[Ptero]

Chris, you've made excellent progress with your vacuum gauge in recognizing its value in drafting. 31.5 mpg is fantastic! After THE BIRD (see my profile album), the vacuum gauge is the most valuable instrument for hypermiling. Used in conjunction, you can do even better.

Unfortunately, you can't drive only by the vacuum gauge for optimum mileage (or a Scangauge, for that matter). While the vacuum gauge does provide a correlation to fuel draw, it only provides verification of an instantaneous state and it cannot provide clues on extrapolated momentum.

Momentum is the kenetic energy of your vehicle, which is a function of your mass and speed minus your aerodynamic drag and rolling resistance. It represents the energy investment you have made through energy expenditure and technique to bring your vehicle to it's present speed. The vacuum gauge indicates instantaneously how well you are maintaining that momentum, but it does not help you decide how to deal with upcoming changes in terrain. You have to make decisions in advance to deal with these circumstances. By the time your actions to deal with changing terrain have consequences, the vacuum gauge will simply validate in real time the energy costs of these actions.

In other words, while the vacuum gauge may appear to be a simple instrument on a flat highway under static conditions, it will become a chess game judge under fluctuating conditions.

For instance, absolute economy mountain driving requires utilizing momentum and power to crest the apex of a grade at the lowest speed and lowest expenditure of energy because on the other side of the grade you will trade the kenetic for potential energy and it will be unnecessary to use any fuel. The vacuum gauge cannot tell you, when you reach the base of a grade, what speed and throttle setting will be necessary to optimize the kinetic energy you can gather to reach the crest of the grade using a minimum of fuel. That requires the human element - the extrapolation of momentum. By illustration, driving by the vacuum gauge's optimum economy indication to the base of the grade will entail greater fuel use than if you had gradually increased speed before the grade to build momentum to assist in the climb.

Look at truck drivers. The heavier the truck, the lower the power to weight ratio. Very heavy trucks have so little power available that they reley primarily on momentum when traversing rolling hills - accelerating on the downhill with appropriate fuel delivery to allow them to crest the next rise without falling into lower gears. On steeper grades, they are forced to fall into very low gears, which requires much more fuel than maintaining momentum where possible.

If you are driving for economy, you limit your pedal and emulate the economy technique of much heavier vehicles. You look ahead 1, 2, 3, 4, 5 thousand feet and determine the speed you want to be traveling at that point, then apply the least amount of constant throttle that will get you to that goal. During this, if your guess is good and you hold your throttle steady all the way to your destination point, your vacuum gauge will indicate the minimum amount of work your engine does to reach that point. If you record or remember the gauge reading, you can change your throttle attack for the next hill slightly to see if you can do better. What you cannot do is look at the vacuum gauge and use it to modify your throttle because you committed way back down the road and the vacuum gauge is now only a validation of that decision. It is not an instrument to help you on the grade you are on. It is an instrument that helps you determine your technique the next grade by telling you how well or how poorly your decision on how to assault this grade worked out.

Cars, of course, experience a much more rapid deterioration of momentum on a grade than heavy trucks. The best technique after momentum is lost is CONSTANT THROTTLE. This is where you hold your foot steady and let the grade determine your speed. I discuss this at length in MY INTRODUCTION TO THE SMART CAR on the Hypermiling thread.

Are we clear? See, vacuum gauges challenge intellect. They're a lot more fun than you thought!