I know this is not recommended by tire dealers, but can I do it?

 

If 315's will work well on 8" rims, you should be fine. You may have to run lower tire pressure in the rear to keep the tires flat and wearing evenly...
but lower in the rear is recommended anyway with 65% of CTD weight on the front axle!

RJ

 

i run 35x12.50s on my stock 6 inch dually wheels they said big nono but ive got 35,000 on them and there doing fine you just hafto play with your pressures a little had it up to 110 mph and tow all the time

 

How did you end up with 17x7.5" rims? The tires will mount up ok but you may loose fuel economy having to run them under inflated to keep the tread wear even. Also the tread life will be shortened. I would ditch the rims and get some 17x8" or 17x8.5". You'll never be happy with those rims.

 

The spare steel rim is 17x7.5".
In addition the 285/70/17 allows for a 7.5-9" rim

 

unfortunately, I'm stuck with these rims. I had them sand blasted and re-powdercoated black. In other words, I am stuck with them.

 

Not a CTD, but my old Nissan Titan with the offroad package ran 285/70/17 tires on 17x7.5" wheels stock.

 

what brand?

 

I can't see it being a problem, people run 12.50's on 8 inch rims all the time, granted usually on a lighter vehicle. I just put 285/70's on my 17x8's and can't see 0.5 of and inch causing any sort of problem. Run em.