A lot of it is in how you drive, when you drive. I've been on a quest for better mileage over my daily commute. It's about 50% city, 50% hwy in terms of time. Before this came up, I was averaging about 12.7, according to the meter. And I wasn't hotrodding much, mostly just keeping up with traffic.
So I started trying different things with the meter reset, to see what made a difference. What I found was interesting.
Easy starts was the big one. These trucks weigh 3 to 4 tons, it takes some juice to get them moving. So I kept the boost under 10psi, just took it easy, and saw a huge improvement. Granted, you'll be getting slow starts, but I found it added maybe two minutes to a 30 minute drive, and mileage jumped up to over 16. I also find that when I'm not caught up in trying to make the fastest commute, I arrive in a much better frame of mind.
Avoiding rush hour traffic. I leave fifteen minutes early, avoid the 8:00am crunch and the stop and go. Boss is impressed that I'm there before he is, my 32 minute commute dropped to 28-29, and mileage jumped up to 18.4 And this is a F350 with 4x4 and dually rear.
Knowing when to plug in. I have to pull a bit of a hill leaving my house. Mileage sucked there, especially on cold but not freezing mornings. So I ran tests of plugging in for two hours, and not plugging in when the night temp was around 40. Without plugging in, five to six minute warmup, I showed 5mpg by the time I got past the hill. That includes the fuel used for warmup - these PS's seem to be a bit thirsty when idling. With a two hour plugin, thirty second warmup, it showed 11 mpg pulling the hill. 2 hrs@1500watts=3kw, @11c/kwh = .33. Fuel saving appears to be around 1/4 gallon, which now is $1. It pays to plug in when the temp is in the 40's.
Bottom line - I got a $75 weekly fuel bill back down to a little under $50, average mpg commute only is high 17's to low 18's, average with side trips is around 15.5. No mods, still driving the same distance, just paying attention to detail.