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  1. #1

    Default Confused about Offset

    On my 79 Zephyr wagon I have the 5 lug conversion. My plan is to run a 17x8 wheel at all four corners. From what I can determine from all of the expertise/recommendations on this forum suggests to run 245/45/17 tires on 17x8 wheels to minimize clearance problems.
    What role does offset have in selecting wheels?

  2. #2
    Moderator wraithracing's Avatar
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    "The offset of a wheel is the distance from the hub mounting surface to the center line of the wheel. The wheel offset is measured in millimeters and results in a positive, negative, or zero offset. Positive offset is when the hub mounting surface is toward the front or wheel side of the wheel. It is common to find a positive offset in newer and front wheel drive vehicles. Negative offset is when the hub mounting surface is toward the back or brake side of the wheels centerline. A negative offset wheel usually has a "deep lip" Zero offset The hub mounting surface is even with the centerline of the wheel."

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    ​Trey

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  3. #3

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    I ran a 17x8" wheel with 30mm offset ('01 Bullitt) with my SVO/Mark VII rear axle on my car. That was not enough positive offset for that axle width, so I had rubbing on the wheel well lips. If I'd run 45mm S197 wheels the same size, I would've been fine.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI T-5 (Mustang LWB)
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  4. #4
    FEP Member Patrick Olsen's Avatar
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    Quote Originally Posted by Ratchet1 View Post
    What role does offset have in selecting wheels?
    Offset and backspacing are two different ways of defining essentially the same thing - how much the wheel is sunken into (or poking out of) the wheel well.

    For most folks doing one of the "normal" 5-lug conversions on a Fox platform car ("normal" = Fox or '94-98 SN95 length rear axles), a safe option is to run SN95 wheels - as you said, 17x8" with 245/45-17s. All of the SN95 17x8" wheels [GT tri-bars, GT split spokes, '94-97 Cobra, '98 Cobra, '99/01 Cobra, Mach1, Bullitt] have 5.72" of backspacing, which works out to +30mm or +31mm offset. If you are interested in other 17x8" wheels, you'd want to look for wheels with an offset in that same ballpark.

    ZephyrEFI's example shows how you use offset to adjust fitment. His wider SVO/Mk VII rear end pushes the wheels out. Installing wheels with a higher positive offset sucks the wheels into the wheel well more to clear the rear fender lips.
    '89 GT convertible - not a four-eye
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  5. #5

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    FYI, the image that Trey posted has the correct definition for offset shown, but the definition for back spacing is incorrect. The left side dimension line should go to the inboard side of the wheel, NOT to the inboard bead location as shown. This makes a 0.5" difference in offset.

    In almost all cases, offset is useless for determining if a wheel will fit on a given application. Look at back spacing instead.
    Jack Hidley
    Maximum Motorsports Tech Support

  6. #6

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    Thank you so much for the information. I have ordered a set of 17 x 8 Bullet wheels, 5.72 bs, from American Muscle.

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