You Guys With Fat Tires . . .

Larry,

In theory, the difference should be 10 mm, (not factoring a rim) ..... 235 vs 245. Those numbers are supposed to denote section width, which is the overall width between sidewalls (unloaded, at the top of the tire, not bulging, like a tire will close to the ground). An 8 inch rim is likely the sweet spot rim size for the 245/17, and 7 inch for the 235/15, so minimal difference there.... maybe 2-3 mm over specification on that.

Note that I said "in theory" ... if you're jumping between brands and/or you already have tight clearances, always look at a spec chart for each tire. Actual section width is more of a suggestion than a rule, depending on how that company constructs the tire, and other factors, such as hitting load and speed ratings... some tires have more layers built in for these factors. It gets pretty complicated. Some tires are constructed with square-shouldered tread (stiffer edges for performance) and others are round shouldered (usually for better ride).

 

Cooper 255/60R15 on a 15X8 with 41/2 backspacing. cleared the spring by 1/4 in on one side and 5/16 on the other side. The suspension bumper was close so I added a 1/4 spacer. In 100 miles I haven't rubbed anything and that included a little hard driving....went with Cooper 215/65R15 on a 15X7 with 41/4 backspacing it was close at the back edge of the tire to frame and so I added a 3/8 spacer. I have manual steering and the steering effort is comfortable. Again no rubbing any where

 

Larry. Another aspect which has been eluded to a few times but not specifically hashed out is to always keep in mind the stiffness of springs(tall mushy stock stuff or shorter stiffer performance) and the bushing materials used. Full droop height and bushing stiffness of a suspension makes a humongous difference in lateral movement, twist, and wrap. Sway bars and panhards can really help matters.

In other words.. if you want to run really tight sidewall/spring clearances for the sake of max stuffing a tire?.. you'd very likely rather be getting out of shape(whether you chose to do it or not) on a dirty sand-filled corner of blacktop with bumps and potholes while running a pretty tight setup beyond what any factory part would enable.

Trust me here.. I've seen and driven plenty of these old front heavy and non-rear sway-barred muscle cars doing heavy evasive maneuvers to avoid hitting parked cars, walls, poles, people, animals and just generally trying to get themselves back in shape to avoid damage or injury. The amount of sideways suspension articulation and violent hopping and twisting around of these old suspension designs will eat up minimal "never had any problems before" clearances in a heartbeat. And no one wants a sudden blowout pushing them well past the point of no return while they're trying to straighten a car back out. Especially at higher speeds when a suddenly dropped tire can mean the difference between a car going topside and life and death.

Not implying that you, or anyone else shouldn't do it.. just to take all the relevant variables into mind and allow sufficient margin of safety for those extreme situations. Pretty hard to look "badass cool" while you're repairing your wreck, wearing casts, or laying in a casket.

 

I had to notch and bend the "tire cutter" on my 1970, to get some more room between the tire and the wheel-well.

 

Did you ever fine the specs of your wheels by chance?