Actual sizes vary by brand. I have both 235s and 30 x 9.5s and the 30s are taller. I think the 235s are 29 and considerably narrower. Jim
As JB pointed out, actual sizes vary by brand. For my tire, the BFG AT/KO (from the BFG website):
235/75R15 = 29.0" x 9.1" (measured on a 6.5" rim)
30x9.5R15 = 29.5" x 9.4" (measured on a 7.5" rim)
This doesn't seem like a huge difference for this model & brand of tire. Actual section width increases/decreases about 40% of every increase/decrease in rim width for this tire. That makes these two BFG sizes almost exactly the same width (9.2" or 9.3") on my 7" rims. I still haven't decided which I really want, but I'm leaning toward bigger, I think ....