My 98 Ram was supposed to get 16 hwy and around 12 city stock from the factory, don't really know, I modified it as soon as I got it home. I do know that it has been getting 26-27 highway (at 65-70 mph) and in the 17 range in the city ever since I have had it. I put true duals with no mufflers or cats on it, added a ram air setup, use 8 mm wires, platinum +4 plugs, 4 degree advanced timing, and K&N air filter. For me at least, the sound is actually a good indicator of the fuel econony. With no mufflers at all and true straight pipes, you can really hear what the engine is doing. A loud roar indicates acelleration, and probably none to good fuel economy, when she's in the cruising 'sweet spot' those pipes will quiet down to barely a whisper as you're going down the highway. I have her engine set up to run at under 140 degrees, so even with the timing advance, I can still run 87 octane with no pinging. I tune it up every year, and usually run marvel mystery oil in my fuel. I usually seafoam it once a year too, only rather than sucking seafoam into the engine through a vacuum hose, I simply add the gallon jug of seafoam to a tank of fuel before going on a long trip. There's nothing in the stuff that would hurt engine or fuel system components, and the concentration is enough to break up any carbon or varnish formations on the injectors or pistons/valves. Oh, and it burns cleanly at this concentration and does not smoke.
I make quite a few road trips, and have found out that speed is a big factor, above 70 mph, the mileage decreases rapidly. If I maintain speeds of 85 mph and above on a trip, I estimated my mpg at around 16-18. And stop and go traffic at speeds under 40 really eat up the fuel too. I have the 26 gallon fuel tank, and if I'm doing mostly highway driving and keep her just under 70, I'll get just around 700 miles to a tank of fuel. Unfortunately, i rarely keep my foot out of it or keep from roasting the tires long enough to realize that kind of fuel economy, lol.
Dave