Back spacing

[patman]

I’m looking at getting a set of mags for my ‘72 and am wondering what the maximum backspacing is for the rear wheels to fit our cars? I’m thinking 5” but I’d rather note guess.

[cny first gen 71]

I think it depends in the wheels,  I have 8 inch on mine with 5 inch back space no problems at all. There have been several discussions about wheels and fit but not sure where they are someone else may chime in on it to help more. I guess there is a wheel calculator somewhere too not sure where.

[Scott S.]

What size (dia) are you looking at ???

As well, how much rubber are you looking to wrap them in ??

[420ponies]

275/60/15 (rear), 5" positive offset. Now for my 285/70/15,(rear) I use a 5 1/2" positive offset. If you're going bigger diameter wheel,well it all changes. In the search topic, look up tire spacing, should come up with something. Lots of different options for our cars.

[patman]

1 hour ago, Scott S. said:

What size (dia) are you looking at ???

As well, how much rubber are you looking to wrap them in ??

I’m thinking an 8 or 9 inch width rim with a 275-60-15 tire

[70white402]

8” is 5”.  Perfect for 275/60-15

[keith72monte]

I have the brand U.S. wheel 15x8 inch with 5 inch back space on three of my Montes no problems. One car rubs slightly  if I hit a big dip in the road at high speed, maybe bottoms out slightly. I figure i need new springs in that car. I think 275 's rear and 245 fronts look the best and are much needed with a big block car. 255's rear are ok too. with 235's in the front. Bigger rear always gives the car the muscle look. I remember a carcraft  magazine years ago and they tested all sizes on a 70 monte. 17 inch wheels made the best handling, If your going with a pro touring setup. Search archives, tons of info

[SPLATT71MC]

I have 18 x 9.5 with 5.75" BS and 275 width tires that fit perfect with no modifications front and rear.

 

[MCfan]

Patrick,  

You are getting (and will get) various answers to your question about maximum Backspace for one simple reason: Backspace depends on rim/wheel width and Offset! In fact, for any wheel/tire combo, the calculation is: Backspace = ((Rim Width +1)/2) + Offset

NOTE: Backspace (usually expressed in inches) is the distance between the mounting surface and the inside edge of your rim/wheel.

For example: For a wheel/tire combo to be centered on the hub mounting surface (zero Offset), a 7" wide rim needs a 4.0" backspace, an 8" wide rim needs a 4.5" backspace, a 9"wide rim needs a 5.0" backspace and so forth.

If you are considering aftermarket wheels of various widths, you need to focus on OFFSET first (as all of the wheel manufacturers do) and then Backspace will follow depending on which wheel width(s) you select.  Offset is necessary to center the wheel/tire combo within the wheel well opening for best inboard and outboard clearance.

OFFSET (usually expressed in millimeters) is the distance between the center of your wheel/tire combo (regardless of rim width) and the hub mounting surface.  Positive (+) offset moves the wheel/tire combo inboard, zero (0) offset perfectly aligns the center of the wheel/tire combo with the hub mounting surface, and negative (-) offset moves the wheel/tire combo outboard.

GM did not build first gen Montes for or with zero offset wheels! Front wheels have the best clearance at +6 mm offset and rear wheels have the best clearance at +12 mm offset. However, rather than have different front and rear wheels, they compromised at +8 mm for their stock wheels.  For example: stock 7" wide rally wheels have a +8 mm offset resulting in a backspace of 4.30" (look it up in the factory specs).  At zero offset, that backspace would be 4.0" but it has to be 4.3" to move both front and rear wheel/tire combos slightly inboard.

There is a great chart at https://tiresize.com/wheel-offset-calculator/ that you can use to determine what Backspace you will need for whatever rim/wheel width and Offset you want to consider.  Just scroll down on that Offset Calculator page until you see the multi-color chart at the bottom.  First select the rim width in the vertical scale, then read across until you see either +6 mm offset for front or +12 mm offset for rear and follow that column back up to read the Backspace you need for that rim to fit you Monte with the best inboard and outboard clearance. 

Car guys love to talk about Backspace, usually without considering the effect of rim width and Offset, but wheel manufacturers always talk first about what Offset you need and then tell you what Backspace you should get depending on the rim width you select and how much Offset (+ or -) you want/need.

Sorry for the long answer, but you can easily determine what Backspace you will need once you select a rim width and specify the Offset.  Of course there are practical limitations to both rim width and tire sizes for our Montes which many of our members can advise you on from personal experience.  Good luck.

