Help with backspacing on 71 Firebird...
 

Looking to run 18x10 rear 18x9 front. The rear is mini tubbed and running Ridetech level 2 all around. I need to figure out back spacing and offset. The tires look like they'll be 285/40/18 and 245/40/18. Any help would be appreciated.

don't wanna run 11's in the rear?

I'd like to, but it doesn't appear they'd fit.

Here's a 275/40/17 on the car now:

http://imagizer.imageshack.us/v2/640...661/ipS0gX.jpg

Also having a hard time finding a tire size for an 11" rim that doesn't have too little sidewall.

What size wheel size is on there now?
How much room do you have between the tire and the outer wheel house?
How much room do you have between the tire and the inner well and suspension components?

On link suspended cars I like to shoot for 1/2" minimum to the outer, 1" minimum to the inner wheel house and 1/2" to axle mounted suspension components.

The wheel that's on there now is a 17x10 if I remember correctly. It has 5.5" backspacing and the tire is a 275/40/17. From the surface of the disc to the wheel lip looks to be 4 5/16". I can roll the lip and get a little bit there. From the front surface of the disc to that lower mount for the coilover appears to be 7". Thanks for the help.

No problem.

On the 2nd gens the wheel lip isn't the problem. It's the outer wheel house above it that gets on the way. If your outer is fine you can go in more.

Since you have a wheel and tire on there that you know the specs on, use them to figure your backspace from there. Find your distances from the tire / wheel to the parts mentioned. One important dim is the distance between the outer tire to the outer wheel house.

Going from 10 to 11 you will just have to increase the back space to 6.5". Since the axle mounted parts move with the tire you can get away with it being close. 1/2" should be fine.

Thanks for your help! The tire just barely touches the lip only on the drivers side on bigger dips in the road. It appears as though just tapping it up a bit or slightly rolling it would take care of that.

I might be mistaken, but I thought I read somewhere that on an 11" rim, the tire would be about 12" wide. I'm assuming this means sticking out about 1/2" on each side of the rim. What's got me worried is that adding the 2 measurements I got from the surface of the disc, I get 11 5/16". It just doesn't appear that a 12" wide tire will fit in there.

Others running my combo seem to be sticking 335/30's back there and even Ridetech said it was possible, I just don't see how.

FWIW I'm running 255/45/18s on 18x8 front and 295/40/18s on 18x10 rear with stock tubs. With minitubs I'd think you'd be able to put a lot more tire in the rear.

The tub isn't the problem any more....it seems that lower coil over bracket as pictured above is.

I would call ride tech and ask about your spring perches,,, the shock looks to be laid way over to the outboard side in the photo... Maybe just the angle but my 70 Formula looks like this....
http://i997.photobucket.com/albums/a...psf2858861.jpg