Right, look for a metal ID tag on either axle pumpkin. If you don't know what the numbers mean, just post them and someone will decode them. The 89 Bronco commonly has a 3.55:1 gear ratio ,so going from 31" to 33" tall tires will not make much difference. If you know what the transmission and transfer case are, you can compute the final drive ratio for any size tire. You should have a E4OD tranny and B-W 1356 t-case. Your mileage may even increase with 33"'s, but you may have some minimal power loss. Changing tire height just changes the engine's peak power band, and a gear swap corrects that. It's usually only neeeded when a drastic change is made in tire height, like going from 31''s to 38"'s.