I was getting about 40 miles to the gallon but all of a sudden I may get 20 mpg. on 1995 Toyota Camry

I took my car to a mechanic just to glance at and they said transmission and motor both sounds good. Not leaking anywhere either so I am so confused???

2 answers
Has got to be a noticeable difference in performance for mpg to be cut in half!! Sure about 40 mpg? What engine? What transmission, automatic or stick, some were manual shift.
Is the check engine light on?