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Discussion Starter #1
I hope all are well and keeping safe.

After much research and thought, I ordered some new sox and shoes for my '66 coupe; now I'm having some doubts.

The car has been lowered, I think the prev owner said 1/2", but I could be wrong (hafta check the paperwork). The wheel openings and wells are stock, no mods. It's currently riding on 245/60/15 (15x8) in back and 225/60/15 (15x7 I think) in front.

I ordered Cragar 390C Pro Street wheels, 15x8 w/ -6mm offset and 4.25 in backspace for the rear and 15x7 w/ -13mm offset and 3.50 in backspace for the front (from Cragar website). I think they will fit fine, but my tire choic may not; 255/60/15 rear and 235/60/15 front.

So, whatya say?

Jim
 

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The optimum backspacing for 15x8 wheels is 4.5" (0 offset) and for 15x7's it's 4.25" (+6 offset). So, with the tire sizes you specified the rears will be about 5/8" further out than optimum and the fronts even worse at about 3/4". Both those specs are highly likely (read damned near certain) to cause interference issues, especially on a '66 that's been lowered to any extent. (Those front wheels you ordered are a particularly poor fitment.) Sorry for the bad news.

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What 66kcar said . No way.
 

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1965 2+2 Vintage Burgundy A-code C4
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Nope, 245/60-15s fit on the rear of mine on 15x7 rims with 4.25" backspacing. With 15x8 you will be all into the fender/fender lip with a stock width rear.
 

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Discussion Starter #6
Hey, no worries as everything is returnable! A hassle yes, but doable.

So as it appears I have ‘screwed the pooch’ with regard to wheel parameters, they will be on their way back as soon as they get here and I load em up in my car! :cool:

Since there is no clearance/rub issues w/ the present wheel & tire combo in front or back, would the bump in tire size from 245 to 255 rear & 225 to 235 up front be doable? Iirc, the rears would be .4” taller and wider and the front .4” wider and .5” taller. If I must, or should, I will exchange em, but if not....

Anyway, no gain without pain, right?!?
 

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Hey, no worries as everything is returnable! A hassle yes, but doable.

So as it appears I have ‘screwed the pooch’ with regard to wheel parameters, they will be on their way back as soon as they get here and I load em up in my car! :cool:

Since there is no clearance/rub issues w/ the present wheel & tire combo in front or back, would the bump in tire size from 245 to 255 rear & 225 to 235 up front be doable? Iirc, the rears would be .4” taller and wider and the front .4” wider and .5” taller. If I must, or should, I will exchange em, but if not....

Anyway, no gain without pain, right?!?
probably not so much
 

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Discussion Starter #8
Nope, 245/60-15s fit on the rear of mine on 15x7 rims with 4.25" backspacing. With 15x8 you will be all into the fender/fender lip with a stock width rear.
Based on that, my present wheels MIGHT be 7” then, as the 245’s fit well within the wells, yes? I guess I really should pull one off and measure, I ASSUMED they were 8’s.

35+ years ago I don’t recall having so much trouble figuring out rims n tires...been outta the game too long I guess.

Forgive my “20 questions” routine, but regardless of rim width, I MUST look for 4.5” backspace for the rear and 4.25 for the front, yes? How about offset?

Again, sincere thanks for all your input, thoughts and recommendations.

Jim
 

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Forgive my “20 questions” routine, but regardless of rim width, I MUST look for 4.5” backspace for the rear and 4.25 for the front, yes? How about offset?
Back space and offset are just two different ways of saying the same thing.
 

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Discussion Starter #10
Back space and offset are just two different ways of saying the same thing.
Watched a few vids explaining back space and offset, so I understand the concepts; too much backspace and you’re hitting the suspension, but you COULD go with less than the maximum depending on the wheel’s overall width and offset of the hub, correct?

Not trying to make a silk purse outta sow’s ear, just a low learner I guess, but would a15x7 wheel w/ 3.5” bs and -13mm offset work in either or both front and rear?:unsure:
 

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Watched a few vids explaining back space and offset, so I understand the concepts; too much backspace and you’re hitting the suspension, but you COULD go with less than the maximum depending on the wheel’s overall width and offset of the hub, correct?

