I'm thinking about replacing the stock wheels on my '00 STS but I don't know the rim size. I need to know the bolt pattern, offset, and the size of the center hole. I believe the stock wheels are 7 inches wide, how wide can I go without any rubbing?
A zero offset wheel has the mounting surface exactly in the middle of the wheel regardless of the width of the wheel. A wheel 7" wide with 0 offset will have the mounting face 3.5" from both edges of the wheel. Positive numbers move the mounting surface away from the center of the car moving the wheel into the wheelwell, negative offset moves the mounting surface in towards the center of the car pushing the wheel outward. In your case, the new wheels are the 13mm wider with 13mm more positive offset, so the new wheel will move back into the wheel well by those 13mm (1/2")
The more positive the offset the shallower the front of the wheel is. Deep dish wheels have lots of negative offset.
A 114.3mm bolt pattern wheel will fit on a 115mm bolt circle hub if you don't mind bending those hardened wheel studs inward a bit. The OEM wheels index on the center bore, not the bolt circle, so any mistake in tightening/bending the studs gives you one or more wheels which rotate off-center.
The rims I was looking at got bought, I'll find something more suitable later. If I get a hub kit that adapts the center hole of the wheel to the hub of the car can I get away with a 114.3 bolt pattern, I'm not finding many that are 115 dead on. If I get a half inch wider than stock wheel I need to make sure the backspacing is less than +35, more like +20 or so to not move the wheel any further under the car, right ?
No you want to stay as close as possible to the stock offset. You can maybe go + or - 5 but anything beyond that will put added pressure on your studs and suspension. You dont want to use any adapters or anything else to get the rims to fit. There are rims that are 5x115 you just have to find them. Alot of people run 5x114.3 rims on these cars which is close but it puts alot of added stress on your studs. I've seen people break studs doing that. And no when looking for rims look for +35 offset or anything within 5 either way (+30, +40).
The width of the wheel doesn't matter - it's the offset. The wheel pictures I posted in another thread are for a 7.5" rim, which uses the same offset as the 7" because the wheel centerline remains the same.
The problems arise when you wind up with a wide wheel and gross offset so the wheels stick out of the well like an urban ricer.
The width of the wheel does matter. If you have a 7" rim with a 35mm positive offset, that means the rear edge of the rim is 3.5" plus 35mm (1 3/8") which is 4 7/8" from the mounting surface (rotor cap face). If you jump up to a 9" rim with the same offset, the rear edge of the rim is now 5 7/8" (4.5" plus 35mm) from the mounting surface so you just pushed your wheel and tire 1" (half the width difference) further back into the wheelwell). To keep the rear of the wheel in the same relative location you would need to decrease your offset by half the difference in width, in this case a 2 inch increase in wheel width would need 25mm less offset. Of course if you do this your pushing the entire difference in wheel width outward towards the fender lip. You need to find some kind of happy medium where you still have space between the rear face of the wheelwell (or the front suspension components on the front end so you can turn without hitting the frame or inned fender areas) and the fender lips so your not tearing up the fenders going over bumps.
If you change the profile of the wheel, make sure the brake calipers still fit between the rotor and the inner face of the wheel.
Well if he's going aftermarket he doesnt want to go any wider than a 8" rim. And 9" rim will either stick out or rub badly when turning. Stick with a 8" wide rim with a +35mm offset and you will be good to go.