How big a tire . . .
 

What's the tallest tire one can run on a '69 K-10 withOUT lift or fender trimming?

TIA.

probably 32X10.5/15 on an 8" rim. Anything taller or wider will catch the front lip of the fender well. (I'm running 31x12.50R15s on 10" wide rims w/ 4" backspacing and with a little flex the tires hit the front lip of the fenderwell while turning.) This is even with new bushing for the stock 3 leaf springs and new poly body mounts and ORDs 1/2" longer shackles. i'm actually think of adding ORDs 1" zero add-a-leafs. I like the look of the wide tires. They are good for the sand.

Before I lifted my 70 K10 I was running 33x12.50/15s on 70s rallys. No hitting anywhere and did some serious wheeling with it.
Not sure of the backspacing on those factory wheels though. I can measure them for you if you want.

So, I should be OK with 31s on 7" rims?

most definately, i have 31x10.50's on a 15x10 wheel. no problems with rubbing

im running 32x10.50 bfg K.O. tires and 15x10 ultra wheels VERY NICE SET UP... no rubbing at all, my truck even sits stock . hope this helps

I ran 33 14.5 MT Bajas on my Jimmy with 15x8" wagon wheels. No rubbing. Going to 10" rims caused slight rubbing on the fenders. (both with 3.5" backspacing) You could probably get away with 4" bs, but you shouldn't have any troubles with 31 10.5 except that a 7" rim is too narrow. Get yourself a set of six lug rallys off an 80's 4x4 or some aftermarket rims.

Why is it too narrow? (This is a genuine question; not a sm@rt-@$$ remark.) Is there danger or is it a "looks" thing?

I really dislike Ralley wheels, and I've never found an aftermarket set I like as much as the Center-cap and narrow trim rim combo I run now.

i don't think you'll have any problems running that tire, i ran 35x 12.50's on like a 7 inch wide wheel (however wide 70's ford trucks are) with no problems good luck

The only reason that 7" wheels would be too skinny is that the wider the tire is the wider the rim must be for the sidewall to be it's strongest. If you put a fat tire on a skinny rim the sidewall is vulnerable to rocks and whatever else they encounter. They will also last longer as there will be less stress on the sidewall.

It also allows you to run more PSI on the tire and still have even tire wire. With the narrow rim, the tread tends to bow up in the middle and you have to reduce pressure for even wear. This reduces the load the tire can carry. It also tendt to push the sidewall against the rim lip which means the load isn't carried on the bead in the proper area. A 10.5" tire should be on a 8-10" rim ideally, but the rim will be better protected offroading with an 8" width.

after reading my post i guess i should clarify cause it does seem a little misleading, you can get away with running those tires on a 7 inch wide wheel (if you are a cheapskate like myself) but if you have the money to spare it would be better to go with something a little wider.