C5 wheels on 70 ElCamino, backspacing?

[oobus]

Well, I picked up a perfect set of 00 thin spokes 17X8.5 w245/45/17's 56mm backspace and 18X9.5 w275/40/18's 63mm backspace for the rear.
Now, from what I can tell, to get 4.25 backspacing on the front, I'll need 2.55" adapters, and to get 5.25 on the back I'll need 2.35" adapters. Car is lowered 2" front and back, stock suspension for now, but ATX uppers and spindles for the front next year, as well as new upper and lower arms for the rear next year. I'll also be running C5 brakes front and 02 Camaro disc's for the rear.
When these tires go, I'd like to put some 255's on the front and go another size wider on the back.
There is not a lot of A bodies running this set of wheels, and KAA is the only one running C5 wheels on an Elky (but he has different offset wheels). Does this look like a good ballpark starting point? Also, exactly how did you mock up your wheels? Using wood seems silly as it is not precise. I don't have acces to a free machine shop so what do you guys recomend.

Matt

[Apogee]

First off, you've got the offset of your C5 wheels confused with backspacing. As for your numbers, where did you come up with them? I would think that if you want 4.25" BS in the front, then essentially you want to end up with what is effectively a centered wheel...therefore you'll want adapters that are equal to your offset dimension minus your rotor thickness (.30" for C5 rotors), or 1.90".

t = 2.20" - .30" = 1.90"
t = 56mm - 7.6mm = 48.4mm

As for the rear, wheel centerline is a 4.75" BS and you want 5.25", or .50" more BS than the offset baseline. If you allow .25" for the rear rotor thickness and .50" for the backspacing you want, I come up with a 1.73" adapter thickness for the rear.

t = 2.48 - .5 - .25 = 1.73"
t = 63mm - 12.7mm - 6.4mm = 43.9mm

As for mockup, I'm running 1/2-20 screw-in studs so I just swapped out the wheel studs for some longer lengths of all-thread and then spun some extra lug nuts and washers onto the all-thread to space the wheel where I wanted it. I did this without the springs installed in the front so that I could cycle the suspension and steering and check for clearance. Just don't forget to factor in the thickness of your rotors when measuring for your adapter thickness.

[MonzaRacer]

http://www.rims-n-tires.com/rt_specs.jsp
use this to figure offsets and spacings and tire sizing.

[oobus]

Thanks for the responses guys. I've been researching this for some time and it looks like the only way to know for sure is mock up everything. Apogee, I think you have it figured correctly. I used someone elses numbers that is running a similar wheel combination to me, but I guess it will be better in the long run to make my own measurements. Plan on tracking the car so it needs to be right.

Well, it takes more time and effort, but I think I'll have a really good tire/wheel and brake package on my car for less than $1,500!