There have been several good points made in this thread about varying ways to measure efficiency.
One that I feel is missing is that some people are discussing "optimal efficiency" in a scientific or technical sense: i.e. the engine is doing the most work per (something), where (something) could be a unit of time, a revolution of the engine, a unit of fuel, or something else. That is the true "peak efficiency" of a vehicle. However, it is not the way to get the best mileage.
Example: For you vehicle to be operating at its absolute peak efficiency, you would have to be at the best spot on your BSFC map (probably peak torque), in your top gear, at WOT (wide open throttle), in perfect environmental conditions, AND you would have to have that much load on the vehicle (i.e. pulling a trailer uphill, or something). In other words, "true peak efficiency" never happens. Nor should we really care what it is (the engineers designing the vehicle probably did, though).
All we (members of GasSavers.org) really care about is fuel efficiency, which can be achieved by following some basic rules:
1) Whenever reasonable, operate in your highest gear, usually 5th
1a) If you vehicle has an automatic transmission with tc-lockup, operate in lockup whenever possible.
2) Keep your engine speed (RPMs) as low as is reasonably possible. The most commonly recommended shift-point I see is 2500RPM, but that will vary from vehicle to vehicle.
3) Keep your vehicle speed (MPG) and acceleration within reason. You don't want to be either the tortoise or the hare. "All things in moderation".
4) Don't use the brakes if you don't have to. "Predictive" driving helps a lot with this. Pay attention to when traffic lights change and to what the cars in front of you are doing. Keep a large buffer space between yourself and the car in front of you--this lets you slow down by simply letting off the gas instead of having to hit the brakes every time there is a minor change in the speed of traffic.
Corollary to 3 & 4) Use smooth motions with the pedals, not sharp jabs. Some people call this "driving like there is an egg under the pedal". Brake sooner and with less pressure, and accelerate smoothly instead of short bursts.
Everything after that is nuance or minutia. There are small tweaks that will bring small gains, and large tweaks that will bring small gains. Much time is spent on this website seeking to eek out the last 2-3% that can be found using these nuances and minutia, so sometimes newcomers may not realize that you need to practice basic efficient driving until you are proficient at that before trying to maximize with these nuances. If you think you always need to be at 50MPH for optimal efficiency, that may lead to poor choices try to get to (or maintain) that speed, and can leave you at a loss for what the best choice is when 50MPH is not a safe (or legal) choice.
Sorry... I'll get down off the soap-box now.
No slight is intended against any of the previous posters.
Peak Torque vs TC Lockup
Linear Mode
