What size tire is it?

Too often that is one of the first responses’ I have to make when asked a question about tire replacement. The reason is that 98% of the time the person asking the question does not provide the details needed to allow me or other tire engineers to make an informed answer to the question.

A quick review of some on-line RV forums show the problem of incomplete tire size information. Current posts mention:
225/75R-16E 235/80R 16s 275/70/22.5 12R22.5 315/80R22.5 235/80 22.5 LT235/85R16E 225/75R/16E 235/80R16E

None of these “sizes” provide what I consider the complete size nomenclature.
Here are some facts from the Tire & Rim Association industry standards book that point out why having all the information is both helpful and important.

P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112mph on a Passenger car
1844# 35 psi 112mph on a SUV or P/U (no Dual)

LT235/75R15 101/104Q LRC
1985#single 1820# Dual 50 psi 99mph

LT235/75R15 LRC
1512#single 1377# Dual 50 psi 85mph

ST235/75R15 LRC
2340#single 2040# Dual 50 psi 65mph

To help understand the nomenclature the “P” or “LT” or “ST” is the type tire i.e. passenger, Light Truck and Special Trailer. The numbers are related to the physical dimensions of the tire with 235 being the overall width in mm. The 75 is the ratio of the height of the tire from rim to tread and the 15 is the nominal rim diameter. Hopefully everyone knows that the “R” means radial construction.

LR stands for Load Range with P type tires being LRB which usually means the max inflation is 35 psi but no one uses that designation as it is considered “standard”. P type tires can be rated XL for Extra Load or “Reinforced” and tires with either of these identifications would have a max inflation of 41 psi marked on the tire. The rest of the letters C, D, E etc are related to the maximum inflation for that size. NOTE not all tires with the same Load Range letter are rated for the same max pressure.

An example would be the LT275/70R16 LRC is rated for a max inflation of 50 psi while the LT305/70R16 LRD is also rated for 50 psi max.
Now what about those numbers and letters such as 101/104Q? The “Q” is the maximum speed rating symbol and the numbers would correspond to the Load Index for Dual application and Single application. There is a table that identifies the load in pounds that correspond to the index. The simple way to think of why this is there and how it can be useful is to think of how you can decide if a different size tire can be used. If the Load Index number is equal to or higher than that would be an acceptable replacement.

All these examples relate to P, LT or ST type tires. I will provide another post focusing on larger Truck-Bus type tires some of which have a completely different dimension nomenclature.



UPDATE 9-17-2011
I guess I was in too much of a hurry to post to be sure I covered all the bases. Thanks to Al for pointing this out.

BOTTOM LINE
Whenever you post a question here or in another forum or in an email or even when asking for replacement information at a tire store Please be sure to include the complete size description

Examples:
Most passenger tires will have this format
P235/70R16 104S

Extra Load passenger tire would look like this
P255/60R15 105Q XL

Most newer LT tires will have format like these two
LT235/85R16 116/120R LR-E
LT265/70R17 109/112S LR-C

This would be on some older non-speed rated LT tires
LT235/75R15 LR-C

NOTE I have not covered TBR tires. I will try and do a better job in that post.