[redpoint5]

I haven't seen anyone directly address some of these misconceptions, but Old Mech's post was very accurate and informative. Here goes...

Originally Posted by Mulder25

Anyways, the more you press the pedal, the higher your GPH will go, regardless of engine RPM, it increases the amount of fuel injected into the cylinder. At no point, can 80% of your max fuel rate be better than 20%... i just cannot imagine that 80% throttle in high gear would be better than 20% throttle in a lower gear.

You would think that less throttle position would also mean less fuel consumption, but that's only half the story! Most people imagine the throttle as a valve on a water facet, with a constant flow that is dependent upon how open the valve is. This isn't the full story though. Engines don't flow a constant stream of fuel, instead they consume pulses of fuel during each injection event.

The throttle is a valve, but it controls air flow. When the throttle is opened, more air is allowed in the cylinder and proportionally more fuel is injected. So throttle position addresses the question of how much air (and fuel), but it doesn't address the question of how often. Fuel must be injected into each cylinder every 2 revolutions of the crank on a 4-stroke engine. At 2000rpm each cylinder is requiring 1000 fuel injections per minute. At 4000rpm, each cylinder is requiring 2000 fuel injections per minute; that's double the amount of injection events!

While dropping down a gear may allow a lower throttle position and less fuel injected into the cylinder, it is increasing the rpm and requiring that lower amount of fuel more frequently. As others have pointed out, more rpm means more energy lost due to friction as these parts move faster (further).

It is more efficient to have big explosion events less frequently, than to have smaller explosion events more frequently. Remember that in a 4-stroke engine, only 1 of those strokes produces power. Double the engine speed and you are doubling the amount of intake, compression, and exhaust events; all of which produce no power but instead waste power due to friction.

Originally Posted by Mulder25

I always try to keep my RPMs low, and my throttle position low. I avoid higher than 3,000 RPM and try to keep my TPS less than 19(starts at 10, goes to 90) while cruising.

By now you should know that keeping RPM low but throttle high is more efficient.

Originally Posted by Tesla

...because the engine is not going fast enough to consume what you are giving it, better to change down and let the engine rev up and breath properly.

You have probably already arrived at the answer to this. The engine "breaths" best when the throttle is most open.

Originally Posted by Tesla

Surely running in a higher gear with low engine speed and a more open throttle would result in lower vacuum & hence lower economy, only one concept can be right.

All correct except the last part. Result is higher economy.