I did a DX to CX/VX tranny swap in September in my Civic Coupe. Not a scientific analysis at all, but it looks like it was worth about 2-3 mpg for me or about 6-7%. I feel it was worth it for sure but I was hoping for 5 mpg going into it. Took about 12 hrs total to complete.
I am finally finding time to attempt my swap (Si + HF 5th ---> full HF) in the coming weeks. I'll also be changing out the clutch while I'm there and hopefully keep the high-strung hoss in one piece next semster driving 100mi/day for clinicals Mak
'85 CRX Si Original EW3 @ 254k...suck it Detroit
'89 CRX HF-Z1 ~ When haste is needed
'97 Civic DX Hatch ~ Formerly 600hp, now 0hp, soon 115hp.