Tire size 68 firebird

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Ok whats the deal here? Ive been searching over hundreds of threads to find a perfect and biggest tire combo I could find for my 68 firebird. I definitely know I want 18 and 19 combo. My problem is I just measured my 275/60-15 that I currently have on back and my measurement doesnt jive with manufacture specs. I have 32 psi and measured 26 7/8" or 26.87 from flat surface ground to top of tire with a straight edge. Man. specs are 27.99" thats over an inch difference?? So we must assume the tire mounted with the weight on it will lose an inch for a real world measurement? I just answered my own question right!!!! So we have to look at loosing and inch to manufactures specs?

[68Formula]

It's going to be a combination of sidewall stiffness, weight, and tire pressure.

Shorter sidewalls tend to be stiffer, so I would expect less loss going to a 18 or 19" combo with the equivalent unmounted tire height. But again, how the tire is built will also affect the stiffness, so you can't even assume tire manufacturer to tire manufacturer the same result for the same size.

[SR71]

a lot of tire companies are publishing static loaded radius measurements these days. this will let you compare how much diameter you lose with the vehicle weight added from one brand to another and one size to another.

I'm not a good enough typist to explain SLR anytime soon. look it up and it will help answer your question.

[Tom Welch]

i have 17" tires all around on my 67 bird. 245 in front 275 in rear, tucks in just fine with no body mods. every tire is going to be a little different. measure the ones you want to use. most tire shops will work wth you if you are serious.

[john5670]

Max tire width for 18" rims on a stock F-body if you want the tire to fit in the wheel well is:

Rear: 255 - 9 inch rim
Front 225 - 8 inch rim

If you try anything bigger you will run the risk of having problems. Before you order rims take measurements because every car is different. I contemplated wider tires for a long time. I read that some people can fit larger tires, but after installing the rims and tires I purchased...... I could have gone no bigger.

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Thanks for the reply> I plan on staggering 18/19 combo. I have a 60.5" rear end. I plan on using stage 3 air ride system and I want to get the ride height as low as I can go and slammit at shows. Whats the biggest tire size and back spacing I could roughly go> i will make measuremnts when I am ready to buy, I just want a close figure. Thanks in advance. Derek

[Tom Welch]

check out Carl Cassanova's camaro if you dont believe it. The wheel offset makes all the difference, I measured many times before purchasing my 9.5" rear wheels and I have never had a rub or scrape. The front rubs a tiny bit, so a modified bump stop fixed that. It works if you plan and measure carefully.

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Thanks tom!

[JustinB]

You can safely get away with a 8" wide front and 9" wide rear. Tire width would be a 245 front and 275 rear. We would adjust the aspect ratio to suit the wheel diameter. The problem you are going to run into with the 19" rear wheel is they are only available in a 8 and 10" wide hoop in two piece billet. A 19x10 is a very tough fit with rubbing on a 67-68 F-Body. Feel free to call us if you need any help.

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Thanks justin. when I am ready to buy I will call u for sure!

[doribak]

so just to understand, on stock first gen firebird (mine is a 69) i can fit 275/40/18 9" on the back?

[bruiser]

Ive got 255/55/18 on my backs on a 9" wheel. They fit real snug and cover the whole wheel well.

[JustinB]

so just to understand, on stock first gen firebird (mine is a 69) i can fit 275/40/18 9" on the back?

Yes, as long as your backspacing is correct.

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You can safely get away with a 8" wide front and 9" wide rear. Tire width would be a 245 front and 275 rear. We would adjust the aspect ratio to suit the wheel diameter. The problem you are going to run into with the 19" rear wheel is they are only available in a 8 and 10" wide hoop in two piece billet. A 19x10 is a very tough fit with rubbing on a 67-68 F-Body. Feel free to call us if you need any help.

Maybe ill go with 17/18 combo instead do to better wheel size availability. Going from 15" will be a big improvement.

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Ive got 255/55/18 on my backs on a 9" wheel. They fit real snug and cover the whole wheel well.

Those are 29" tall and 10" wide! How low are you? Do you have any pics?