Measuring wheels and tires the
By: Kevin Oeste
Photos By: Kevin Oeste
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Selecting wheels and tires is a
crucial element when it comes to styling your ride… as every pro
builder will tell you, there’s nothing more important than the car’s
stance and attitude, both of which are directly to the wheel and tire
If you’re going to run wheels and tires that are the same size as
stock, than the choice is simple. But if you’re going to upsize the
combination, there are a couple things to keep in mind when measuring
the for the right sizes.
There are a couple terms that apply to wheel size… the main ones are
width, diameter and bolt pattern, and then there’s backspace and
offset. Diameter is the overall diameter of the wheel, and width
refers to the width of the wheel. The bolt pattern is the number of
bolts and the measurement of the bolt circle. Ford cars from the
1960’s usually run a 5 hole, 4.5” bolt circle, where GM used a 4 ˚
bolt circle on intermediates and a 5 inch circle on full size cars.
You can check it with a bolt circle gauge, or you can measure across
the bolt holes to determine your size. Just measure from the edge of
one bolt hole, SKIP A HOLE, and go to the CENTER of the next hole.
The next size measurement is the backspace. This is the distance of
the wheel face to the backside edge of the wheel. This measurement is
crucial because it determines the clearance of the wheel in the well
and the amount of the wheel sticking out of the well. You can measure
a wheel by running a straight edge across the rim bead and dropping
another straight edge down to the wheel face and measure that
distance. In this case, the backspace is 4.5 inches.
Offset refers to the amount of wheel behind the face and the amount of
wheel in front of the face. A positive offset puts the face towards
the outside of the wheel, a negative offset has a deeper dished look.
Zero offset is centered in the rim.
Tires are another story. We’re running these BFGoodrich G-Force TA KDW
tires, and they’re available in 93 different sizes! The G-Force KDW is
a high-performance tire capable of sticking hard in the turns, but
also delivering a smooth ride on the street. Plus they look cool! The
numbers go like this: The first number is the section width in
millimeters. The section width is measured from sidewall to sidewall
with the tire unloaded. These are 225’s, so the width is 225
millimeters, or about 8.85 inches
The next number is the aspect ratio. This is the height of the side
wall expressed as a percentage of the tread width. For simple math,
say you had a 225/50 tire, the sidewall would measure 50% of the
section width. 50% of 225 is 112.5mm, or 4.4 inches of sidewall. The
last number is the rim size. You can double the sidewall size and add
the rim size to get the overall tire diameter. Our example has a
sidewall of 4.4… double that makes 8.8, plus 17” rim size and you get
25.8”. That’s how high the tire stands off the floor.
Luckily, companies like BFGoodrich publish tech sheets that have all
this info printed in them, and you can download them off the internet,
but it’s important to know what the numbers mean. It’s also important
to size the tires so you have similar sidewall heights ¯ or the rear a
little taller than the front ¯ to make the car look natural and
All these measurements are easy to take from a wheel, but what do you
do when you are trying to measure a car to see what wheel will fit and
We found this tool called the
Percy’s High Performance.
Wheelrite is a wheel and tire simulator which bolts to the hub and
lets you fit a wheel and tire to the car and actually check the
clearance before you order your wheels. Chances are, if you order the
wrong wheel size, you either bought it or you’re waiting for a
replacement, so this tool is nice to have to get things right the
first time. Here’s how it works.
First, you bolt the
Wheelrite to the hub, and make sure you have the
brake rotor and any spacers installed.
Next, you set the diameter measurement on the front of the tool. We’re
running 17 inch tires, so we set it to 17 inches.
Next, you set the overall width of the
wheelrite to simulate the width
of your wheel and tire. The
Wheelrite has a scale so it’s easy to set
the width. Under this scale is another for backspace. Loosen this knob
and slide the two scales to match your wheel size to see if your
existing wheels will fit. This wire is a simple tire simulator… using
the dimensions from the tech sheet, form the wire to match the profile
of your tire. IN our case, it’s a 4.4 inch sidewall height and a 8.8
Now you can see if your chosen combo will fit! Make sure the
won’t hit anything or rub anything when you spin it! Be sure to
measure this with the car at ride height.. we installed these spacers
to set the wheel in the approximate ride height position. Be sure to
turn the wheel back and forth and check that clearance, too. If you’re
getting new wheels made, set the
Wheelrite to fit the car best, then
take measurements off the
Wheelrite and order the wheels to match.
Now we still have to center our rear axle under the car, but Our
Wheelrite has plenty of clearance when set to 18” diameter, 8” width,
and 4.5” backspacing with a BFG235/50/18 tire mounted on it. To check
it, we have an actual wheel and tire of the same size… check it out!
The wheel rite is right on.
Custom wheels are expensive, and the
Wheelrite helps make sure you
don't end up owning a set that doesn't fit! SOURCES
PERCY'S HIGH PERFORMANCE, INC. 450 BUSINESS PARK ROAD LINN CREEK, MO
(573) 346-4409 888-737-2970
Percy's High Performance