 

[patman]

10 hours ago, MCfan said:

Patrick,  

You are getting (and will get) various answers to your question about maximum Backspace for one simple reason: Backspace depends on rim/wheel width and Offset! In fact, for any wheel/tire combo, the calculation is: Backspace = ((Rim Width +1)/2) + Offset

NOTE: Backspace (usually expressed in inches) is the distance between the mounting surface and the inside edge of your rim/wheel.

For example: For a wheel/tire combo to be centered on the hub mounting surface (zero Offset), a 7" wide rim needs a 4.0" backspace, an 8" wide rim needs a 4.5" backspace, a 9"wide rim needs a 5.0" backspace and so forth.

If you are considering aftermarket wheels of various widths, you need to focus on OFFSET first (as all of the wheel manufacturers do) and then Backspace will follow depending on which wheel width(s) you select.  Offset is necessary to center the wheel/tire combo within the wheel well opening for best inboard and outboard clearance.

OFFSET (usually expressed in millimeters) is the distance between the center of your wheel/tire combo (regardless of rim width) and the hub mounting surface.  Positive (+) offset moves the wheel/tire combo inboard, zero (0) offset perfectly aligns the center of the wheel/tire combo with the hub mounting surface, and negative (-) offset moves the wheel/tire combo outboard.

GM did not build first gen Montes for or with zero offset wheels! Front wheels have the best clearance at +6 mm offset and rear wheels have the best clearance at +12 mm offset. However, rather than have different front and rear wheels, they compromised at +8 mm for their stock wheels.  For example: stock 7" wide rally wheels have a +8 mm offset resulting in a backspace of 4.30" (look it up in the factory specs).  At zero offset, that backspace would be 4.0" but it has to be 4.3" to move both front and rear wheel/tire combos slightly inboard.

There is a great chart at https://tiresize.com/wheel-offset-calculator/ that you can use to determine what Backspace you will need for whatever rim/wheel width and Offset you want to consider.  Just scroll down on that Offset Calculator page until you see the multi-color chart at the bottom.  First select the rim width in the vertical scale, then read across until you see either +6 mm offset for front or +12 mm offset for rear and follow that column back up to read the Backspace you need for that rim to fit you Monte with the best inboard and outboard clearance. 

Car guys love to talk about Backspace, usually without considering the effect of rim width and Offset, but wheel manufacturers always talk first about what Offset you need and then tell you what Backspace you should get depending on the rim width you select and how much Offset (+ or -) you want/need.

Sorry for the long answer, but you can easily determine what Backspace you will need once you select a rim width and specify the Offset.  Of course there are practical limitations to both rim width and tire sizes for our Montes which many of our members can advise you on from personal experience.  Good luck.

 

Thanks, I appreciate this. It’s very helpful!

[Whons]

I just put new wheels on my 71. rear: 15 x 8 with 4.5 backspace tire: 275/60/15.    Front 15 x7 with 3.75 backspace, tire: 235/60/15

[420ponies]

Always liked those rims on any car.Nice!!

[Whons]

Thanks James.

[FL 72 Monte]

6 hours ago, Whons said:

I just put new wheels on my 71. rear: 15 x 8 with 4.5 backspace tire: 275/60/15.    Front 15 x7 with 3.75 backspace, tire: 235/60/15

Looking good I like it!!!

[patman]

23 hours ago, Whons said:

I just put new wheels on my 71. rear: 15 x 8 with 4.5 backspace tire: 275/60/15.    Front 15 x7 with 3.75 backspace, tire: 235/60/15

Looks great! What brand wheels are they?

[Scott S.]

I believe they are American Racing "Vintage" wheels. 

[patman]

1 hour ago, Scott S. said:

I believe they are American Racing "Vintage" wheels. 

Did you buy them from American or a shop? They don’t offer a 4.5 back space on the website. Unless I missed it…which is entirely possible! 

[Whons]

I bought them from Wheelsforless.com    they are in Texas. Check them out. Great prices, free shipping, and no tax.

[Scott S.]

3 hours ago, patman said:

Did you buy them from American or a shop? They don’t offer a 4.5 back space on the website. Unless I missed it…which is entirely possible! 

I still have the factory rallye wheels on Montelishi..... I just remember seeing them somewhere.

 

[Whons]

Scott I bought them from a shop called  wheelsforless.com   the 8" wide ones on the rear do have a 4.5 backspace. I measured them before I had the tires mounted.

[Scott S.]

Hey Wayne, it's actually Pat (Patman) looking for them..... But they are a nice set.