Is that the manufacturer's claimed diameter? If so, you'd probably be closer to actual at 30". You'd probably notice some loss of mileage on the highway, but you should have better acceleration. I don't know that it will be as noticeable at speed as on take off.
Years ago, I put 4.10s in my 92 bronco with the stock 235/75-15s and 3.55s which were 29" tall. It accelerated more aggressively, and mileage didn't suffer that much. It was a speed density unit (old EFI) and didn't get good mileage to start with, so that might not be saying much. I installed them in anticipation of putting 32" and a lift after that. It was still geared a bit lower than stock, but not quite as extreme. After I converted it to mass air and put built 351 in it, it probably could have used 4.56s.
With a stock engine, I think you'd be just as happy with 3.73s and probably not notice any mileage difference. If there were a gear set between those two, if would probably be ideal for your tire size.
Last edited by Windsor; 08-19-2009 at 10:21 PM.