I have a 90LX hatch...but mine is a 5spd.
The best mpg I ever got was 30.
It averages 27-30 depending on how I drive.
I installed a ranger header,2-1/2 catback exhaust with
a FlowPro muffler,cold air intake into the fenderwell with
a cone air filter,an MSD 6A ignition with coil,Mr.Gasket 8mm
plug wires,split fire plugs,and Mobile 1 20w50 synthetic motor oil
and tune it regularly.
I haven't gotten around to changing the trans and rear end
to synthetic yet.
Some may argue about some of the parts I have installed,but
the numbers speak for themselves and I'm happy with it.
Of course,yours being an automatic,your results will vary.