Kinetics, coasting, and you

[Loren]

You can't quantify how much mpg difference it will make over a tank of fuel because you don't know how much time any given person spends sitting at idle. But, you can reliably state that everyone gets "zero MPG" while their car is at rest idling.

Without the quantity (how much time is spent idling?) we can't quantify.

To quantify with any degree of accuracy would be a hassle, too. One would either need to carry a passenger with a stopwatch and a notepad and do several lengthy experiments or take video of same and spend hours reviewing it.

Those of us who have spent time with a ScanGauge honing our driving technique for best MPG already know the answer.

[sbergman27]

Originally Posted by Loren

You can't quantify how much mpg difference it will make over a tank of fuel because you don't know how much time any given person spends sitting at idle.

Last week, individuals certainly seemed capable of making back-of-the-envelope estimates defending their own level of DFCO usage. But this week it is impossible for a person to make a ballpark estimate of how often his engine is at idle?

But, you can reliably state that everyone gets "zero MPG" while their car is at rest idling.

Which is *technically* true, but *practically* meaningless until it is put into the context of total quantity of fuel used.

ScangaugeII does automatically keep track of total fuel used, total miles, and average speed driven on a per day basis. (Among other things.) Say you drive 50 miles in a day, use 1.6 gallons of fuel, and have an average speed of 30 mph. For idling to represent 10% of total fuel consumption, you'd have to be moving for 40 minutes at an average speed of 75 mph, be stopped at idle for an 60 minutes, and get 34.7 mpg while you are moving, for a combined value of 31.25 mpg.

It's certainly possible for a person to ask himself if he thinks he spent 60 minutes idling, and only 40 minutes driving that day... without having to hire on a dedicated stopwatch operator. ;-)

I pulled those numbers completely out of the air, of course. The point being that you can get creative and extract very interesting and useful information out of your ScangaugeII data if you don't just throw up your hands and say that it's impossible, or too hard.

Those of us who have spent time with a ScanGauge honing our driving technique for best MPG already know the answer.

Or at least you think you do. I've seen too many claims of a switch from 5w30 to 0w20 oil yielding an FE improvement of 5%, or jacking tire pressure up to 60 psi yielding 10%, to take anecdotal evidence seriously. Considering wind, temperature, fuel composition (E0, E10, winter, summer) rain and snow, hills, passenger load, etc., fuel mileage logs are mostly noise, and people attribute day to day variations to whatever is on their minds at the time that they expect to make a difference in a particular direction. It's a lot like primitive people's attributing a drought to the displeasure of a particular god.

-Steve

P.S. I should probably also mention that from what I've read of what you've posted on the topic, you seem to drive in a very sensible and reasonable way. Probably a lot like I do, in the city at least. Few people spend as much time on the highway as I do. Highway driving is much simpler to analyze, BTW, which is nice.