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.
Yep, anything to help the air intake and exhaust.
If you don't want to spend a lot of cash, start
with a new air filter and sparkplugs/wires.
Clean the throttle body.
A performance muffler would be the next step......
A noisy muffler is not going to bring the fuel economy up enough to make it worthwhile, if at all, especially when the mileage is this usually bad.
If you don't know when the car was last serviced, then check the air filter, check the sparkplugs, and check for trouble codes. You didn't say how many miles are on it, but I would suspect that it also has an original oxygen sensor that is well past the replacement interval.