[piers.singer]

First thing would be to get some instrumentation (such as an MPGuino, seeing as your car won't be OBDII) that will give you an instant fuel consumption read-out. Then you can decide for yourself what speed works best to achieve your target fuel economy.

Finding an overdrive for an FWD car (which I assume the Integra to be) is pretty much impossible, if any exist at all, so your best bet would be to substitute a different gearbox with more suitable ratios and/or a taller diff ratio. It might also be possible to swap over a diff only, but due to the compact nature of FWD transmissions, the plausibility of such an undertaking will depend almost entirely on your ability to be adventurous and get away with it.