well... u should be able to get a mileage increase just going to a taller tire... as it will make your gearing taller... slower acceleration but lower RPM for a given speed...
however... i dont know how much extra MPG you can expect squeeze out of a honda like ours... depending on what type of engine you have....
the reason why i say this is that there is a page that probably many of you have seen..describing the ZC/Si hybrid trans...and the guy in question ended up trying something like 3 different 5th gears...and he put some miser gear in for 5th... said the gap between 4th and 5th sucked...it dropped his RPMs way too much... and (here is the kicker)... his MPG didnt change worth shit....
u can always swap in a HF drive train

he he ... it seems that honda had to make some pretty drastic changes in order to squeeze out that insane mpg figure... gearing... mpfi with a really crappy ass cam to match the gearing.. and a 8V head... and its probably tuned to run pretty lean...
if you drive in a conservative manner... somewhere in the neighborhood of ~35 mpg should be very easy on almost any of our cars...
does anyone really need much more so bad that its worth killing the performance of the car ?
oh well..anyways..enough speculation...the HF does prove however that if you do some somewhat drastic tweaks that insane MPG is very possible.....
by the way... if im not mistaken, the name sake of the Civic is the old no longer used CVCC engine design... aka "Compound Vortex Controlled Combustion"... aka charge stratification....
where the main intake valve would let in the regular fuel mixture...which was set up to be extremely lean... so lean that the car wouldnt run properly on it...but there was a significantly smaller valve located as such that a small portion of rich mixture could be placed near the spark plug.... giving an uneven distribution of atomized fuel ... allowing it to ignite the rich mixture around the spark plug and burn off the rest of the surrounding lean mixture... causing the car to run normally despite the otherwise too lean mixture... producing lower emissions and much higher gas mileage....