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· Registered
749 Posts
Discussion Starter · #1 ·
:WTF :laughing: Yeah, I know it's silly, but I've been called worse.

I bought a set of four GS replica wheels on e-bay from OE wheel. WOOHOO!!! That's
the best $500 I have ever spent! And what a deal!! Well..I guess if I was replacing a wheel
on an actual GS I'd be pretty pissed but that's neither here nor in my driveway...but I'm happy
as a clam! {maybe even a clam-ray!}. { :huh: }
Anyway, I went with 9.5" all the way
around because I have three very nice 275/40 Goodyears that need to be "used" and I was pretty
sure they wouldn't fit the 11's. I also picked up two free 245/45 EMT's on Craig's list earlier and planned on
using them on the front. Even though they came off a car with an alignment problem, they're both
feathered on the inside edges and one is patched, they have 40% tread on the other 80% of the tire so
they'll do for awhile..beggars cannot be choosers. {although they probably shoudn't drive 'vette's either}

The car looks way better than I had imagined and I am STOKED.
I can't wait to lower it now. I really like the way the 245's look on a 9.5" wheel and where the
fender lip will be in relation when it's lowered. I want to replicate that, on a larger scale, on the
rear using the 11" wheels...albeit someday. Hence the questions below.
Here are some pics for reference...

Below are some figures and calculations for my "Someday, when the wife isn't looking and the Goodyear's are bald"
plan that I'm working on..see if it makes any sense at all.

1988 Z-51/52 Wheel/Tire

Front Wheel: 17"x9.5" 56mm Offset 7.4" Backspace
Front Tire: P275/40ZR-17

Rear Wheel: 17"x9.5" 56mm Offset 7.4" Backspace
Rear Tire: P275/40ZR-17

1996 Grand Sport {Coupe} Wheel/Tire

Front Wheel: 17"x9.5" 56mm Offset 7.4" Backspace
Front Tire: P275/40ZR-17

Rear Wheel: 17"x11" 50mm Offset 7.93" Backspace
Rear Tire: P315/35ZR-17

My Car at Present {88 Z-52 Coupe}

Front Wheel: 17"x9.5" 56mm Offset 7.4" Backspace
Front Tire: P245/45ZR-17

Rear Wheel: 17"x9.5" 56mm Offset 7.4" Backspace
Rear Tire: P275/40ZR-17

Wheel Widths:

11" {279.4mm} Overall Width: 11.5" {292mm}
9.5" {241.3mm} Overall Width: 10" {254mm}

Backspace in Millimeters:

7.93"= 201.42mm
7.4"= 187.96mm

Offset in Inches:


Sidewall Height:

275/40 > 110mm {40% of 275mm}
315/35 > 110.25mm {35% of 315mm}
245/45 > 110.25mm {45% of 245mm}
285/40 > 114mm {40% of 285mm}

Conclusion based on known facts and a few wild assumptions:

Rear tires should be 285/40 on 11" wheel to approximate the "look" of 245/45 on
9.5" wheel, the only difference being that the 285/40's would have a 4mm taller sidewall and be 8mm
taller overall.

11" GS wheel has roughly 1/2" more backspace than stock 9.5" wheel, therefore the remaining
1" additional width is on the outboard side of the wheel.


Are my calculations correct in regards to the "look" of a 285/40 tire on an 11" wheel?
Are they also correct about where the extra width of the 11" wheel is "going"?
Are there any drawbacks to mounting tires on wheels that are wider than nominal, for the tire? Beside the
obvious curb issues since the tire isn't protecting the wheel, I'm wondering about the bead
failing when flat. Not that I,d run them that way, but it would seem to be an issue. And finally,
will this combo "fit" inside {sorta kinda} the lip of the rear fender? I think they will.

I could answer the last myself by hanging a plumb off the rear fender and measuring to the
existing wheel....but that would be assuming my assumption about the 11" wheel is correct.

Any input?

P.S: I went with machined lip, chrome nuts, stems and caps 'cuz I know how some of you feel about 'em..and that's just the kinda guy I am. Yeah, I know they look like Hell, but it's worth it.

{Insert big fat smiley face here}


· Registered
134 Posts
The black wheels look great!:thumbsup:

How did you work out the backspacings?
I calculate 6.95" backspace for the 9.5" wide wheels, and 7.47" for the 11" wide wheels - either way, there's around 1/2" difference and therefore the 11" rear wheels will stick out 1" further than your 9.5" rears wheel do presently.

I agree a 285/40 on an 11" wide wheel will replicate the look you have on the front.
I would just check with a tire shop to see if it's an acceptable fitment. The gut-feel-o-meter says they're too narrow (front and back) but then I'm no tire expert.

· Registered
749 Posts
Discussion Starter · #3 · (Edited)
Thanks for the compliment Thunder Kiss, and thanks even more for checking my calculations. Your 86 is lookin mighty nice itself! :thumbsup:

I pulled those backspace figures from another website and it looks like there's a good chance they're wrong, since yours came up a little different. I guess the good news is that the results were same, 1" outboard difference from the 9.5" wheel and I think they'll just barely fit the way I want them too.
I took everything to a respected local shop to have them mounted and the kid doing the work did say the tire looked a little narrow for the wheel which prompted a "look in the book" to find that an 8" wheel is "nominal" for the 245. After consulting with a superior, we proceeded as if there was no consequence. {?}

My exact calculation for duplicating the "look" actually called for a 283mm tire, and would have 110mm sidewall. Given that the 285/40 is 2mm wider {than 283} and has a 4mm taller sidewall than 245/45, means that the sidewall in the rear shouldn't have quite as much "lean" as it does in the front.

I really like the way it would look, my trips to 150mph are non-existent and I certainly don't plan on running them flat.....but I would need them to stay on the rim if disaster should happen to strike.

Still happy to have more input.


· Registered
749 Posts
Discussion Starter · #5 ·
Thanks Chuck :thumbsup:

I think I'll try to get around to lowering it over this combo here pretty soon, {:laughing: "soon" is a relative thing, grasshoppa.} see how it looks and go from there. The fronts are gonna wear out quick, even though my adjustments are good and I'm running them slightly over-inflated to try and move the wear towards the center, so that gives me a bit to make up my mind before I have to buy new ones.


Now I need a big can of red paint..........and a small can of black paint....and a little charcoal gray. :D
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