I found some old style Cragar wheels but I am unsure if they will fit the 56. The wheels are 15x8 4.5 backspace and 5 on 5 pattern. They have new 235/75r15s on them. The tires may be too tall so I have reservations. Can anyone help?

 

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Can you arrange to get one and try it out at a local tire shop? If not,
This stuff gets kind of complicated.
First, as you likely know your 56 Olds bolt pattern is 5 x5 (five studs with five inch spacing so that part is OK as far as the new wheels.
Second, The 235x75x15 tires are just a hair under 29 inches tall and are 9.25 inches wide. That 9.25 is the section width with the tread width being somewhat more narrow.

The more complicated part: With an 8 inch rim and 4.5 inch backspace you will have 4.5 inches of wheel width behind the mounting surface and 3.5 of wheel facing out. Now add .625 of an inch to each one of those to account for section width of tire. so you will have 4.5+ .625 or 5.125 in. of wheel and tire behind the mounting surface and 3.5 +.625 or 4.125 inches facing out from the mounting surface. And with a diameter of 29 inches you would have 14.5 inches sticking up above the center of the hub. For sake of measurement assume 9.25 inch section width all the way up.

Now you have to get up under the fenders and measure how much clearance you have vertically and horizontally and see if the wheel/tire combo you are looking at will fit.

The rear is pretty straightforward. A piece of 1x1 lumber 29 inches long and marked at its center and another piece 9.25 inches long. Hold the long piece up against the mounting surface of the hub with its center at the center of the hub. Mark the 9.25 inch piece at the 4.125 inch measure and have that part sticking forward. The 5.125 inch remainder faces the rear. Slide the short piece vertically to see if it will clear the closest body/frame parts in the wheel well.

Now go to the front using the same sticks but in addition to calculating vertical clearance turn the wheel to full left lock and measure to see if new wheels and tires will hit the frame or the opening of the wheel well. Do the same at full right lock.

The closest radial size to the original 56 Olds tire is reported to be 225/70/15 which is 27.5 inch diameter and 8.8 inch width so you would be going up 1.5 inches in diameter (.75 in. in radius) and just less than a half inch wider. Actually your measure of concern would only be .75 inch radius as at any given point along the circumference of the tire it is only .75 in. further out from the center of the hub.

It makes my head hurt a little to think of this stuff. Hope this is helpful and not too confusing.
Jerry