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Figuring Out What Tire & Wheel Will Fit

Please note these references are for the rear wheel wells where many of you are trying to stuff 2” more of tire than could possibly fit.  The basic principles do apply to the front also but you have the additional problem of the tire wheel combo hitting different things when you have the wheels cut hard in either direction, so take that into consideration also for the front

To See if the Tire / Wheel combination you wish to use will work (see drawing for reference).Follow the Formula Below.

You will need to…

1. Place one side of a carpenters square on edge & flat against your brake drum or rotor with the other end facing out to the narrowest part of your wheel well opening inner lip.  Measure the distance from the drum surface to the wheel well inner lip. This is Measurement 1. (On most stock setups this is less than 6” with a stock rear.)

2. Get the cross section width measurement for the tire you wish to use. This is not the tread width but the width of the fattest part of the tire about in the middle of the side wall.

3. Get the measurements for the wheel width you wish to use and its mounting surface offset. Offset is the measurement from the outside of the rim to the mounting surface as measured from the back of the wheel.

To find if the combination will work:

A. Deduct the wheel width from the cross section width and divide the remainder by 2. For example a tire with a 10” cross section minus an 8” wheel = 2” divided by 2 = 1”.

B. Deduct the wheel offset from the wheel width and add the remainder to the final figure from A above.

For example you want to use an 8” wheel with a 3 ½” offset, 8 minus 3 ½ = 4 ½ plus 1” from A above = 5 ½”.

C. Deduct this number from Measurement 1 above. For example if Measurement 1 above is 6 ” minus 5 ½” from B = ½”.  This number is your clearance. Anything less than ¾” will rub the wheel well in certain situations (such as going around sharp curves as your leaf springs will flex to the inside of the curve).

Regardless of whether you have a wheel with positive or negative offset the formula is the same. (Positive offset places most of the wheel off center to the outside, deep dish wheels all are positive offset, negative offset places the wheel off center to the inside, most front wheel drive cars use this offset.)

Front End Alignment Specs

Caster                         0° ± 1/2°

Camber                       1/2° ± 1/2°

Toe-in, inches              1/8" - 3/16"

Front Wheel track width change

Our disc brake kits adds            1" per side

Heidt's drop spindles adds       5/16" per side

Heidt's drop spindles w/ Wilwood calipers adds1/4" per side

Steering Box Ratios

Stock steering box              22:1

Stock box w/ quick ratio  16:1

605 power box                    14:1

Rack & Pinion              15:1   [Jim. Meyer] New Concept 1               14:1  12.7:1 optional