I found this forum while searching for HHO information. My test vehicle is something that most of you will probably hate me for, though.
I have an HHO generator installed in a 1984 Dodge D150 longbed pickup truck. 225 CID slant 6 with a A833 4 speed manual transmission. The Chrysler lean burn computer has been removed and the ignition has been upgraded to points and condenser. The Carb needs a rebuild.
Without the HHO, it's getting about 17-18 MPG over the road. With the HHO, it's getting right at 20 MPG. I have noticed that with the HHO generator running, the truck has more power to pull hills. I'm hoping that once I get the carb rebuilt (parts are on order) I'll be able to increase the fuel economy even more...and get my eyes to stop watering at a traffic light.
I've had several Dodge trucks with the 225 in them. When running correctly, they can easily get 19-20 MPG. BUT, that was when gasoline was really gasoline. I'm trying for 25+ MPG out of the truck.
17-18 is pretty impressive on its own. Is that slant six just a very efficient motor for that truck, is the whole truck designed well for efficiency (tall gears, anyone?), or do you practice hypermiling driving techniques?
The slant 6 is a pretty efficient motor. I've had several in various cars and trucks with the 225 in them. 20 mpg hwy for a 1/2 ton truck with a 225 and a manual trans is about right if it's running correctly. Not sure about with the oxegenated stuff they call gas now, though. My current truck needs a carb overhaul, so the mileage is down. I think Dodge quit making the slant 6 because it never broke. Can't sell too many cars when the engine lasts 300,000 miles with just basic maintenance.
As for hypermiling, I don't consiously practice it. I coast to lights and let the transmission slow me down instead of speeding up to lights and slamming on the brakes and then having to accellerate hard to pull away. But I do that more to save excessive wear on the vehicle than for fuel economy. I also travel at road speed instead of zipping around everyone.
Other than experimenting with the HHO, I guess that the only thing that I really actively do to increase fuel economy is that I refuse to purchase a vehicle with an automatic transmission. Automatic transmissions raise engine RPMs 600-1200 RPM for any given speed until they hit overdrive (if they even have overdrive). That drastically drops fuel economy for in town driving and for highway driving if they don't have overdrive.
You're right, the manual transmission definitely makes a difference. I can see 20 being pretty normal for that truck, even with E10 fuel.
Modern automatics have closed the gap pretty well for average driving styles. For drivers specifically trying to get better economy in city or mixed driving, the manual transmission still can't be beat, although I think modern automatics tend to come with a taller overdrive gear than modern manuals.
If you coast (or brake) up to those lights in neutral instead of in gear, you'll save some fuel.