Compare measured tire footprints with section width and tread width

The short answer

Section width and tread width describe a tire that is mounted and inflated but carrying no load. A footprint, also called the contact patch, is the area where the tire meets the road while it carries weight. These are different measurements, so neither width tells you a footprint's length, width, outline or area. A footprint figure only means something together with the conditions behind it: load, inflation pressure, surface, speed and measuring method. Compare two footprints directly only when those conditions match.

Measurement	What it spans	Tire condition	Typical source
Nominal section width	Casing from sidewall to sidewall, excluding ribs and projections	Unloaded, on a specified measuring rim, at recommended inflation	Size marking (nominal) and manufacturer dimension tables
Overall width	Same span plus ribs and projections	Unloaded, on the measuring rim	Manufacturer dimension tables
Loaded overall width	Overall width with the tire carrying a stated load	Loaded, on the measuring rim	Some manufacturer dimension tables
Tread width	The tread face only	Unloaded	Manufacturer dimension tables
Footprint length, width, outline and area	The region actually touching the surface	Loaded, on a defined surface, using a defined method	Test reports and imprint or sensor measurements

What section width, overall width and tread width mean

Bridgestone's truck and bus tire data book (Version 24.1) defines the three widths separately. Nominal section width spans the unloaded casing and excludes ribs and other projections on the sidewall. Overall width includes those ribs and projections. Tread width spans only the face of the unloaded tread. The data book also says its dimensions are taken at the recommended inflation pressure, 24 hours after the tire is first inflated. That waiting period gives the casing time to stretch and settle before it is measured.

The number in a size marking is the nominal section width. In a passenger size such as P225/60R16, 225 is the nominal section width in millimeters. In a truck size such as 11R22.5, the leading 11 is the nominal section width in inches. A measured width belongs to a specific rim, which is why manufacturer tables name the measuring rim next to their dimensions.

Example: published widths for one truck tire

The Bridgestone data book lists the R268 Ecopia in size 11R22.5, load range G (material number 248-783), on an 8.25-inch measuring rim. Its overall width is 11.2 inches and its loaded overall width is 12.3 inches. The data book bases these dimensions on the measuring rim, and it bases the loaded figure on the tire's maximum dual load.

Under that load, the tire's overall width grows by 1.1 inches. That figure still measures across the tire body. It does not measure the strip of tread touching the ground. These table columns give neither a footprint width nor a tread width, so you cannot read a contact-patch dimension from them. Each value is valid only with its own rim and load.

What a footprint measurement describes

A 1989 Goodyear paper by Pedro Yap calls the interface between tire and road the footprint. A footprint can be described with four separate quantities. These should be recorded separately, because no one of them can be calculated from another.

• Length: the extent in the direction of travel.
• Width: the lateral extent, across the tire.
• Outline: the shape of the boundary, which may be rounded, squared off or irregular.
• Area: the surface covered. The report must say whether this is the area inside the outline or only the marked contact, with gaps between tread blocks left out.

Conditions that change a footprint

A footprint depends on how the tire is loaded and inflated and on how it is measured. In Yap's tests at a constant load of 4,250 pounds, raising the inflation pressure reduced contact area for both the bias and the radial tires tested. Pressure within the footprint was also uneven: contact pressure varied along its length and across its width. Two footprints with similar outlines can therefore load the road quite differently.

Method and speed matter as well. Yap used a moving flat bed with a strain gauge. Longitudinal contact paths were sampled at several lateral positions across the tire. The bed moved at 0.10 mph, and dynamic effects at higher speeds were beyond what the equipment could measure. Results from a static or very slow test should not be treated as a description of the footprint at road speed.

Reading a published footprint figure

VDI describes a static ink-imprint method. The tread is inked, and the tire is lowered onto paper ruled in one-inch squares. Using this method, VDI reports 36.75 square inches per tire for its police-package Ford Crown Victorias on P225/60R16 tires at 32 psi.

That number applies to those cars, that tire size, that pressure and a static imprint. It is not a general figure for P225/60R16 tires. A heavier vehicle, a different pressure or a rolling test could produce a different footprint on the same size.

Why length times width is not footprint area

Footprints are rarely rectangles, so multiplying maximum length by maximum width does not give their area. Hypothetical example: an outline fits inside a rectangle 6 inches wide and 8 inches long. That rectangle covers 48 square inches. To report the area inside the outline, you have to say how the boundary was traced. To report the marked-contact area, you have to say how the blank areas between tread elements were excluded. The 48-square-inch rectangle gives you neither figure. It only gives an upper bound.

How to compare two footprint measurements

• Match the method first: static imprint, slow rolling bed or another technique.
• Match the load per tire and the inflation pressure, including whether the pressure was set cold or hot.
• Match the contact surface and the test speed.
• Match the area definition: area inside the outline or marked contact only.
• Only then compare length, width, outline and area, one quantity at a time.
• Do not fill a missing footprint value with the nominal size, section width or tread width. Mark the field as not measured.
• Keep the dimensional comparison separate from conclusions about grip, wear or handling, which the dimensions alone do not establish.

A worksheet for recording footprint data

The following template keeps every value tied to its conditions. Section width, overall width, loaded overall width and tread width each get their own fields, so none of them gets mistaken for a footprint dimension.

Tire / model:               
Size marking:               
Rim (width, type):          
Load per tire:              
Inflation and temp basis:   
Surface:                    
Method (static / rolling):  
Speed:                      

Footprint length:           
Footprint width:            
Footprint outline:          
Area + definition:          
Units:                      

Section width (source):     
Overall width (source):     
Loaded overall width/load:  
Tread width (source):       

This worksheet is for documenting and comparing measurements. It does not establish whether a tire fits or is suitable for a particular vehicle. For fitment, load capacity and inflation, follow the vehicle and tire manufacturers' current specifications.