressure of 80psi.

It's usually a good idea to upgrade to valve stems rated for higher pressures, and also to make sure the wheel itself is designed to handle those pressures, ESPECIALLY if you choose D or E rated tires.

A lot of common pickup tire sizes also have a "P EXTRA LOAD" tire available which is a "P" tire that is designed to handle more weight. Often this is a much better alternative to an LT tire as you get the best of both worlds, but it isn't always available.