As for your gearing question: do you have a fuel economy gauge?
If you do, you could get a
rough idea of how much a gearing change would affect your fuel economy by comparing MPG at a constant speed in, say, 3rd, 4th & 5th. Plot RPM vs fuel consumption, and then extend the plot into your theoretical new, lower RPM range for that speed.
It will not predict
perfectly what the drop in RPM will accomplish (because engine efficiency for a given load changes as RPM changes, so you're changing multiple variables). But it'll give you a ballpark idea.
EG:
from:
http://www.metrompg.com/posts/rpm-mpg.htm