'68 Firebird 18" Wheel Backspace and Tire Size Advice Needed

[projectfolly]

I need some advice with regard to wheel width, backspacing and tire size, since I don’t want to buy stuff twice. I seem to have a bad habit of picking the wrong combo of parts!<o:p></o:p>
I’m at the point on my ’68 Firebird project where I need to pull the trigger on wheels/tires.<o:p></o:p>
Yes, this topic has been covered in many threads, but in my experience, every car is a little different, and merits due diligence when picking the right combo.

The Firebird has stock rear wheel tubs, Detroit Speed front control arms, 2” drop front springs and 3” drop rear springs. I’m also usingAFX front spindles which supposedly have a 1” drop built in. The ride height shown in the front, was arrived at by using spring spacers under the DSE springs. Sits about level front to rear, but hasn’t settled yet and I still have to install interior, exhaust, fuel, etc, so I’m sure it’ll sit a little lower once compete, with a full tank of fuel and a few miles.<o:p></o:p>
The Pontiac Rally IIs shown in the pic are 15 x 7 with P235-60-15 Pirellis. These will not work with my brakes, which are stock C5Corvette, which drives the need for 18” wheels. <o:p></o:p>
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In keeping with the Pontiac theme, my wheel choice is 18” USMag Bandits. <o:p></o:p>
From what I’ve gathered on all the great threads here, is that 18 x 8’s will work on the front, and 18 x 9s will work on the rear, even with stock wheel tubs.

I’m still not sure what backspacing will work. My initial impressions are 4.5 on the front and 5 on the rear. Comments? Suggestions?

Tires. Not sure what size to go with at all. My requirements are no rubbing, and given the crappy roads here in southeast Michigan, I’d like to run a tire with a little sidewall for those times when traffic forces me into a chuckhole.<o:p></o:p>
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Any help, ideas, info would be greatly appreciated!<o:p></o:p>
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[67rally]

In general (every car is different), the widest you can go is 245/40/18 up front and 275/40/18 rear. 18x8 w/4.75" bs and 18x9 w/5.25" bs (or 18x9.5 w/5.5" bs) will center the wheel in the wheel well giving the most clearance between the tire/suspension and tire/fender.

The bs you listed will give you an extra 1/4 of suspension/wheel well clearance, but will push the tire out towards the fender/quarter. If that's the case, you may want to drop the tire sizes down to 235/40/18 and 265/40/18. Of course the narrower the tire you choose, the more clearance you'll pick up all around.

[TheJDMan]

Do you know a friend that has some 18" wheels that you can borrow for test fitting? That would allow you to do some measuring to see what would work best for your build. Just FYI, I'm running 275/35r18 up front mounted on 18x10 with 6.6" bs. These tires fit without any fender or inner fender modification. Now that wheel will likely not work with your suspension, I'm running a DSE subframe but I think with careful measurement you can fit a 18x9 wheel.

[rickpaw]

I'm running similar suspension set up (SPC arms/AFX spindles)/leaf springs rear, stock subframe. I have 18x8 up front with 5" BS; and 18x9 rear with 5.375" BS. I'm running a bit more BS than most, since I want to be able to adjust the BS with spacers. Tires are 245/40/18 front and 275/35/18 rear.

Here's a picture of the car.

https://static1.pt-content.com/images/pt/2014/08/DSC_0969_zps92b3846e-1.jpg (http://s616.photobucket.com/user/rickpaw/media/67%20Firebird%20LS%20swap/DSC_0969_zps92b3846e.jpg.html)

I ran 17x8 wheels with 4.5" BS before and the tire would sit flush with the fender lips, and occasionally rub.

[projectfolly]

Thanks for the input guys. This is a great help and your cars all look awesome!
Unfortunately, I don’t know anyone with 18” wheels (with the same bolt pattern) I could borrow. Pity, test fitting would be the way to go.
I was wrong in referring to the wheels I picked as “Bandits”, actually they are of the US Mag Retro series, Trans Am – U429.
I priced a set of today and they are about $4100 + shipping, with a 6 to 8 week lead time.

Looks like I’ll be moving to my next choice, ET Team 3 Minilite style wheels.

[xavier296]

For what it is worth, I bought a set of wheels/tires for my 68 'maro that did not work. I spent a lot of time researching sizes and seeing all the cars running 9 inch front rims and thought I would be conservative. 18X8 with 4.75 BS (3.25 Front space). My car is lower than yours, but even adjusted all the way up, the 245s hit the stock fenders. My brakes didn't add any extra width, but 3.25 out was too much. I ended up with 18X7 with 4.5 BS (2.5 Front space), 225 tires. I needed the wheel to be inset more.

