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Wheel sizes, offsets, backspace

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  • Wheel sizes, offsets, backspace

    Can anyone explain offsets and/or backspacing when considering alternative wheels for the RetroBirds? I've been through various/numerous website for calculating measurements, but I still don't have a concept of what it all means. My basic question is, regardless of the wheel diameter, if a stock wheel has a 60mm offset, what is necessary to adapt a 38mm offset to fit?
    Please use only one or two syllable words.

  • #2
    This is something I had put together some time ago and was in the Thunderbird Garage, but is one that can't be accessed at the moment. I hope this will help. Please note that I did not write this, I only put together thoughts from others, who deserve the credit for the information:

    Rim Sizes, Offsets, and Tire Variants

    Please note that this is a condensed and edited version of the above thread. You are HIGHLY encouraged to read it for further information as it was written as well as very useful pictures. Kudos to those who shared this information.
    I bought the Dayton 80 spoke Rims 17x8 and want to buy new whitewall tires 235/55/17. The tire shop told me today the tires would not fit because of the control arm on the front just over the tire would not allow enough room. I told him lots of people here have upgraded to the 235/55/17 . He didn't care and told me he would not put that size on my car. So here is the question. Has anyone ever had a clearance or rubbing problem with a 235/55/17 on the Dayton 80 spoke rim in 17x8
    If going with a stock 17x7.5 wheel with 235x55x17 tires, what offset range will work on our cars. It's hard finding those with 60mm offset.
    • It depends on how "wide-track" you want to go. With stock wheel size (17x7.5", 60mm offset) and stock tires (235/50/17) on both of our 2003 cars, the outer surface of each tire is inboard from the top outer surface of each wheel well by 1" to 1 1/8".
    • Each 5mm reduction in offset moves the wheel outboard by about 0.2" (exactly 0.19685"). Increasing the wheel width to 8" would move the outer surface of the wheel outboard by 0.25". So; an 8" wheel with 40mm offset moves the outer surface of a stock tire outboard by about 1.05", which makes it almost exactly even with the top outer surface of the wheel well. There are several Volvo, Ford (Focus and Fusion) and Jaguar wheels with 5x108 bolt pattern and offset in the 49mm to 60mm range. I have Jaguar Antares wheels (stock width and offset) on my car.
      There's room to move the tires/wheels outboard and still have clearance for tires (the amount depends on the tire/wheel combo), but every fraction of an inch moved outboard means more road spray grit and grime higher on the car behind the tire. Mud flaps and/or more frequent cleaning may be part of the price to pay for that wide(r) track.

    Do you really mean "no more than"? Maybe I'm confused, but the stock wheels at 62mm is more than 49 or 42mm.
    • To answer the question about which way the numbers go- The stock wheel is a 60mm offset. Regardless of width or diameter, a 60mm offset - the centerline of the tire remains the same as designed by the manufacturer. So as the numbers go down, the wheel & tire go outboard.
      So the 49mm that I recommended for the 8" wide wheel would be inboard 7mm from what you now have on your car, a hair over 1/4". One inch = 25.4mm. So compared to stock, your 8" wide, 42mm offset wheels are just shy of an inch outboard. (24.35mm)
      When you change the width from 8" to 8.5" the 49mm offset becomes 23.7mm outboard from stock. The 42mm 8" wheel you're running should be 24.35mm outboard from the stock 17x 7.5" stock wheel. almost an inch.

    My recommendation would be no more than a 49mm offset on an 8" wide wheel. 42mm on a 7.5" wide wheel. I've been accused of being conservative in this area but I'm OK with that, my tires don't rub. The 18x 8" Ford fusion wheel has a 55mm offset- I think that would look really nice & still clear nicely. Others have suggested the Jaguar wheels -what I run. There's a 7 spoke jag wheel out there that looks sharp either painted or chromed but I can't remember "what" they call it. Volvo also makes some nice wheels that "fit"
    And I know you like the Jag wheels with the 60mm(ish) offset. Or is this like a golf score where "more is less"? And for others info, I'm currently running American Racing Mavericks at 18x8 with 40mm offset and no rubbing.
    No problems with the 55's on my car but I have adaptor/spacers behind the Mustang wheels I have on.
    Spacers could be another resolution as they come in various thicknesses. Many racers use them without problems. Billeted aircraft aluminum are available at most speed shops and on e-bay. I believe that the offset for these original Ford Bullitt Wheels was "0". In my case the reason for the adaptor/spacer is the difference in the bolt pattern between the Mustang which was/is 4.5" v 4.25" on our Retro Bird hubs. I bought the rims and Pirelli 235/55/17 P-Zero Nero tires used on e-bay for $400 complete.Rather than change the 4 hubs, I purchased the billet aircraft aluminum adaptor/spacers on e-bay which pushed the wheels out even further than I really wanted to. There was no problem on the front end, but I did have to change the rear struts to the Lincoln LS struts to get the additional height I required to solve clearance issues. So now the footprint stands four inches wider than normal.

    There are several Jaguar S-Type with factory 60 offset.
    When I was looking, it appeared wheels recommended for Jag S-Type would work on our Birds. There are quite a number. Also some Volvo factory and aftermarket wheels appear to fit. IIRC most of those wheels were in the 40 offset range.
    Mine are 8 1/2 x 20 with 40mm offset and I think they fit fine and it could be lowered a inch with ease .
    Been running 235/55/17 Vogues and Dayton wheels like yours since 2004. I have had zero problems
    Life without passion is no life at all.
    Sean Combs


    • #3
      Thank you for that delightful explanation. I think you just single handedly caused the price of stock wheels to double.
      Bird is the word.....
      Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats. H. L. Mencken US editor (1880 - 1956)


      • #4
        Originally posted by RustyFrye View Post
        Thank you for that delightful explanation. I think you just single handedly caused the price of stock wheels to double.
        Well, if a reasonably clear and definitive explanation of how to look to other, less expensive wheels and if they will work on our 'Birds, we will all benefit.


        • #5
          I bought my 04 with 45 offset wheels already on it no extra modifications have original size 50s not 55s I think they are great . minimal lean in corners and feel tight. they go out right to the edge of the wheel wells and make it look kind of tougher. I do however wish I had mud flaps.
          Last edited by hippy_mailman; Mar 9, 2024, 05:02 PM.
          figured I needed something here


          • #6
            What offset would I need to add (if any) for a 6.5" diameter wheel with a stock 38mm offset for the NuvoBirds?


            • #7
              There are an abundance of wheels that will fit a 5x108mm bolt pattern for the NewBirds
              for anyone interested to do a deep dive into alternative choices


              • #8
                For my need-to-see-it learning level, this is the better site I've found to understand how different wheel configurations and tires will fit on the 'Birds

                I use the 235 50 17 60mm offset for the original setup, then enter specs for a new wheel/tire combination I am considering.