Gear ratio for better fuel econo.

[swampbuck]

I have a 2000 suburban 2500 with a 6.0L. It has 4:10s, narrow 35 inch tires, a K&N intake and a true duel exhaust. No matter how easy I drive it (70-75mph down the e-way for long distances) all I get is 10 mpg. I don't plan on towing anything heavy and I have a 3:73? front end (but will need to find a matching rear) I could put in but would that help? Btw I'm turning about 2000rpm at 55-60 mph. Is there a chip that would really help? I drive almost no city, 99% back country roads (55) and highway (70-75). Any help would be great.