i have a 1985 k5 blazer with 33x12.50 tires on it
the truck has the inside mounting bracket for a spare tire
whats the largest tire/wheel combo i can use ( and still have the top fit) ?

 

I have a 31x10.5 tire in the back and its pushing the height of the bracket, I'd assume that is the biggest it can take.

Seeing as that was the biggest offical size that the truck came with is the 31 I don't think that the 33 will fit.

But you should be able to modify it a little bit to get it to fit.

 

yeah 33s maybe if its a little worn down. i had a bald 35 inch that measured 33 1/2 and it wouldnt work. the tailgate almost closed. i ended up just bolting it to the middle of the floor in that back. that way you dont have that huge blind spot

heres a picture of what i believe is a 31x10.50 in the back of the truck and you can see how close it is to the tailgate. i photoshped it down onto the 35 inch tire to give you a better comparison