I want to replace the transmission on my 91 HF to gain maximum fuel efficiancy. In 91 55mph was the freeway speed limit and so the 5th gear RPM was set to run at 2.5k @ 55mph which gave about 45 mpg.

Right now when I am at 65mph (current speed limit) in 5th gear my RPM's are at about 3.5k. I want to bring that down to 2.5k at 65mph in 5th gear. so to my question...

What is the best way to achieve this?

Are there any other transmissions that I can use from newer than 91 Civics that will bolt up to the HF engine? For example, will an EK Transmission bolt up? or could I have a shop replace the gear set for the 5th gear?

 

not sure exactly what your options are but remeber unless you do a conversion you'll have to stick with another cable trans.