So far I have not heard of any aftermarket final drive gear which will increase the final drive. Maybe you can refer to your Honda/Acura dealers & check whether there are OEM final drive gears from other Civic models. It may or may not fit in.An increased final drive will increase the max speed but will decrease overall torque & pick up speed. Whereas decrease in final drive will improve torque & pick up speed while decreasing the max speed.To improve fuel consumption, the most practical way is to tune your ECU to produce less power output & torque output. Some aftermarket ECUs enables you to switch between different driving mode. Example, frugal mode, sport mode, track mode. Down tuning of power output will ensure better consumption. Of course you must also step less on the gas pedal & have a relaxing drive like a old man.