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Discussion Starter · #1 ·
Hello all - thanks in advance for any help/insight.

I have a 1995 firebird formula. Currently it has 17 inch wheels up front and 18 inch wheels in the back. The previous owner installed these wheels - they are aftermarket corvette wheels. I need new rear tires, so instead of replacing the rear tires, I want to swap out the rear wheels for some 15x10 wheels and sell the other wheels. (I believe the front wheels are 17x8 since 255's fit perfect and the rear wheels are 18x10 since 275's are a little stretched - if it matters).

I have found some guidelines on the web in regards to offset/backspacing. I see for a 10 inch wheel...

Tire outside fender: +32 mm offset = 6.76 in. backspace

Limited backspace front = +50 mm

Tire inside fender: +55 mm offset = 7.67 in. backspace

Limited backspace rear: +62 mm

What I am asking now is that is any offset around 32 mm - 55 mm okay? I would like to do this without having to do any work to fenders or anything.

I have found some wheels with +46.00 mm offset (7.250 in. backspace) are these good to go with? They are 15x10 and race stars brand. They also say the lug nut seat style is "shank" - is that what I want or will I need new lug nuts? or are any lug nuts good as long as they are the 12 x 1.5 ? The wheels are the correct 5 x 4.75 inch bolt pattern.

I am doing this because I would like to get some more tire showing (I do not like the 18 inch wheels in the back - especially with the 275's). So I am going to be getting some mickey thompson sportman st tires if this works out. I do not have many performance parts currently so I am not worried about the T speed rating - I wont be racing the car. I do want to do this modification though, so any insight here would be greatly appreciated.

Also is there any real benefit of 15x10 over 15x8 wheels? Since I am not racing I would not be opposed to putting 15x8 on the rear for now. I will be keeping the front wheels the way they are until those tires need changing. For 8 inch wheels is anything between 15 mm - 38 mm offset a safe bet? This question can be ignored if 15x10's are the way to go.

I am looking into 295's (295/50/R15) for the 15x10 wheels and looking into 275's (275/60/R15) for 15x8 wheels if I do that instead. I see much debate online about whether 275's fitting on 8 inch wide wheels. do they fit or no?

I am sorry if I am leaving some information out - I tried to be as thorough as I could. I hope this will be an easy answer for someone. I know by leaving the front wheels the same on the car I will have different wheels. I do not necessarily like this - but since the front wheels are black with the chrome trim, I am hoping some black wheels on the rear will look okay and keep the car looking good. Is this purely cosmetic? Not really. I would like to have the 15x10's so I can have some drag tires on it in the future. I have exhaust and intake down already, but will not be racing it until I get some more modifications done. Just thought this was a good time to do the tires.

Thank you very much!
 

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I run 15"x10" Weld Pro-Stars, 7.5" backspace (about a 52mm offset). To run those, I needed to grind the fins off the rear brake calipers. Not all 15" wheels will even work in the back. See photo in signature. You probably don't want the 28x10.5-15(W) tires though. That backspace puts the outer edge of the tire just under lip of the fender. Using a narrower tire like a 275 or 295 will move the outer edge of the tire inward a bit.

The acceptable wheel width for any tire is set by the tire manufacturer. Search the tire sizes you are considering on a site like Tire Rack, and click on the "specs" for each tire to see what the manufacturer recommends for wheel width.

Lug type is determined by the wheel design. Shank style lugs are used on many drag wheels. If the wheel you are looking at says you need shank style, you need new lugs. If the shanks are long enough, you may also need longer studs, like I run.

http://www.camaroz28.com/forums/members/injuneer-1133-albums-injuneer-s-94-formula-1580-picture-clearance-et-drag-28x10-5w-15-6362/

http://www.camaroz28.com/forums/members/injuneer-1133-albums-injuneer-s-94-formula-1580-picture-caliper-grind-15-pro-stars-6372/
 

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Hello thanks for the assistance, I LOVE your setup! - I wish I was doing that!

Did you have to grind the fins off because the wheel was 15 inches diameter? I am just wondering if the 7.25 in. backspace on the wheel I am considering will help me avoid having to grind the fins off or not. To me, it does not look like the fins had to be grind because of the wheel diameter, but I really do not know - I just wanted to ask.

If I do a wheel that is 10 inches wide - I will be okay doing a 275 or 295 tire then?

Does my problem see to be in the 10 inches wide or the 15 inches diameter? Which one do I change to avoid having to do anything like that?

edit: I see online that some people say 93-97 f bodies are okay with 15's. But some people say they do not, I am assuming it depends on the wheel since you have a 94 and had to do some grinding. Will a 15x8 with a 25 mm offset be better here?

Thank you.
 

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The brake fins hit the center disc of the wheel. It will hit no matter what the width or offset is. This is specific to the Welds, both Pro-Star and Draglite. Summit and JEG's sell one-piece cast wheels that look just like the Pro-Stars, but I think they clear the brake fins. In the front I have to run a 5/16" spacer so the center disc of the wheel doesn't hit the spring clips on the brake pads. And that's a 15x3.5" wheel.
 

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Discussion Starter · #5 ·
Thank you! Really appreciate the quick help. Now I just have to decide what brand to go with - I really dont mind grinding if I have to. I will make sure to post in here after I am done so other people can benefit from this. Again, Thanks to Injuneer for the help!
 
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