I used to sell a repair a lot of tires in my younger days, I'm a little embarrassed I had to google this to jar my memory. A tire that is 235/75R-15 the "75" means the tires height is equal to 75% of it's width. So a 235/85r-15 is narrower and taller than a 235/75R-15. So as all of us 70's teens wanted 50's on the rear of our wanna be Camero's.
The first number (235) is the over all height of the tire, which on a car has everything to do with the accuracy of your speedometer. The "R" simply means radial tire, don't know if they even make a bias ply tire anymore. The "15" is the wheel height, as in 15 inch wheel.
I'm sure all of you don't need to know this but it was nice for this old guy to refresh his tire knowledge from my 20's.
I agree with JW's thoughts but I don't think going from a 75 to a 85 or even a 70 would be a problem. But I would vary much from that.
Edited to add after seeing Denvers post, my comment is strictly dealing with passenger tires. But I still feel very comfortable with 235/75R-15 on my 16' trailer for as much as I can load in it.