I have a 94 4runner with the 3.0 V6, manual trans, 31s with 4:56 gears and auto hubs. I get 15-16 MPG at 75-80 on long trips with cruise control set.

What can I do to improve on this?

My old 88 4Runner had about 16-17 mph with 22RE, 5 speed 31x12.50 Boggers and 4:11 gears and manual hubs.

I was thinking putting on Manual hubs, and changing to synthetic gear oil. Will this improve my MPG?
what else can I do?