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!
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Ptero
Last edited by Ptero; 10-25-2008 at 06:11 PM..
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Which is better when going up mountains?
I know economy really goes out the window on this one, but I figure I'd throw this out there..
higher rpm's with high vac #'s or lower rpm's with lower vac #'s?
I use higher gear which gives lower rpm and lower vacuum reading. I just took a trip over the weekend that was 610.4 miles round trip and used 12.423 gallons of gas in an '88 Escort. This comes out to an average of 49.13 mpg and about 100 miles of this was mountains in eastern TN and western NC on I-40. I was normally driving 55 and ocasionally reaching speeds of 60-70. There were two passengers in the car with me and a tool box in the trunk that weighed about 100 pounds. The only time I used a lower gear was when I needed the power.
Okay, So the best MPG while cruising for a particular vehicle is the speed at which vacuum is the HIGHEST?
In other words:
65mph @ 14 vacuum > 50mph @ 12 vacuum
ERIK
You want the highest vacuum reading in the highest gear possible. Your example would mean that the 65 MPH would be best, but it's not likely that is the way it would actually work out, because you'll probably be at lower vacuum at 65 because the engine will probably be working harder to maintain 65 MPH than 50 MPH. The load on the engine is what controls the vacuum.
Hypermilers like abbreviations I don't know why - but it has allowed the term CODFISH to exist Goofy
I didn't have anything useful to add to this thread except a new abbreviation... I got this one from working on computers all the time... it's a techie term, adapted to this environment, and pretty much explains most issues with cars:
Problem Exists Between Dash And Driver's Seat
The actual phrase I translated into that was "PEBKAC" "Problem Exists Between Keyboard And Chair", which we used as a nice way of telling each other "Man, this computer user is a fsckin idiot."
Usually applies when people talk about things they don't understand, then get offended when you tell them they are making it obvious that they have no idea what they're talking about.
You spent more on your new tonneau than a used SG would cost. Guess which one I would have recommended as the best way to improve your overall MPG?
So wrong MetroMPG. I had two pickups and put tonneau covers on both. That box is like a big parachute back there. BIG diffrence in milliage.
__________________ "The Stone Age did not come to an end because we had a lack of stones, and the oil age will not come to an end because we have a lack of oil" ; His Excellency Sheikh Ahmed Zaki Yamani (Saudi Arabia Oil Minister from 1962 to 1986)
I have 215,000mi on my Aspire, I can barely get acceleration at 10in. I have to drive pretty much at 6-8, I still get good mileage, but is this an indication my motor pretty much had enough, no oil consumption, just seems tired??
__________________ "The Stone Age did not come to an end because we had a lack of stones, and the oil age will not come to an end because we have a lack of oil" ; His Excellency Sheikh Ahmed Zaki Yamani (Saudi Arabia Oil Minister from 1962 to 1986)
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I don't know why - but it has allowed the term CODFISH to exist 
Goofy 
