How to calculate tire size?

Tires and Wheels (Mopar Hubcaps Too)

  1. matthon

    matthon Well-Known Member

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    I have various wheels I'm considering for the front of my 67 Barracuda, and I'm trying to figure what tire size will fit for each rim.
    SBP with KH brakes.

    14x6 Cragars, 4.125 backspace, currently with 215/70/R14. Crusty, rusty.

    14x6 Cragars, a smaller backspace, 3.75 or 4, currently with 205/70/R14. Like new, although I'm not a fan of the backspace.

    14x7 chrome, not sure of backspace, bare rims. Rusted centers, but may paint black with dog dish.

    I also have a set of sorry Keystones I'd like to use, but I'm not sure if they are safe.

    How do you calculate the best size that will fit?

    Also, I have one 5.5 wheel for a spare, what's the smallest tire you can mount on a 14x5.5 wheel?

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  2. roccodart440

    roccodart440 Well-Known Member

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    I don't really understand what you are asking.

    Look at the section width of the tires you are considering. They should be at least .25" greater than the actual width of the wheel. The more sidewall width you run beyond the width of the wheel and worse it will handle.

    As far as height, above 26" in the front can start to get pretty tight, especially depending on the ride height.

    As far as the spare, you can mount a tire at or slightly narrower than the overall wheel width and make sure to check the diameter of your spare tire well. I don't carry a spare anymore. CAn't fit rear spares anymore and AAA is much more convenient.

    Last, I beg you to reconsider running 14's.
     
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    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      And especially the rims. hey look pretty shot and your Barracuda looks like a nice one.
       
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      • Tinmannz

        Tinmannz Well-Known Member

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        • 66fs

          66fs Well-Known Member

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          I usually use the Tire Rack specs. I usually search by size and go from there.
           
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          • roccodart440

            roccodart440 Well-Known Member

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            Summit has an amazing tire size filter.
             
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            • mvh

              mvh FABO Gold Member FABO Gold Member

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              As a general guideline regarding backspace, you want to keep the rim centered where the factory rim was centered — that's where the most clearance is. So if the factory 4.5" rim has 3" backspace, and you want to install a 7" rim, split the difference (7 minus 4.5 equals 2.5, 2.5/2 = 1.25) and add it to the factory backspace to get the backspace you need (3 + 1.25 = 4.25). If the backspace is too small, the rim will sit farther out than stock, which is likely to cause clearance problems on the lower front corner of the opening with a wider-than-stock tire (on a Barracuda). For example, I had a set of original TorqueThrust mags, 14x7 SBP, but they only had 3.25" backspace when mounted — the widest tire that would have cleared would have been a 215 (maybe), and I already had 215s on my factory steel wheels — I wanted 235s.

              The other problem is that a lot of cast 14" rims won't clear the calipers with proper backspace — this is true of many slot mags.
               
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              • trapster

                trapster Well-Known Member

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                • grimreaper

                  grimreaper Cross Member

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                  • diymirage

                    diymirage HP@idle > hondaHP@redline

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                    go with the keystones
                    that way if something goes wrong, at least you're going out in style
                     
                  • 69_340_GTS

                    69_340_GTS Well-Known Member

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                    Put the 14 x 7 on the back with 225/70R14, put the 14 x 6 on the front with 215/70R14. You can use a 195/7514 tire for a spare on the 5.5" wheel. But it won't quite fit in the well, depth-wise. You need to keep all the tires about the same diameter. These are all around 25"-26" dia.
                     
                  • matthon

                    matthon Well-Known Member

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                    This is what I meant to ask, with all my babbling. I noticed the smaller backspaced Cragars were closer to the front of the lower fender.

                    I wish the Keystones were in better shape, I'm not sure if they are unsafe, no visible cracks.
                    As far as the tires go I would never drive on those.

                    I went to 15s in the rear, SBP, going to a taller gear and got the rims at a great price.

                    Keystones look about as wide as the chrome wheels I have, probably the same offset. I would blast and spray paint, they don't need to be shiny chrome.
                    Need a center cap.

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                  • AJ/FormS

                    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                    I have a 68 Barracuda, for street romping.
                    On the front; It has been wearing 14x7s since about 2005 with a near-zero offset, I think it is 3.75 backspace. It wears 235/60-14s.
                    I have to run them at 28/29 psi for long tread life. But whenever the mood strikes me, I pump them up a tad; but honestly, on the street, I got used to the front end skating, and the low-pressure roll-over. Street tires in my chosen size, driven on the street,lol, just do that, I learned to deal with it or slow down..
                    The KH calipers will not nestle inside ALL wheels. The Rallys have the shape to fit them perfectly.
                     
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                    • matthon

                      matthon Well-Known Member

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                      How close are your tires to the bottom front fender?

                      That's the same backspace as my shiny Cragars, which stick out too far, imo, and come close to the bottom front fender.

                      I think your tires are a bit shorter, but not by much.

                      I'll have to check the backspace on the chrome rims and see if they fit the KH.
                       
                    • 383Scampman

                      383Scampman Well-Known Member

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                      The tire spec will give you overall width at its widest point . Backspacing , as was posted before, should be between 4 1/8- 4 1/2" . If the backspace is too much , tire hitting on the inside , shims are available . If the tire is hitting the outside you have to change rims . 14" rims are more forgiving than 15" rims but the selection is aweful . 15" tires are getting difficult . 16",18"& 20" are the rage today but look wrong on an A-body , IMO , out of proportion . I would start with a 235/60R-15 rear and same up front . If you want you can go 205/70R-15 front . Bear in mind that a 70 series tire is taller. I would do a mock up first . Get steel rims and . Put junk tires on them and see if you like it . Everyone's taste is different . Good luck .
                       
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                      • roccodart440

                        roccodart440 Well-Known Member

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                        14's are more forgiving how?

                        There is every size imaginable in 15" and then some.

                        If the aspect ratio doesn't change it's very hard to tell an inch difference in a wheel. I.e. a 28" tire on a 15" vs. a 29" tire on a 16".

                        15-17 look fine on a an Abody. I will agree if you exceed that it can often look out of place, but not always.

                        Tire and wheel style and sizing looking correct on a vehicle is very dependent on the whole package. THe best tire and wheel combos fit the over all theme of the car and also fit properly under the car, including the all important but often neglected or misguided ride height
                         
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                        • 69FBCuda

                          69FBCuda Life in the Fast Lane

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                          Measure from the center of the hub to the fender at the lower front, then swing that radius to see the allowable arc.

                          28" tire
                           
                          Last edited: Feb 7, 2020
                        • AJ/FormS

                          AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                          Are you asking me?
                          lol
                          I installed 245/50-15s on my car one summer ( a real bugaboo, I assure you), and when I backed off the drive-on hoist and hit the brakes, sad day, the tires rubbed the front corners. So I drove straight back up the ramps, dug out my tin snips, and adjusted those corners,lol.
                          However, by this time, my car was lowered quite a bit, And I had sucked some caster into her. I think the lowest point of the K was just 5 or 5.25 inches off the asphalt.
                          With the 235s from the previous post, the K was closer to 6 and the 235s never touched those corners, ever.
                          Over time, the K has been adjusted down again, but obviously no contact is being made.
                          In my opinion, you'll have to be a pretty adept and practiced driver to take full advantage of a 235/60-14, in city driving. Most of the time, you don't know what any given corner will have in store for you, as to it's surface. So you'll be slamming around the turns at maybe 50 or 60, perhaps 70 percent, cuz sliding in traffic is highly dangerous. Well if that's true, then you wouldn't actually need a 235 , I guess.
                          But they're a pretty good back-up for when you come in too hot and gotta slam on the brakes. So for that reason they can stay on my car. What I mean is I gotta brake a lot,lol.
                           
                          Last edited: Feb 7, 2020
                        • belmateo

                          belmateo Well-Known Member

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                          This has all tire sizes as well as section width based on tire diameter.

                          Tire Height Chart
                           
                        • famous bob

                          famous bob mopar misfit FABO Gold Member

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                          Dam , this looks just like a new 68 form S 383 fastback , I bought in Tulsa at Tink Wilkersons chry/ply . . Traded it in for a 69 coronet rt the next yr at Jack Clark dodge , main and chartel in okc . Wish I still had the barracuda , they said it lasted 3 hrs, before being sold ------------------
                           
                        • trapster

                          trapster Well-Known Member

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                          I run 215 60/14's on Stock rallyes all day.
                           
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