My dad's '97 CC SB with 3.55 gears gets 18-20 when he drives it (he's a gentle driver) He gets about 15-16 in his '02 F350 CC LB dually with 4.10s. Both are stock. The difference in fuel economy with the different gear ratios is significant. The gear ratios aren't the only differences between these two specific trucks, but the different gear ratios do account for a lot of the difference in fuel economy.
I would expect to get at least 15-16 out of any PSD.(unless its a 6.4 with a DPF) Many acheive better fuel economy, but it really depends on the specific truck, and most of all your driving habits.