Adventures in Hypermiling

Hi, I'm MIT Mommy.  I am an MIT alum, a mom and I brainwash engineer my kids.  No wait!  There can be only one.  Will the real MIT Mommy please stand up?  The truth is that this impersonator is the ever geeky Angela at mommy bytes.

While the real MIT Mommy is off guzzling gas on vacation, this MIT mommy obsesses over gas mileage with her spreadsheets.  Ever since the days of Lotus 1-2-3 (for those who remember the DOS days), I have kept spreadsheets for every car that I've owned.  I track every tank of gas going into each car, including the number of gallons and price per gallon, in order to track gas mileage and gas price per mile.  Here is a glimpse into my insanity.

When gas prices skyrocketed last year, I realized that I could create my own chart of gas prices because I had years of data!  Here is the price per gallon for regular gas in New England over the past 8 years.



From this chart, you can see that in December 2008, gas prices returned to their lowest levels since 2003, but they are creeping back up again.  When gas prices hit $4 per gallon last year, a friend of mine told me how he has been getting 20% higher gas mileage by hypermiling.  There are moderate to extreme techniques to hypermiling, but the one that fascinated me the most was to coast down hills in neutral. 

In my car (a BMW 325 xiT), there is an instantaneous gas mileage gauge at the bottom of the tachometer as well as an average mileage readout.  When I coast down hills in gear, the instantaneous gauge is pinned to the extreme left (100+ mpg?).  But when I coast in neutral, the gauge reads somewhere around 80 mpg.  From these observations, I assumed that it took more gas to spin the engine in neutral because of the nominal load of the engine friction.  However, it only spins at idle speed of 600 rpm, as opposed to 2k-3k when in gear.  All those extra cycles eat up more gas even though the transmission is helping the engine, right?  Obviously, empirical testing was in order.


(I took this photo while driving, do not attempt this at home unless you are a professional MIT mommy )

It is absolutely true that you get better gas mileage when you coast down hills in neutral.  It could be that you go much farther without having to apply gas again than you would in gear.  It could be that the gauge isn't telling the whole truth.  Either way, I have taken to coasting down hills in neutral whenever I can.

There is a long rolling hill on the way home from work that I used to just drive in gear, applying gas without thinking about it.  Now I know that when I see a particular Blind Driveway sign, it's time to shift into neutral.  I usually start out around 40 mph and coast up and down this rolling hill, with my speed varying between 35 and 45 mph.  I leave enough space between the car in front of me because nothing annoys me more than cars that brake going downhill and accelerate going uphill, the complete opposite of hypermiling (would that be hypomiling?).  This road ends going uphill, where I have to stop and turn.  If there are no cars behind me, I try to coast all the way to the stop sign.  When the car slows to 25 mph, it feels deathly slow.  At 20 mph, you can see me nudging my seat to get the car to move faster.  But it does eventually reach the stop sign, even though the last 50 feet seem to take hours.  Usually a car shows up behind me, forcing me to use some gas and saving me from hypermiling insanity.

In all the other hypermiling areas, I fail miserably.  I can't accelerate from a stop slowly (other than being obsessive with spreadsheets, I like to do things fast fast fast).  I don't think it is safe to draft larger vehicles or to over-inflate tires.  It is way too much bother to shut off the car at lights.  But I do like taking ramps at close to full speed, wouldn't that be the point of having a BMW?

Hopefully, the gas prices won't spike during the real MIT Mommy's travels and she will have fun but sane stories to share with us.  Happy trails everyone!