The 31" tires will account for another 10.71 miles/100, since the 31's are 10.75% bigger than the stock 28's. A lazy O2 sensor will acount for about a 1-2 mpg loss, they get lazy with age and wont trtigger a code. If you replace the O2 sensor replace it with an oem one, or one that is a direct plkug in one. The universal ones have to be spliced in, and if the wires aren't the exact same length, the ecu can register a false code.
i also took out the cat and stock muffler and replaced that with a 2" test pipe and flowmaster.
This wil also account for a 2-3 mpg loss. Contrary to popular belief an open exhaust isn't always the best thing, especially for the Toyota engine, there isn't gobs of HP being choked off by the oem exhaust. After rebuilding my '90 3.0 with headers and a highflow cat and open muffler I easily lost 3mpgs, I was getting about 13-14 combined when before the exhaust I would get right at 17. I replaced the highflow muff with a 3 chamber turbo and regained the lost mpg's. An open exhaust takes away the low end torque of these engines, which is where a lot of fuel economy is lost. If your not launching at the 3500rpms, or are constantly cruising above 90mph, I would suggest installing a new cat either a standard or a hi-flow one from magnaflow, this should help to build some back pressure which is needed(some) for these engines.
I would think that the '88 5-spd w/a 3.0 in good working order should get around 21 highway and about 15-17 city, depending on the driver. Your 12-14 is about what 2nd gen 3.0 Runners w/auto tranny get.
