Just get a 94-96 G20T LSD tranny and you will get the best gas milage possible.
Actually you can get a 91-93 g20 tranny that is non LSD like your car. Change the bellhousing or drill the hole for the flywheel senser. This tranny has the same gears as a 94-96 G20T tranny but it has the 4.083 gears vs the G20T 4.176 gears. So the 91-93 will give yiou the best gas mileage as long as performance is not you number one goal.