transmission swap

can i put a 92-95 dx tranny in a 98' ex civic without any modifications

why would you want to?

gas mileage

your car should get good enough mileage.... but if your dead set on doing a pointless swap IMO... Yes it should Bolt to your engine... im not sure if both trannys are hydrolic though.... youll need to check into that.. why dont you put a tranny out of a DX of the same year? that way you know it will work?

yup, they're both hydro and will swap over fine. And I completely agree that's it's kind of pointless. You're not going to gain that many mpg by doing it. You're better off just making sure you car is up on it's maintenance (replace air filter, fuel filter, spark plugs and wires, valve adjustment, check the O2 sensors to see if they're fouled out). And make sure you keep the engine in lower rpms, but high enough where it's not bogging.