That's not bad

 

It all depends on your driving habits.. Some days I'll eat up a tank driving with a heavy foot and get around 15mpg. And days when i baby it and keep at 2100rpms or lower and can average 18-19mpg.

 

I'd go with 4.56s even. You see such a huge performance gain with gears. You will be happy you went to them. I'm running 5.13s on a 35" tire and am turning 2400 at 60 mph with convertor locked (4l80e wit 0.75 od gear ratio) go on Richmond gear and check out their calculator. Ittl make your mind up with what gears you run. I assume you have the 10 bolt rear end, I wouldn't go to low of a gear (5.13s) not much pinion contact. But definitely go with the 4.56s at least and see the smile you have on your face in the rear view mirror when your passing all the diesels from light to light.

 

im running 4.10s with 6speed and 30.6" tires. 2000rpms @ 67mph.