If you have an auto tranny that is about right for the 3.0 in truck.
If you have 4.10's gears w/31's(stock is 4.56 for manual and 4.88 for auto), your mpg could be off by 10-11%

Mpg also depends on your driving style, jack rabbit take-off will kill the mpg in Toyota trucks.

It could also be time for a minor tune-up.

You could also have a sticking/stuck exhaust valve, how is it performing?

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed