[Navy Lifer]

OK, so you've read the stickies, etc....did you find the offset/backspace calculators?

Start with a couple of things....this discussion is about 8.5" rear axles only. RMS has a "narrow" rear axle--of the 2 sedan versions used between 1991-1996 (narrow only 91-92), it is 32mm narrower (total width) than the axle used on some Chevy sedans 1993-1996 with the revised wheel opening configuration. This works in your favor, but it may still limit your options at the 9.5" wheel width--depending on tire size choice and offset/backspace options available. You get an extra 16mm (5/8") per side to work with, in other words, but the body opening of the RMS probably takes most if not all of that away. To be clear--to do what you are wanting to do, you DO want to use the axle housing in the car now. There is nothing good to be achieved by using the wider housing with the RMS body design.

Stock Impala SS wheels are the reference starting point for your evaluation of what is possible.

17 x 8.5" bead width - total wheel width edge to edge is close to 9.6" due to rim design (extra material on inside, wheels designed for clip weights)--typically total width (such as a steel wheel) will measure 1" more, so an 8" wheel will measure 9" edge to edge.

+6mm offset = 5.06" backspace

You will find that these wheels with stock SS tire size (255/50-17) fit & clear RMS with no rear clearance problems (assuming car is "square" side to side).

275/40-17 has been used on stock Impala SS wheels by many, even though specs for this tire size typically calls for minimum 9" width (example--2000-2002 Camaro SS used this tire size on 17 x 9 wheels). These (Camaro) wheels are high-offset (+50mm), and used a spacer or adapter to fit on vehicles with more conventional suspension designs with lower wheel offsets, such as the B-body. This is driven by FRONT suspension design, brake rotor hub projection, etc.

There are some aftermarket wheels (American Racing, Coy's, and a number of others) that do provide a range of fitment for B-body (5x5, width choice, offset needed) to accommodate your quest.

Start by determining what amount of room you have to work with when an Impala SS wheel & tire is fitted to your car, and calculate from there.

Every 1/2" of additional bead width will add to the positive offset calculation by just over 3mm, so a 9" wheel should probably have +10mm offset. Many of the wheel suppliers making B-body wheels (those mentioned) often choose to express the wheel specs as backspacing rather than offset, so you would find a 9" wheel (10" total width) would be offered with a 5.25" or 5.5" backspace to position the wheel (and tire) safely within the available space the RMS offers.

Another thing you should determine before going too far into this endeavor is how much body clearance difference your car has at the rear--many B-bodies are NOT even, side to side, and since we're dealing with nearly 20 year-old (minimum) vehicles, if the car has ever been hit, and not knowing condition of rear suspension links or body mounts, you need to determine whether the amount of room you will have to work with is as equal as possible on both sides (is rear axle lateral position OK, in other words) and determine what can be done to correct it if you find it is NOT equal within 1/4" (ie. 1/8" either direction).

Probably very little if anything I've really contributed that hasn't already been stated in other posts over the years regarding this general topic.

What wheels are you actually considering? Give us some links to see what's available and look at what is offered in width and offset (or backspace).

 

[Navy Lifer]

Stock wheels already hit the front sway bar--actually it's the tires that make contact. Just the nature of the design, and only an issue at full lock, unless you end up running a wider wheel and/or tire--so expect it to be part of the "normal" for these cars.

American Racing re-introduces the Vector....I had 'em on my 73 Nova (14x7) 40 years ago--wish I had some pictures! Interesting history--the design of the wheel originally came (or was "borrowed") from GM, and was the wheel used on the 1973 Corvette XP897 "Aerovette" mid-rotary engine prototype that never came to fruition. Later, it was given a production "go" on the 80's Buick Regal Turbo & GN. It doesn't appear that this wheel is being offered as a 17", however/anyway.

As for the Rally wheels from Year One, my concern is that the offset/backspace is not favorable--the extra 1/2" of width is all toward the outside (putting the wheel closer to the body opening sheet metal). This is a zero-offset wheel, and ideally it would need to be at +15mm or +20mm OS (5.5" backspace) or no less than +10mm OS (5.25" BS) to be safe. And I could be wrong....

Really, it would depend on just how much you can gain, if anything, by massaging/rolling/trimming the sheet metal flanges on the inside of the rear wheel openings.

 

[Navy Lifer]

they're out there.....

For example:

Coy's C-67

17 x 9

5x5 bolt circle

5.5" BS

The nice thing about a greater BS wheel is that IF you find there is room to move the wheel out, you can, using a proper spacer and longer wheel stud setup, to fine tune the wheel/tire "fit" to the wheel openings.

 

[Navy Lifer]

limited 17" selection?

Tire Rack:

255/50-17 - 2 choices starting at $133 each

275/40-17 - 32 choices starting at $91 each (size OP wants to use)

285/40-17 - 7 choices starting at $169 each

There are additional sources offering all 3 sizes in other brands.

I think it won't be a problem to get tires for 17's for a good while.....how much more "choice" is needed?

 

[Navy Lifer]

wouldn't 275/40 be way too small a diameter?...285/45 will give the exact same diameter as stock tires...that's what i'm looking at

do you think a 285/45/17 tire on a 9.5" wide, 4.5" bs wheel fit in the rear?...or would i need to go 5" bs?...looking to push it out as far as I can to stay flush, but avoid rubbing...I guess I can roll the fenders too

Well, there have been more than a few who have used "inappropriate" tires on their cars--tire load rating is something that not many pay attention to these days, it seems. A "too small" tire, in diameter, is a conscious choice, right or wrong--but you are correct, in my view.

Wheel bead width becomes more critical as tire aspect ratio is reduced, so the 275/40-17 tire may fit a stock SS wheel, but it is pinching the sidewalls more than the tire manufacturer intended.

If a 275/45 or 285/45 option existed, yes, they would be far better--as to your wheel question, a 9.5" wheel is going to be 10.5" wide, so 4.5" BS means there will be 6" of wheel outboard of the hub flange plane. A stock Impala SS wheel (5.06" BS) has about 4.5" of wheel outboard of the hub flange plane--can the body handle an extra 1.5" of wheel in that direction? I doubt it. I'm not comfortable even with a 9.5" wheel with 5" BS....5.5" BS would be safe, but still depends on tire size selected.

As I suggested earlier, "too much" backspace can be corrected with a proper spacer/wheel stud setup, but if there is NOT enough wheel BS, it is not normally possible to remove material from the wheel mounting pad to pull it inward.

Where someone wants to really have the ability to stuff a wide wheel/tire into a B-body sedan, my first suggestion would be to use the narrow housing if the car doesn't already have it.

This might be a dumb question but does the overall diameter effect offset/backspacing or just the width.

Offset/backspacing is a wheel dimension calculation. Tires only impact this when the size selection is going to cause a rub or the wheel is too narrow or too wide for the tire selected--based on manufacturer recommended wheel width.

 

[Navy Lifer]

The tire doesn't care, as long as the wheel & tire are compatible (bead width vs specs for tire size)--wheel offset/BS simply dictates the position of the wheel/tire assembly laterally.