[QUOTE=onsturn]Ok so 8" wide and 4" backspace on the front and rear will work on my 69 using 18" or 20" wheels? Do I have that correct? Thanks Again[/QUOTE]

the factory wheels are 8" with a 4" backspace

so you know this works width wise...

and it usually gets easier with larger wheels.

the only thing you need to watch for on the wheels is interference with the calipers...

 

cthulhu38974.8244444444

OK Thanks again

I have a 1968 blue Vette with 245/40ZR18 93W in front and 275/70ZR18 103W in the rear. The tires are NITTO NT 555. The wheels are billet from Intro Wheels. They are the Pentia model.  A good reference is Corvette Enthusiast, August 2004 Issue, Page 62. The article is about C3s and how to fit wheels and tires. The wheels I have are the rolled-lip which have equivalent widths of 18x9 and 18x10 upfront and rear, respectively. You can go to www.introwheels.com and see my car in there (1st page).  In the article they talked to Dave Rolland who owns Intro Wheels. The setup they came up with in the article is different than mine  and it had a 4.5" back spacing. The wheels fitted in the article were 235/45ZR17 in the front and 255/40ZR18 in the rear. I presume that my back spacing is the same and it is inherent within the wheels. By the way, my wheels fit within the wheels wells. I have not had any problems with rubbing in the front when the wheels turn. The setup if far superior in terms of handling (straight acceleration and curves) than what I had before, which was 15x8.5 rims with 255x60 Dunlop radials. With those I always had rubbing in the front when turning.

JMB what did you use as a backspace? I was planning to use 18x8 in front with a 225/40/18 and 20x8 in the rear with a 255/35/20. Both with a 4" backspace. This tire combo will give me the same sidewall tire height to even out the look. The rear tire is the same diameter as the stock F70/15 so my speedometer will be accurate. It is a bigblock car with Vette Brakes GT coil springs so the front end is 1" lower than stock. The smaller diameter front tire should fit nicely.

onsturn: I also have the 427 block in mine  and it is slighlty lower in front. The fitting in terms of the wheels in the article I mentioned seems to have a better fit with a 4.5" backspace. That is why I am pretty sure my wheels have the same backspace. The backspacing is built into my wheels. The article also addressed a 4.25 backsapcing and a 5.00 backsapcing. But then you have to be careful with the trailing arms, at least in later years Vettes. The 4.00 backspacing may have your tire extending too far out of the wheel well.

It seems that your 225 upfront may be too small. Have you considered going at least to a 235. My 245s fit perfectly.

onsturn: Yes my car has the stock trailing arms. It also has the stock rear stabilizer bar for big blocks. I have no rubbing with the backspacing built into my wheels either front or back.

You really do not need to stay exactly with the same overall tire height. Tire manufacturers recommend, however, that you stay within + or - 3 percent of the original tire diameter to have accurate readings of your speed. You may want to check an article in Tire Rack on this. The change allow is about 3/4" of the tire diameter. However, tire load capacity needs to be preserved.