hempsy Posted February 25, 2022 Share Posted February 25, 2022 I am planning on putting 15 X * rally wheels on my 1971 SS454. I am going to put 275/60R-15 on the rear and 235/60R-15 on the front. What wheel backspacing do I need on the rear and front for them to fit in the stock wheel wells ? Thanks Hempsy Quote Link to comment Share on other sites More sharing options...
MCfan Posted February 25, 2022 Share Posted February 25, 2022 There have been several threads on this topic recently but perhaps you are not a regular visitor/reader of this site. The use of the Wheel Offset Calculator at the tiresize.com site plus many real-life examples have shown that the offsets (not backspace) that keep non-stock tire/rim combos centered within the front and rear wheel well spaces of first gen Montes are as follows: Front offset = +6mm (+.25 inches); Rear offset = +12mm (+1/2 inch) Note that offset it measured from the dead center of the rim (independent of rim width) to the rim's mounting surface while backspace is measured from the outer inboard edge of the rim to the rim's mounting surface and is therefore totally dependent on the rim's width. So, for a 7"wide rim, the backspace that equates to a +12mm offset is 4.5" inches and for a +6mm offset is 4.25 inches For an 8" wide rim the backspace that equates to a +12mm offset is 5.0 inches and for a +6mm offset is 4.75 inches If you want 15x7 rallys on the front, you'll want 4.25 inch backspace. If 15x8 rallys in the front, use a 4.75 inch backspace. If you want 15x7 rallys on the rear, you'll want 4.5 inch backspace. If 15x8 rallys on the rear, use a 5 inch backspace. While the backspaces given above are close to ideal for our Montes, a difference of 1/4" will probably not get you in trouble, especially with the smaller tire sizes. I run 15x7 rallys with 255/60s on the front and 15x8 rallys with 275/60s on the rear and you may use different width rims on front and rear, also. BTW, the backspace for a stock/factory 15x7 Monte rally is 4.30 inches (+8mm offset) for both front and rear (which is a workable compromise between the +6mm and +12mm offsets shown above for front and rear, respectively). Good luck. 1 Quote Link to comment Share on other sites More sharing options...
714024SPEED Posted February 26, 2022 Share Posted February 26, 2022 My rear tires are also 275 60 15 I used a 15x8 wheel with 5" of backspace and they fit with no issues 2 Quote Link to comment Share on other sites More sharing options...
MajorPain99 Posted May 1, 2022 Share Posted May 1, 2022 What about 15x7 4.25" on all four corners? Quote Link to comment Share on other sites More sharing options...
MCfan Posted May 1, 2022 Share Posted May 1, 2022 3 hours ago, MajorPain99 said: What about 15x7 4.25" on all four corners? Those rims would be essentially the same as stock rallys which are 15x7 with 4.30" backspace (+8 mm offset). What tires are you planning to run front and rear or all around? I ran 245/60-15s on stock rallys all around without clearance issues anywhere. I have also run 255/60-15s on the front on 15x7 rallys with 4.5" backspace (+12 mm offset) but a 4.25 backspace (+6mm offset) will also work well. Where you will run into trouble with that 4.25" backspace is on the rear if you try to use a really wide tire (like a 275/60) You will run into unsafe clearance problems with the lip of your wheel well opening unless you plan to trim it enough. I wouldn't suggest putting a wider tire than a 255 on a 7" wide rim anyway but it's been done. Good luck. Quote Link to comment Share on other sites More sharing options...
MajorPain99 Posted May 2, 2022 Share Posted May 2, 2022 Honestly was gonna do 195's all around. Trying to get it rolling for cheap right now. Wheels and tires are low on the priority list. Quote Link to comment Share on other sites More sharing options...
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