I got 44mpg ATL-Richmond non stop with a D15B7 and a CX trans on a stock bodied '92 CX. Stock 13" steelies with 3 different kinds of tires on it and 2 different pressure ratings... and loaded down with another passenger and a weeks worth of stuff for 2 people. The car was full.

So... get a CX transmission. That will boost it. Just kiss performance goodbye.