Saturday, March 26, 2011

Acceleration to highway speeds

I was out trying some hypermiling early this morning around town, attempting different methods, and after a couple loops of a disappointing 80 mpg (if you can get 70 mpg on occasion in regular driving, 80 mpg is outright depressing when you're the only car on the road!), I discovered I had a little scrap of paper and a pen in my bag, and decided to head out for some acceleration tests.

I'd been very curious in particular about how to accelerate onto the highway. Anyone attempting good mileage will tell you what a buzzkill it is to accelerate to 60 mph and watch the hard earned mileage bleed off your screen.

So I set out for a limited number of highly unscientific tests, but whose results were clear enough to produce some useful (and for me, surprising) results.

I chose to start accelerating from 10 mph since that would simulate moving slowly as you might at the start of an onramp, but also give you an idea of accelerating from relatively close to zero. I did it from two locations to give myself some data from the roundtrip. You can tell from the numbers below that the first direction is net downhill, and the second net uphill. Both accelerations were on slight uphills leading to the highway. It might be of interest to know that the first one, with net downhill, had a tougher initial acceleration than the other direction, but still came out way ahead.

I decided to measure my acceleration by the percentage of gas pedal depression, programmed into my Scangauge as an XGauge. The margin of error is about 0.5% for this, since it's really very difficult to keep a perfectly steady depression percentage, even when staring at the data right in front of you. As a subjective measure to help everybody else in the world who doesn't have a way of precisely measuring this percentage, I'll break it down like this:

  • 35% depression = stuck behind a granny/school bus driving, or the stereotypical annoying Prius driver holding up traffic
  • 39% depression = normal acceleration for 90% of people out there
  • 45% depression = the asshole who's gunning it past everyone, instant mpg readings in the teens

I reset the Prius' consumption screen just before accelerating, and used that to measure mpgs, since it would be impossible to reset trip data while moving on the Scangauge and then switch to the "Gauge" option quickly enough to make sure the percentage of gas pedal depression was correct. After accelerating I'd switch to cruise control as soon as I reached 50 mph. Other details listed in "notes" in chart below. I picked some signs to go by as my final points, and this is what I got:

I'd like to first address one thing: notice in the downhill run, 45% gas pedal depression, run 1, I got bizarrely better mileage than any of the runs. I have a feeling that the car was in warp stealth at one point in the trip that significantly raised its mileage. I don't know why it did it that time and not on the others. In any case...

Secondly, as a point of interest, there were numerous times in parts of the downhill run where cruise control triggered a no arrows state! Unfortunately the excitement proved too much, and I forgot to check battery current, but RPMs were low at this point (900 something if I remember right), and was not just a warp stealth transition. It would stay like that for 10 seconds.

For the results, even with very few data points, it seems that when a net downhill drive was coming, light acceleration produces the best results. When a net uphill is coming, a very aggressive acceleration is best to get you up to speed. Remember that both onramps were uphill, so I was already up to 50 mph by the time I was even reached the downhill sloping portion of the downhill drive, and yet still light acceleration was better. Of course, as you can see, the difference in end results is so minuscule between your grandma and your local high school idiot, I guess a more practical way to interpret the results would be to not give a damn and just drive the thing!


  1. Dear Mike with a Prius,

    Very interesting observations! I wonder -- are these lessons re: uphill and downhill driving habits specific to the Prius, or can non-Prius drivers take full advantage as well?

  2. Dearest Jason (pronounced clearly with shadow vowels),

    Yes, this would apply for any car. Maybe one day for a little fun we can bring the Scangauge to work in your magical beast (your car, if that wasn't clear).

  3. This is very much in line with my findings, whether you get up to your desired speed at either a slow or fast rate of acceleration, the net amount of fuel used in the end is about the same. Think of it this simplified way, if you accelerate at a slow rate and take twice as long to get to 50 mph, you're comsuming fuel at half the rate for twice as long, in essence, about the same amount of fuel consumed. Yes there is some variation in the ratio of the power developed for the fuel used (g/kWhr), but not that significant especially with the 2010 Prius model onwards. So you might as well gun it or Jack Rabbit start if you need to, as the other fuel saving techniques save far more fuel, i.e. accelerate to a speed that doesn't require subsequent braking, limit your top speed.

  4. Agreed, Brian. I'd add that this is for clear traffic on open road, like a highway. In town, light to moderate acceleration is best, since we encounter so many more stoplights, unexpected cars turning out in front of you, etc. Aggressive acceleration in these situations just ends up being wasted energy as we constantly brake to avoid these obstacles.