I'm getting around 20-21 MPG in my 1991 Toyota Tercel.  I bought the
car 4 months ago and it promptly blew a head gasket.  I replaced it,
had the head resurfaced, put in tons of new seals (incl. the valve stem
seals) and did everything correctly when reassembling the engine.  It
now runs great, sounds great, starts great, never stalls.  But it gets
terrible mileage, right after I did all the work on it, it got 24 and
every tank after that has been in the 20-21 range (accounting for the
speedo that is 10% too fast).  I'm gonna check the compression this
afternoon.

Things to note:
The cyllinder next to where the head gasket blew had a bit of surface
rust on the walls, though it had the same texture by feel as the
others.
The engine was replaced 4 years ago withone of those 30K mile engines
from japan, so there is no telling really how many miles are on it.

If the compression test comes out decently, what might be the cause?