Not trying to make a silk purse outta sow’s ear, just a low learner I guess, but would a15x7 wheel w/ 3.5” bs and -13mm offset work in either or both front and rear?:unsure:
It's not only backspacing/offset that's critical, it's the relation of it to the wheel width, and then tire size on top of that. A 15x7 with only 3.5" b.s. on the fronts would require a rather narrow tire in order to fit, not the direction you obviously want to go. There's good reason why you would rarely (if ever) see Cragar 390's on an old mustang, although on the rear you could fit 225/60's no problem with the 15x8's you ordered. The major issue is on the fronts. I think @2nd 66 knows the 65/66 specs by heart, I have to look them up. :)
 

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Discussion Starter #12
Yikes, what a mess!
Okay, thanks for sticking with me on this, VERY much appreciated.
I’ve found another candidate; Vision 147, 15x8, 0 offset, 4.5“ backspace...so this should be good for the rear, no? And for the front a 15x7, +6mm offset, 4.25“ backspace. They are exactly what is listed on the chart posted by “4ocious” in Post #3, but there are no offsets on the chart; is +6 on the front an issue? That said, I BELIEVE these are the same sizes presently on the car, so 225/60 front and 245/60 rear should be okay....right? 😬
 

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Yikes, what a mess!
Okay, thanks for sticking with me on this, VERY much appreciated.
I’ve found another candidate; Vision 147, 15x8, 0 offset, 4.5“ backspace...so this should be good for the rear, no? And for the front a 15x7, +6mm offset, 4.25“ backspace. They are exactly what is listed on the chart posted by “4ocious” in Post #3, but there are no offsets on the chart; is +6 on the front an issue? That said, I BELIEVE these are the same sizes presently on the car, so 225/60 front and 245/60 rear should be okay....right? 😬
Again, backspacing and offset are just two different ways of expressing the exact same thing. Go by either method, doesn't matter as long as the same wheel is being measured. (Most go by backspace around here.) By the chart, a 245/60/15 on a 15x8 with 4.5" backspacing is pushing the envelope a little for the rears. However, depending on ride height, spring stiffness, and/or rolling the fenders, you might get away with it. 225/60/15's on 15x7 with 4.25" bs should fit a factory fresh stock ride height car, but if the front is lowered a bit (Arning/Shelby Drop, cut coils, age etc.) from stock, you might need to dial in some negative camber or perhaps roll the front fenders for clearance as well. I've seen repeated recommendations from folks here for 215 or 205 width tires on 65/66 performance oriented front ends.
 

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Conceptually, think of a 7" wide wheel with 4" backspace. Now add an 1" width all to the outside. The backspace remains the same but the offset changes.
This might help:

763272
 
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Conceptually, think of a 7" wide wheel with 4" backspace. Now add an 1" width all to the outside. The backspace remains the same but the offset changes.
For the OP's purposes, all that needs to be known is that an 8" wide wheel with 4.5" backspacing will always have a 0 (zero) offset, and a 7" wide wheel with 4.25" backspacing will always have a +6mm offset. :)(y)
 

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Discussion Starter #16
Conceptually, think of a 7" wide wheel with 4" backspace. Now add an 1" width all to the outside. The backspace remains the same but the offset changes.
This might help:

View attachment 763272

Again, backspacing and offset are just two different ways of expressing the exact same thing. Go by either method, doesn't matter as long as the same wheel is being measured. (Most go by backspace around here.) By the chart, a 245/60/15 on a 15x8 with 4.5" backspacing is pushing the envelope a little for the rears. However, depending on ride height, spring stiffness, and/or rolling the fenders, you might get away with it. 225/60/15's on 15x7 with 4.25" bs should fit a factory fresh stock ride height car, but if the front is lowered a bit (Arning/Shelby Drop, cut coils, age etc.) from stock, you might need to dial in some negative camber or perhaps roll the front fenders for clearance as well. I've seen repeated recommendations from folks here for 215 or 205 width tires on 65/66 performance oriented front ends.
Thanks “CJM“, very useful info.

“4ocious” makes a valid point re: recommendations seen on this and other forums for ‘65/6 tire sizing. As the car DOES wear 245/60‘s & 225/60’s presently with what looks to me as plenty of room in the wells above the tires and towards the the fender, maybe it has NOT been lowered at all (gotta find that build info)? I need to take some actual measures and then wipe the slate clean by dragging all this ****e to FedEx!!:rolleyes:...good thing I’m mostly retired.😉

Have a great evening folks, pick up tmrw!
Jim
 

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So to recap,
15x7x4.25 squared with 225/60/15
if you want
15x8x4.5 or better yet 4.75 in the rear only with 235/60/15 or MAYBE(depending on the tire) 245/60/15
 

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Must you really have 15" wheels? I love the look, but current tire options are limited the future is bleak for anything sub 17". If it's 15's or nothing, rock on!
 

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I run 17x8's with a 245 square and had to pull/roll the quarters and fenders out quite a bit with a roller also lowered. If you don't have paint on the car you care about you can make anything fit with some creative rolling/trimming/positive camber.
 

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Discussion Starter #20
Must you really have 15" wheels? I love the look, but current tire options are limited the future is bleak for anything sub 17". If it's 15's or nothing, rock on!
I run 17x8's with a 245 square and had to pull/roll the quarters and fenders out quite a bit with a roller also lowered. If you don't have paint on the car you care about you can make anything fit with some creative rolling/trimming/positive camber.

Maybe on a new build I’d do 17’s, but this car is done and I don’t wanna get into any mods, just change the look to more my liking, so 15’s it is.
I do believe the rears have been rolled a bit.
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