On the back, I initially bought 18X9 with 5.25 BS, 275 tire. These did not fit with unmodified rear wheel lips. I left the springs off the coil overs and cycled the suspension up all the way and hit in many places. I ended up running the 18X8s above in the rear, and they fit well. I have since modified the rear wheel lips by removing 5/8", and can now run the 275s and the 9 inch rim.

I say all of this to make a point: I thought everyone else's experience would mirror mine, especially if they had the same suspension and year car. I was wrong. And now I have two extra wheels and tires than I wanted to buy! I wish you were closer, I would let you try all mine out to your hearts content.

For anyone also researching, this is a 68 with full ridetech suspension, stock subframe, stock width rear end, and TRSD 4 wheel disc brake kit. Wheels are Rocket Booster Hyper Shots.

[csouth]

For what it is worth, I bought a set of wheels/tires for my 68 'maro that did not work. I spent a lot of time researching sizes and seeing all the cars running 9 inch front rims and thought I would be conservative. 18X8 with 4.75 BS (3.25 Front space). My car is lower than yours, but even adjusted all the way up, the 245s hit the stock fenders. My brakes didn't add any extra width, but 3.25 out was too much. I ended up with 18X7 with 4.5 BS (2.5 Front space), 225 tires. I needed the wheel to be inset more.

On the back, I initially bought 18X9 with 5.25 BS, 275 tire. These did not fit with unmodified rear wheel lips. I left the springs off the coil overs and cycled the suspension up all the way and hit in many places. I ended up running the 18X8s above in the rear, and they fit well. I have since modified the rear wheel lips by removing 5/8", and can now run the 275s and the 9 inch rim.

I say all of this to make a point: I thought everyone else's experience would mirror mine, especially if they had the same suspension and year car. I was wrong. And now I have two extra wheels and tires than I wanted to buy! I wish you were closer, I would let you try all mine out to your hearts content.

For anyone also researching, this is a 68 with full ridetech suspension, stock subframe, stock width rear end, and TRSD 4 wheel disc brake kit. Wheels are Rocket Booster Hyper Shots.

I don't own and F Body, but a 7" wheel seems narrow even for a stock suspension. I will say you are correct that all of these older cars are definitely different.

OP, Personally I would rent this tool from Rich

https://www.pro-touring.com/threads/100546-Wheel-Works-Wheel-Fit-Tool-Ultimate-kit-for-rent-65-00?highlight=rent

[projectfolly]

108917
For what it is worth, I bought a set of wheels/tires for my 68 'maro that did not work. I spent a lot of time researching sizes and seeing all the cars running 9 inch front rims and thought I would be conservative. 18X8 with 4.75 BS (3.25 Front space). My car is lower than yours, but even adjusted all the way up, the 245s hit the stock fenders. My brakes didn't add any extra width, but 3.25 out was too much. I ended up with 18X7 with 4.5 BS (2.5 Front space), 225 tires. I needed the wheel to be inset more.

On the back, I initially bought 18X9 with 5.25 BS, 275 tire. These did not fit with unmodified rear wheel lips. I left the springs off the coil overs and cycled the suspension up all the way and hit in many places. I ended up running the 18X8s above in the rear, and they fit well. I have since modified the rear wheel lips by removing 5/8", and can now run the 275s and the 9 inch rim.

I say all of this to make a point: I thought everyone else's experience would mirror mine, especially if they had the same suspension and year car. I was wrong. And now I have two extra wheels and tires than I wanted to buy! I wish you were closer, I would let you try all mine out to your hearts content.

For anyone also researching, this is a 68 with full ridetech suspension, stock subframe, stock width rear end, and TRSD 4 wheel disc brake kit. Wheels are Rocket Booster Hyper Shots.

Thanks for sharing your experience. You bring up a good point with regard to cars all being different. We all know that when our cars were built, build tolerance was no where near what it is today. Add onto that 40 + years of abuse, repairs, subframe shifting, parts swapping, and we have a moving target.
In the case of my car, the siutation is compounded by the installation of repro quarters, outer wheel houses and tail panel all of which, are of questionable quality. I'd be willing to bet that the dimension of the quarter panel wheel openings differ from side to side significantly.
Thanks for the offer to try out your wheels/tires, if I was closer, I'd take you up on it for sure!

Chris, thanks for the recommendation to use Rich's wheel/tire tool. Thats probably the smartest thing I can do at this point. I looked into buying a similar tool made by Percy, but it had very poor reviews.

