If 31X10.50's came stock on Broncos, why do mine rub on the inside if you turn it all the way, and also make my speedometer be off? 1990 XLT with stock rims.

 

As for the speedo, are you certain it came w 31's stock? 31's were a factory option while 235/75r15s were standard. Look at the door sticker to determine factory size, and see:
http://www.homestead.com/90bronco/axlecode.html
U can see the stock tire and wheel size on the door jamb sticker there.
As for the rubbing, it's all about the steering stops. I very much doubt the factory(s) worried about adjusting these much, some came threaded out more than others.

 

yep..and your springs may have sagged a bit.

 

your tires might be rubbing on the radius arms. This will happen when you are turning at full lock.

 

I had an 89 Bronco XLT with same size tires(31x10.50), I didn't have the broblem of rubbing. Your rig probably came stock with 235/75r15 tires(not sure). Could be that the rims(if they are stock) are too narrow for the tire size. I currently have a 90 Full size Bronco EB that came with 33" x 12.5" tires on it when I bought it, and had the same problem with rubbing. Then I replaced the rims with 15 x 10" rims and new 33x12.5 tires which solved the problem of rubbing. As far as the speedo, the gear that plugs into the tranny maybe bad or needs to be replaced to compensate the tire size. By the way did you Change the gear ratio? When I changed the gear ratio in the 89 from 3.55 to 4.10 in the rear-end, there was a change in the speedo. The differrence was about 5 to 10 miles. Also, the different tire size will throw off the speedo. My 90' EB speedo was very eratic(jumping around too much), turns out that the teeth were stripped on the gear. I still have to update that speedo gear to compensate for the tire size & gear ratio.

Hope this helps