Bart:
There's a fuel filter underneath the lift pump on our 12 valves. It's designed to catch the coarser trash from the fuel tank before the fuel is lifted up and run through the second filter under the hood. When the fuel is supplied to the injector pump, any excess is returned to the tank via the "overflow valve," and as this valve ages it also weakens. In addition to its other functions, the overflow valve helps to maintain a consistent pressure (pulling this imperfect explanation from memory) over the whole fuel-delivery system. If the valve weakens too much, your fuel pressure drops off and your mileage will suffer.
My overflow valve had upwards of 150,000 miles on it, so -- when my mileage/performance began to suffer -- I assumed that the overflow valve might have been the culprit. Changing out the valve takes about ten-fifteen minutes and costs about fifty bucks, so it's a no-brainer.
As far as the rest of the maintenance components mentioned (pre-filter, fuel filter, air filter, etc. ) everything's a "continuum," from an efficiency standpoint. Your filters get progressively dirtier from the day you install them, and you could replace them every month if you wanted to see optimum performance from your vehicle. The reasons for not doing this are obvious: The filters are, themselves, expensive, and their optimal service intervals were figured out by engineers smarter than I. What I suspect, however, is that those service intervals were calculated in the era of 50¢ diesel fuel, and with that same fuel now 6-700% more costly, I wonder if it makes sense to change/clean them with greater frequency. . ?
The point I was trying to make in my earlier post is that there are a lot of decisions that effect our gas mileage way, WAY!, more critically than, say, the installation of a 4" exhaust. I can't help being tickled by the guys who wile away long hours (and I include myself in this group, more often than not... ) fretting about poor gas mileage, but they've hung everything but The Great Wall of China on their rigs (camper tops, racks, brush guards, lights, winches, tool boxes, aux fuel tanks, etc. , etc. )
Fancy exhausts, exhaust manifolds, larger air filters (etc. , ad nauseam) are expensive and amount to a gnat-fart in a hurricane, when confronted with the challenge of resurrecting fuel economy from a vehicle that's undergone the above-described, after-market "transformation".
I love my rig, but it's carrying a ton of tools and another ton of aftermarket crap that no wind-tunnel ever blew smoke across in Detroit.
Attention: TDR Forum Junkies 
