1. TBC Review

Tire Essentials

sizing designations

What are the numbers and letters on a tire?

overall width

Maximum width in cross-section of unloaded tire including protruding side ribs and decorations

section width

The linear distance between the outside sidewalls of an inflated tire without any load (exclusive of protruding side ribs and decorations)

section height x 100 / section width

What is the formula used to calculate the Aspect Ratio?

section height

The distance from the rim seat to the outer tread surface of an unloaded tire.

overall diameter

The diameter of an inflated tire without any load

loaded radius

The distance from the wheel axis of rotation to supporting surface at a given load and stated inflation pressure

deflection

Free radius minus loaded radius

loaded section height

The loaded radius minus half of the normal rim diameter. The distance from the rim seat to the outer tread surface of a loaded tire

normal rim diameter

The diameter of rim seat supporting the tire bead

free radius

The radius of the tire/wheel assembly that is not deflected under load

rim width

The linear distance between rim flanges in contact with the road

P-Metric, European Metric

What are the two most common passenger tire sizing systems?

P-metric

What type of tire sizing system is used here: P215/65R15 95S

passenger car tire designation

What does the P represent? → P215/65R15 95S

section width in mm

What does the 215 represent? → P215/65R15 95S

aspect ratio

What does the 65 represent? → P215/65R15 95S

radial construction

What does the R represent? → P215/65R15 95S

rim diameter in inches

What does the 15 represent? → P215/65R15 95S

load index

What does the 95 represent? → P215/65R15 95S

speed rating

What does the S represent? → P215/65R15 95S

European Metric

What type of tire sizing system is used here? → 185/70R14 88S

section width in mm

What does the 185 represent? → 185/70R14 88S

aspect ratio

What does the 70 represent? → 185/70R14 88S

radial construction

What does the R represent? → 185/70R14 88S

rim diameter in inches

What does the 14 represent? → 185/70R14 88S

load index

What does the 88 represent? → 185/70R14 88S

speed rating

What does the S represent? → 185/70R14 88S

True

Some metric sizes don’t show the aspect ratio in the same format as standard sizes.

82

When the aspect ratio is not given, assume it is what?

82

What is the aspect ratio of this tire? → 155SR13

82

What is the aspect ratio of this tire? → 155R13

82

What is the aspect ratio of this tire? → 155R13 78S

divide by 25.4

How do you convert millimeters to inches?

6.1

How many inches is in 155 millimeters?

load index and speed rating

What does a tire’s service description consist of?

load index

an assigned number that corresponds with a load-carrying capacity

75 to 105

Passenger car tire load indexes generally range from what?

LT Metric, LT High Flotation, and LT Numeric

What are the tire sizing systems for light trucks?

false, it does

The sizing systems for the LT metric sizing system does not mirror the P-metric system The P-metric system; it follows a different structure and set of criteria.

light truck description

What does the LT represent? → LT235/75R15 C

section width in millimeters

What does the 235 represent? → LT235/75R15 C

aspect ratio

What does the 75 represent? → LT235/75R15 C

radial construction

What does the R represent? → LT235/75R15 C

rim diameter in inches

What does the 15 represent? → LT235/75R15 C

load index

What does the C represent? → LT235/75R15 C

LT High Flotation tires

What combines lower aspect ratios and high floatation to yield better traction on sand and soft soil found in watery off-road situations?

overall diameter in inches

What does the 31 represent? → 31x10.5R15LT/C

section width in inches

What does the 10.5 represent? → 31x10.5R15LT/C

radial construction

What does the R represent? → 31X10.5R15LT/C

rim diameter in inches

What does the 15 represent? → 31x10.5R15LT/C

light truck tire designation

What does the LT represent? → 31x10.5R15LT/C

load range

What does the C represent? → 31x10.5R15LT/C

multiply by 25.4

How do you convert from inches to millimeters?

266.7

How many millimeters is 10.50 inches?

older commercial vehicles

What is the LT Numeric sizing system primarily used for?

section width in inches

What does the 7.50 represent? → 7.50R16LT/D

radial construction

What does the R represent? → 7.50R16LT/D

rim diameter in inches

What does the 16 represent? → 7.50R16LT/D

light truck designation

What does the LT represent? → 7.50R16LT/D

load range

What does the D represent? → 7.50R16LT/D

MAST Tire Fitment Guide

What can you use to find answers about tire dimensions or sizing systems?