'70 Suburban fuel milege

[Christopher]

I have a '70 Suburban with a completely stock 292 L6 and 4 speed manual transmission, with towing gears (rpm is around 3000 at 65 mph) I would like to increase my fuel milege, it is aound 14 mpg now driving very carefully at 55 mph. What would be the best way to improve the milege? I would like to have a higher gear ratio so the engine wouldn't be revving so high at highway speeds, but I still want to be able to tow with it (it is a 3/4 ton and tows like a tractor now, I love towing with it) I probbably will put headers on it, any other suggestions? Thanks a bunch, Christopher