Here's some good reading and testing of different size tires. I am running ROH Drift R 18x8 w/ a 245/40/18 tire - 49lbs and ROH Drift R 18x9 w/ a 275/35/18 tire - 51lbs
Anyhow, I've calculated that if I go with a 245x40 18" tire up front (or 245x35 19") I get the same rolling diameter as stock. But with that I would want to mate it to a wider rear for a little extra meat. In which case, a 275x35 18" (or 275x30 19") would be the closest match, roughly 0.1" shorter than the front.
The catch here is, will a 275x35 mount on an 8" wheel without problems, or will that be an issue? These wheels have a 42mm offset if I remember correctly. Will that in itself be an issue with such a wide rear tire? (with or without rolling the fender lip)
The OTHER option is to go with a 235x40 18" front (or 235x35 19") and a 265x35 18" (or 265x30 19") on the rear which should mount on an 8" wide wheel just fine. This would be a comperable rolling diameter from front to rear but slightly different than stock so the speedo might be off slightly. The drawback here is, these are both very hard to find sizes compared to 245's and 275's.
I remember seeing someone that had mounted 275's on the rear of stock 17x8 GTO wheels and had no problems to speak of, but wondered really what the drawback is going that wide on a narrower wheel and if it was a danger in any way? The simple fact is, I might have to go with the 235's and 265's like it or not, but that'd really limit tire options.
Results of yesterday's wheel & tire test fitting w/ ROH wheels:
Factory wheels & tires:
17x8 w/ 245/45/17
55lbs each (wheel & tire)
5x120mm stud pattern
48mm backspacing
17's
Front:
245/45/17 is still the best fit
255/40/17 fit but had to use a 8.5" wide wheel, the required backspacing hadthe wheels/tire sticking out too from the body line
275/40/17 didn't fit, using a 9" wide wheel, the tire was rubbibg the strut. Again using a different backspacing would have the set sticking out too far
ROH Drift R 17x8 w/ 245/45/17 - 47lbs
ROH Drift R 17x9 w/275/40/17 - 49lbs
Rear:
275/40/17 fit no problems on a 17x9" wheel
These stick out just a little more than the factory tires, but I think they still look great!
Recommendation for 17" wheels: Use your factory tires and and put them on 17x8" custom wheels or 17x8" wheel front w/ 245/45/17 tires and 17x9" wheel rear w/ 275/40/17 tires
Personally I would just reuse the factory tires and put 17x8" wheels at all four corners. This will be cheaper to do and looks good.
18's
By far the best way to go!!
Front:
18x8 w/ a 245/40/18 tire - fit is perfect
18x8.5 w/ 255/40/18 - again sticks out too far
Rear:
18x9 w/ a 275/35/18 tire - fit is perfect
Recommendation for 18" wheels: Without a doubt the best set is 18x8 up front with 245/45/18 matched with 18x9 out back w/ 275/35/18. This set looks great, fits porportionally and lets you pick up some width out back. The ride quality is also great. Or use 18x8 and 245/40/18 at all four corners.
ROH Drift R 18x8 w/ a 245/40/18 tire - 49lbs
ROH Drift R 18x9 w/ a 275/35/18 tire - 51lbs
19's
Front
245/35/19 on a 19x8" wheel
Rear
275/30/19 on a 19x9" wheel
Recommendation for 19" wheels: GO BACK TO 18"!! While the look great on the car the ride quality is horrible. If you could care less about feeling every bump in the road and want all the look, these are for you. If you just gotta have them then use 19x8 w/ 245/35/19 front and 19x9 w/ 275/35/19 rear or the 19x9 w/ 245/35/19 all the way around.
I have some pics from the fitting I just need to get them off of my disc. I'll get some pics of my Goat with the set up I'm going with and personally recommend,
18x8 w/ 245/4018 front and 18x9 w/ 275/35/18 rear. If I'd been working at Pontiac this is what I would have sold the car with!!
By the way, anyone interested in ROH or other custom wheels contact me I now have pricing and availability info!!
18X8 on front with 245/40's 18X9 on the rear with 275/35 Michelin Pilot Sport AS's
Unfortunately Boyd put a +48 offset on the rears and they rubbed, so I rolled the fenders. Boyd was willing to make another pair but I was tired of waiting. It's been about three months already. I think a +55 would be a perfect offset for these on the rear and Boyd could definately do that.
NEW BOLT PATTERN FOR GENERAL MOTORS 5x120mm
WIDTH OFFSET WIDTH OF TIRE
7.0… +26/+65 215/225/235
7.5 +33/+61 215/225/235/245
8.0 +39/+55 225/235/245
8.5 +45/+48 235/245/255
9.0 (MAX) +48 245/255
REAR ONLY
8.0 +36/+65 245/255
8.5 +42/+65 245/255
9.0 +48/+65 255/265/275
9.5 +55/+64 265/275/285
Offsets greater than shown above will rub on rear fender lip (25mm fender lip).
Another fella posted this: "I have a set of wheels from ROH, the rears are 18x9 51mm 275/35 and front are 18x8 41mm 245/40, with Nittos, no need to roll. BFG or Dunlop you may need to roll?"
The Arrowhead Performance Yellow Jacket 2004 GTO prototype. Thos are Hurst inspired wheels. We got 18x8' rims with 245/40/18 Toyo meats in the front and 18x9' rims with 270/35/18's in the rear. The exhausts exit out each side just before the rear wheels.
Hope this helps.
