19mpg is pretty decent for your Tahoe. Here's a summary of a few things that can help, and do help in my 2002 GMC Sierra 5.3:
1. Tire pressure increase. I run 80psi all around, but my tires are rated for that high pressure. You can probably go as high as your tires stamped max pressure and not have any consequences, just improvements.
2. Heavy on the gas pedal, low on the RPMs. That is not really possible with an automatic; in my automatic truck, I get the best mileage by accelerating somewhat briskly, and letting off the gas pedal to make it shift then getting on the gas pedal again to continue accelerating.
3. I don't idle any more than necessary. This one is pretty obvious.
4. I try to maximize DFCO (Deceleration Fuel Cut Off) usage. When going downhill, coasting, or approaching a slow/stop, my foot is entirely off the gas pedal and the car is in gear, and the road is turning the engine -- basically, it's engine braking, but I do it in a high enough gear that it doesn't slow the car much. In the meantime, the engine doesn't need gas to keep turning, since the road is turning the engine; and so the computer shuts off the fuel.
This is easier in my VW which has a manual transmission and I'm pretty sure DFCO happens all the way down to 1000 rpm. I think it may stop DFCO at 1800 RPM in the GM V8, I read that number for a Cavalier's 4 banger. Eventually I'll get a ScanGauge II and find out for myself. Anyway, while approaching a red light or coasting downhill I often downshift to try to keep it above 2000 rpm.
5. I try to drive steady by making the best use of synchronized traffic lights, not slowing for turns (fun!), and leaving space between me and the next vehicle. Any time you use the brakes, you're discarding energy that you already paid for by spending gas to accelerate before. So, instead of going 60mph on a 40mph road and having to stop at every traffic light, I go 45 and sail through green lights. When a car in front of me is braking and accelerating to keep close to slow traffic in front, I go at a steady speed and let the gap close and open.
6. Put it in Neutral while stopped at red lights so that it's not using fuel to idle against the torque converter.
Here's some things I haven't tried yet. On the highway, it's surprisingly easy to draft tractor trailers; read the "
To draft or to slow down" thread. There's EOC (Engine Off Coasting), where you shut the engine off while driving; this probably is not practical or safe in our trucks. There's Pulse & Glide, where you accelerate to a higher speed than you want, then coast (in neutral, or using DFCO, or EOC) until you're going slower than you want, then (if necessary) restart and accelerate again.
Also, there's aerodynamic mods. I haven't done any yet, but there's various ways to cover the grill, use aero hubcaps, etc. I have removed my FM antenna, primarily for reasons other than aerodynamic drag reduction.
If you're interested in getting a different vehicle, and you really like the seating/driver position in the GM full size truck/SUV, check out the VW Rabbit/GTI/Jetta. I was amazed when I sat in the Rabbit and it felt exactly like sitting in the truck except my knees were straighter. The seat felt similar and my arms went to exactly the same positions.
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