[First Bird]

Your Firebird is awesome. You have a lot of nice work into that car.
On my 1968 Firebird I have 18x8.5 and 18x10 wheels. Stock front and rear suspension. C5 brakes front and rear.
245/40 and 275/35 tires. The Percy tool worked fine for me. It's flimsy but did the job very well. I liked how I could bend the wire to simulate the tire.
I haven't included backspacing since you want to nail that, for your car.
Good luck

[pontiac455]

I run set Boze wheels on my 68 firebird in front are 18 x 8 with 4.7/8 BS with 2" drop QA1 adjustable spring and globe west upper and lower control arm. And the back are 18 x 9 with 5.5 BS non tube stock suspension, the only thing my rear end is 9" ford it is a 1/2" shorter on each side, then the original 8.2 rear end. I did buy a measure tool it was very helpful to me, b/c my wheel were custom built. My tires in front are 235/40 and back are 255/40. I went with smaller tires all way around but could went bigger in back like a 275/40 i was scare and i have 2" drop in back [A109170109171109172109173109174

[rickpaw]

Nice looking birds guys.

[projectfolly]

Yes indeed, great looking 'birds!
Thanks for posting the pics and the advice.
Great wheel and tire combos too. Gets me to re-thinking my wheel choice.

Pontiac455: the "Pontiac Crest" center caps on the Bose wheels are very well done. Were they custom?

With all the help, I've been able to narrow down the choices, however, just to be sure I pulled the trigger on the Year One wheel/tire tool. I know a member here rents his out, but I decided it'd be a worthwhile investment, since I've put myself in this situation before and more than likely I will again on other projects.
More to follow...

[mikewoods]

Great news! Can't wait to see what you come up with. Where in Metro Detroit are you located? I work in Auburn Hills.

[pontiac455]

Pontiac455 The wheel were custom built by Boze wheel out of California and the Pontiac Crest was engrave by them

[projectfolly]

Great news! Can't wait to see what you come up with. Where in Metro Detroit are you located? I work in Auburn Hills.

Hello!
I live and work in Warren.

[projectfolly]

Right. As mentioned, I bought the Wheel Works tool.
Based on recommendations/experiences (shared here) in reference to tire size, I bought 2 Michelin Pilot Super Sports to experiment with, (1) 235-40-18 to try on the front, and 275-35-18 for the back.

I'm still playing with different offsets, but it looks like the 235-40-18s will work on the front without a problem using a back space of 4.5 to 4.75, however, even with the weight of the car, I wasn't able to get the suspension to compress much at all, so more work will have to be done there, maybe pull the shock out.

For the rear, I don't think the 275s will fit without wheel well lip modifications as I have stock wheel lips. I pushed it inboard as far as I could, down to about a 1/4 clearance from tire side wall to inner wheel well, and it looks like the tire will easily hit the wheel well/quarter panel outer lip when suspension is in full compression. Yep, Xavier296, you warned me of this, but I'm a slow learner!

Thinking about trying a 255-40-18 for the rear and see how it fits. More to follow.

In the meantime, more project pics...

[projectfolly]

Hello All!
Thought I’d close the loop on this thread, and share what I ended up putting on the Firebird.

ET Team III LT III wheels:
Front: 18 x 8 Backspacing 4.75
Rear: 18 x 9 Backspacing 5.25

Tires:
Front: 235-40-18
Rear: 255-40-18

Yep, by some measures, conservative sizing, however, the car has stock wheel tubs and wheel lip openings.

Finally getting into the final stages of the project, but waiting on a few parts. Driving it short distances to root out any gremlins that pop up. With respect to the wheels/tires, no rubbing!
Thanks to all that provided input!

[Jk918]

Looks great very informative info in this thread

[rickpaw]

Wheels are looking great. Any close up pictures?

[projectfolly]

Thanks guys!
As requested, shots a little closer.

[Zyman]

What is your backspacing with the 245-40-18 front and 275-35-18 rears?

[Zyman]

Rickpaw - What back spacing are you running on the 18x8 front and 18x9 rear. Are you running a 275-35-18 rear or 275-40-18, the 35 aspect is about 0.2 inches shorter than your 245-40-18 front.

[projectfolly]

What is your backspacing with the 245-40-18 front and 275-35-18 rears?

Hello, Sorry for the slow response. I haven't been on here in a while.

Anyway, to answer your questions:

Front: 18 x 8 Backspacing 4.75
Rear: 18 x 9 Backspacing 5.25

If I'm honest, for the rear backspacing, I think I should have gone inboard a little more. When I have my wife and kids in the car as well as a full tank of fuel, when crossing intersections with a lot of crown or dips. I will get slight rubbing on rear outer sidewalls of the tires.
No issues with the front.


Tires:
Front: 235-40-18
Rear: 255-40-18