[Billett]

My 6.0 lwb got 14 without a tune so I'm sure you can hit 20 with some decent mods and a good tune on a 5.3/4.8. I'm shooting for 18 once mines back from the shop. Look into glide driving that the eco-guys do in priuses. Its actually more efficient than driving with the proverbial eggshell under the pedal.