Quick fix for gas mileage!

June 18th, 2008 by Chris

Yes, there really is a silver bullet. Well, at least a rather shiny metallic bullet of some kind.

I have recorded the number of gallons and trip mileage for EVERY tank of gas I ever put in my car (361 tanks as of today). This is a habit that I picked up from my Mom, which generally serves little or no purpose other than monitoring sudden changes in engine efficiency. It is not my Mom’s fault (at least directly) that I bothered to enter this data into a spreadsheet and analyze it. Anyways, it is because of this record that I am able to somewhat conclusively report that there is an easy way to improve your gas mileage.


It’s really quite simple: Drive slower.

For the last 3 tanks of gas I have been making a conscious effort to drive at the speed limit and to avoid quick starts and stops. It’s clear that accelerating slowly and drifting into stops will reduce the pressure on the gas pedal, but how much difference can it really make?

I drive a 1997 Acura Integra which at the last fill up had reached 108,200 miles. Over the entire life of my car, it has averaged 29.1 mpg. The sample deviation is 2.29 mpg so we’d expect 95% of all mileage to be between 24.5 and 33.7 mpg (assuming a normal distribution, which looks pretty good from the histogram).

My last three tanks of gas have averaged 34.1 mpg (32.1, 34.9 and 35.4 mpg respectively). That’s a 17% improvement at face value. I think it’s fair to say that the statistical likelihood of this from pure chance is quite low*. Two of these are amongst the 5 best tanks of gas in the life of the car (39.5 holds the record**), the third is in the top 10%. In the interest of full disclosure I DID just have a major service at about 107k (timing chain, valve adjustments, spark plugs, etc), which undoubtedly contributes to some degree. Of course, I’ve had major tuneups before and these results still indicate significantly improved efficiency on average.

I’m also improving the mileage (but probably not the blood pressure) of my fellow commuters as I seem to end up with a fair number stacked up behind me now. I find myself waving to the SUVs and trucks that pass me climbing up the big hill on my trip home. I assume they must have Exxon-Mobil stock options.

* I’ll leave it to the real stats geeks to figure out the statistical ‘strength’ of this claim.
** This could be some kind of recording error, or a long road trip with lots of highway driving. This was relatively early in the life of the car when we would have been more prone to doing that. The top five are: 39.5, 37.0, 36.3, 35.4 and 34.9

4 Responses to “Quick fix for gas mileage!”

  1. Bill Ruhsam Says:

    Congratulations. You’re turning yourself into a Hypermiler

  2. Stephen Says:

    I am so tempted to say “Duh!”, but I won’t. I applaud the blog entry. Between the obvious RPMs saved and the not so obvious wind-resistance factor, slowing down is a great idea. Another one of my favorites this time of year is at lower speeds don’t use AC if you can stand it. Windows down is better. The slow speeds don’t add enough drag for most cars to trump the AC. At higher speeds light AC is probably more efficient. I’m sure a ton of other simple tricks could be googled given the gas price ‘crisis’.

  3. annie Says:

    Ooh, a mid-morning z-test!

    Given the numbers you provide, there is statistically less than .0002 probability that your mileage on the latest 3 tanks represent the population of mileages over the life of the car.

    However, in terms of empirical generalizability, you’d have a much stronger argument if your test sample is larger than 3.

  4. Chris Says:

    Thanks Annie! I suspected it was pretty unlikely. I’m working on sample number four now… 🙂

    I suppose I could go back to ‘more aggressive driving’ for a few tanks to show that I can make the mileage go back to ‘normal’.