Bob Drake Catalog

800-221-FORD (3673)

HUBCAPS

AB

C

’s Of Wheels

WHEELS

Usually a wheel will be listed as 14 x 6 or 15 x 8 etc. The

first number is the diameter of the wheel and needs to

be the same as the last number in the tire size. The last

number is the wheel width.

There is also another number which is very important.

This is offset or backspace. If you take the tire you just

bought and put it in the fender opening you will notice

that the middle of the tire probably is not centered over

the mounting flange of the hub. This is where offset

comes in.

When a wheel listed as 15 x 8 offers a backspace of

4½" this means that 4½" of the wheel is inboard of the

mounting flange and 3½" is outboard to total 8". A 15 x

10 with 4½" backspace would have 5½" to the outside.

B

C D

A

VENTS

WHEEL DIMENSIONS

Wheel dimensions can be somewhat confusing so we’ll explain them for you.

A. This is the overall width of the wheel. It is the total of C & D and

usually 3/4" to 1" greater than B.

B. This measurement is across the wheel between the mounting beads.

This is the area that the tire sits in. When you see a wheel listed as

15 x 8 then dimension B is 8".

C. This is measured from the mounting flange to the back edge of the

wheel. Sometimes called backspace.

D. This is measured from the mounting flange to the front edge

of the wheel. Sometimes called front space.

*The difference between B & C is called offset. If D is less

than C this is negative offset. Then if C is less than D it

is positive offset.

VENTED vs SOLID

Original style steel wheels are often called vented.

This term comes from the openings or windows

between the wheel center and the outer rim.

Solids are just that - solid. They do not have this opening.

Let’s run through a quick exercise. Don’t take all of

these numbers as gospel because your measurements

can be different, especially on tires. Remember back

in Step 2 when you took measurement #1, #2 & #3?

Let’s say that #1 is 6" and #2 is 5" for a total of 11".

The 225/60R15 we talked about had a cross section

of 87/8" (we’ll call it 9"). This means that 4½" of tire is

inboard and 4½" is outboard. Dimension #2 is 5". That

only gives you ½" clearance between the tire and the

fender using a centered wheel. If you use the 15 x 8

with a backspace of 4½" that we talked about earlier,

here’s what happens: The front side of the wheel is

now 3½" which is ½" shallower than it would be on

a centered wheel at 4". In essence the outer wheel

has been moved back ½" extra so you now have 1"

clearance between the tire and the fender. Working

out the numbers you also have 1" clearance on the

backside - Perfect.