Sounds like an air leak, likely in the intake system, or possibly cracked/disconnected vacuum lines.  I would start with checking for leaks along the intake system and vacuum lines.  There are several ways to check, first do a visual inspection of the accordion hose and vacuum lines for cracks.  If nothing jumps out at you, grab a small hand held propane torch, crack the valve a little(do not ignite the propane) and run it along the intake system and vacuum lines, if the engine idles up, you have found the leak.  If you don't have it, here is a link to a'93 Toyota FSM.

If you haven't changed the gears, or are not compensating for the taller 31's, your mpg is about 10% higher than what the odometer/gallons is currently equaling.