I have a 99 dx with a d16y8 in it, I have a 97 d16y5 motor, tranny, and ecu to swap in. The goal is to get better gas mileage currently I get about 28mpg city and want to increase it to 36mpg city, I know the y5 gets about 34mpg city.
Do I need to get a 97 hx wire harness? And anything else?

 

You have a ton more research to do. Obd2a vs obd2b, unique cabin wiring of the hx, etc. Not as easy as you think.

 

Nope.