Wheel width/offset/lug pattern for 1973 Century GS. What is spec to replace front wheels?

Discussion in 'The Hides' started by Warren Jarrett, Feb 13, 2018.

  1. Warren Jarrett

    Warren Jarrett Well-Known Member

    Does anyone know the specs for whatever wheels were stock, on the 1973 Century? I want to shop for after-market wheels, for the front. So I want to know the "specs" to quote, and accurately specify the rim configuration. I suppose width, offset, and lug pattern is what I need to know.
  2. pbr400

    pbr400 68GS400

    Several sizes were available, the biggest was 15x7 with 4 3/8” offset. They’re all 5x4 3/4 bolt pattern.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Actually 4 3/8” back spacing. Offset is different. Offset is measured from the center of the rim width and can be positive or negative. Being a 15 X 7” rim is actually 8” from edge to edge, the center of the rim width is 4” from the back edge. That makes the offset 3/8” positive.
  4. Warren Jarrett

    Warren Jarrett Well-Known Member

    OK, I have always been confused by offset and backspace. So let me just ask this. If a 15x7 rim fits just right with a 4-3/8 backspace, what about the backspace for a 15x8 rim on the same car? Do we keep the same backspace spec, or do we adjust it for the different width rim?

    And, for use with a 235-15 tire (or maybe a 245/70-15), will either a 15x7 or 15x8 work equally well?
  5. Harlockssx

    Harlockssx Brother Graw Mad

    Most modern car applications don't even factor in backspacing, they'll simply use width & offset to determine fitment, but also a lot of them will run coilovers, which alleviates a lot of your backspacing clearance issues...Obviously all are important, but there's decades of reference for these cars to pick from!

    A 235 will fit fine on a 7" wide wheel, but has a little less bulge on an 8". I've run 245's on 7's many times in the past, you just get a little more of a ballon appearance to the tire sidewall.
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Wider rims require more back spacing to keep the wheel centered in the wheel well. If the rim width increases by one inch, the back spacing should increase by 1/2 that. 4 3/8 + 1/2 = 4 7/8". When you do that, notice the offset remains the same, 3/8" positive. I would just round that off to 5" back spacing. Should work just fine.

    The wider rim makes the tire more stable on the rim. A 235 or 245 would be fine on either rim. The 245 tire will be 10mm wider and .6" taller. It might fill the wheel well better. See what tires came on the 73 Century stock. Try to keep the same tire height to avoid changing the speedometer calibration and overall gearing.
  7. Warren Jarrett

    Warren Jarrett Well-Known Member

    I am trying to buy (on Ebay) a used pair of front wheels for my 1973 Century. The reason I want used is because the car looks rough, but is a sleeper. Mechanically it is perfect and completely restored. So my rear wheels are already Cragar S/S, they look a little old, with a little rust, but they look great. I don't want new ones in the front to look too much better.

    But the size I need to look for is confusing me. The front tires are now 215/70R14 on the stock rally wheels. The rear tires are now 235/75R15 Cragar S/S. I think I want 15" rims this time, with lower profile front tires. I like the contrast of big high profile tires in back with smaller lower profile tires in front. Is this something that most people feel looks goony, or can people here relate? This is what the first owner, who died and left me his car, liked and I respect his 1970s/80s taste.

    Maybe to keep this "contrast" (bigger rear wheels AND tires), I should stay with 14" front wheels. In this case, what width and backspace (or offset)?

    14 inch or 15 inch, what do you think, for what I am trying to accomplish? And, what range of rim widths are appropriate, 6", 7", 8" or all of the above? And what backspace or offset will be fine, for whatever width rims fit? Does anyone have good advice for me?
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The rim size, be it 14", 15", 18" or whatever size has NO impact on back spacing requirements. Back spacing requirements will change with different rim WIDTHS. I tried to answer your question in a previous post, and you are asking the same question once again. Forget about offset. As I explained previously, offset is different than back spacing, and it is more difficult to measure. Have a look at this youtube video. It will help.

    Again, the 15 X 7 Buick Rallye wheel had 4 3/8" of back spacing. If you wanted to buy an 8" wide 15" wheel, or 18" wheel for that matter, you would need to increase the back spacing 1/2". That would be 4 7/8". That will keep the center of the wheel positioned in the wheel well, the same as it was for the 7" wide wheel.

    Rim width you want will depend on the section width of the tires you want to run. Wider tires need wider rims. The 7" rim will accommodate a tire up to 255 mm. Keep in mind that tire section width and aspect ratio impact the overall tire height. What looks stupid to me is short tires that don't fill the wheel well. Your current 14" tires are 25.85" tall. Your rears are 28.87" tall, about 3" taller. How does that look?
    Last edited: Feb 18, 2018
  9. Warren Jarrett

    Warren Jarrett Well-Known Member

    Thank you again, Larry. Yes you did answer my questions last time, but then I rephrased them to encompass a few more aspects. Again, you answered these also. So, I understand now. And the video helped further.

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