My 91 typically returns 33 MPG in a mixed mode environment - I have gotten over 40 MPG on rare occasions. Typical highway mileage is 36-38 @ 65-70 MPH with no air.

I think your stated mileage is low enough that I'd have somebody take a look at it - but realize that you are going to spend far more $$$ in diagnostic/repair costs than you are ever going to get back in a 4-5 MPG improvement.

Tim