1990 Civic Hatch, I'm looking for mods that provide better MPG
 

I am back here after a 6 year hiatus. You can search my old posts about my collection of Geo Metros (hence my user name) In the interim I have had some fun with a classic gas guzzler which I recently sold and replaced with a 1990 Honda Civic Si.

The new car is in great shape and with the exception of adding cruise control it is stock. The one thing that stands out to me is that the top gear seems too short, at 60 mph the engine is doing 3000 rpm. Is there a swap or a mod I can do to get a taller 5th gear? I am thinking this would improve my mileage, are there any flaws in that thinking? My commute has very little elevation changes, mostly flat highways. I would be interested in hearing about any modifications that have been effective.

How does power feel doing 60 at 3000? If there's more power to spare on tap without downshifting than you feel like you need then there's a good chance that a taller 5th or final drive will help. This is pretty likely with the Si. I think I remember that someone has done it to an early Si before so you might be able to find the example in old posts and see what was used/how it worked out.

Find a hf manual trans acceleration will w sacrificed but fe will increase.