[Diesel_Dave]

Just to get a perspective on things, we're talking about 2,356 mi on two 37.2 gal tanks. That equates to a 31.7 mpg average.

From my own experience with a modern diesel, I can tell you that you're really pushing things to go that far. I'm not saying it's impossible--just saying that it's going to be a real challenge (which is where the fun comes in of course!)

The way I see it you can divide all the tools available to reach your goal into the following categories:
1) Drivng mods
2) Engine efficiency mods
3) Aero mods
4) Reduced rolling resistance mods
5) Reduced accessory load mods
6) Gear train efficiency mods

1) Driving mods
This is probably where you can gain the most. You do have 2 things working against you. First, driving mods are easier when you're on familiar roads--over time you learn when to shift, what's around the next curve, how the different red lights are set up, etc. Second, 55 mph as an average speed is actually quite high. I think the average speed in the EPA highway drive cycle is about 45 mph (as I recall). In order to average 55 mph, you're going to have to spend quite a bit of time up at at least 65 mph--aero gets really significant when you get up that fast.

In keeping with your spirit of keeping low emissions, keep in mind that if you do pulse and glide either with engine off coasting or coasting in neutral, your catalytic converter will likely run at a relatively cold temperature where it functions very poorly. This doesn't affect your ability to pass an emissions test but would increase your actual emissions produced.

2) Engine efficiency mods
Ruling out things that either increase emissions or cost a significant amount, there's not much that can be done here. Maybe a different intake, better flowing exhaust, some low viscosity oil or something like that, but that won't buy very much FE.

3) Aero mods
There's quite a bit of benefit that can be had here (especially considering the high speeds). Grill block, belly pan, smooth wheel covers, and fender skirts can all give you some benefit. The biggest aero benefit on a pickup is what's over the bed, but it sounds like you've ruled that out. That's going to make life difficult. You might want to reconsider this. I know you're trying to preserve functionality of the truck, but I think there may be ways to have an aerocap of some kind that would be easily removable by one person (such as a cap in 3 sections or ribs covered by canvass).

4) Reduced rolling resistance mods
Tires are about all you got here. They'll buy you a little but not a ton. There's maybe a tiny bit as well from weight reduction as well, but there's not a whole lot of non-essential parts that you can take off the truck. And with the high speeds, the rolling resistance will play a lower role than aero.

5) Reduced accessory load mods
There might be a little to be gained here too. An electric fan comes to mind. With those high vehicle speeds, it will be off most of the time. Of course, you'll have to balance this with the grill block. You might be able to do some smart alternator switching, too.

6) Geartrain efficiency mods
There might be some gains here, of course these can get kind of pricey. You might be able to run a differnt tranny or differnt rear end gear that would lower your rpms some at cruise speed. Synthetic fluids might buy a tiny bit as well.


I'm sure I'm probably missing a few things, but that how I see things. I wish you all the best!

P.S. You'll find a few pickup haters on this site, but I think most folks will appreciate what you're trying to do.