Rear end swap?

[primerib]

My truck is a daily driver. It has a 250 and a 4.11 rear end. The mileage is pretty lousy (about 13 mpg). I'm looking at accepting a job that would require me to drive 230 miles of highway a couple of times a week. The old girl is wound-up awful tight at anything above 50 mph, and this stretch of road is a 70mph zone.

If i were to swap to a better ratio (like a 3.08) would my mileage be significantly better or would it not make much difference.

Thanks