Myth Busters - MPG Trend Over Time
 

Decided on this slow afternoon at work to put to the test the claims by a few on here that MPG will increase dramatically (i.e. more than 5 mpg) after the break in period. I'll admit I was very curious myself as mine has gone up only 1-2 mpg over 3,000 miles.

I went on Fuelly and downloaded the data of the first 10 2012 Focus owners I found with at least 30 fillups, meaning they're probably approaching 10,000 miles.

I made sure to start with the first fill up they entered, which I assume is very soon after they bought the car new. And I went up to the first 50 fill ups if they had more than 50.

Here is a graph of what I found. Pay no attention to the spaghetti, but look at the straight lines which are the linear trendlines through each set of raw data. You will see that in this relatively small sample MPG increased in the long term by an maximum of 3 mpg. The other folks whose MPG trended upward saw maybe 1-2 mpg increase.

But the majority in this sample, 6 out of 10, actually saw their MPG go DOWN over the long term, by as much as 4mpg!

Anyway, look for yourselves. I realize this is nowhere near statistically significant and will think about spending more time on this to add data sets in the near future.

http://im1.shutterfly.com/media/47a2...D720/ry%3D480/

I'm guessing these are a mix of highway and city drivers? It would probably be more accurate if their driving styles/conditions were the same. The MPG for city drivers would probably vary from traffic conditions.

True, but all I'm looking for is the long term trend. Shouldn't matter if they're city or highway drivers. City drivers' mileage will vary more widely on a given tank but over 40 tanks or more the short term variance will disappear and we'll see over the long term if their mileage is trending up, down or steady.

I agree with TiGero. Thanks for all the work, but I'm not sure how good the data are if you have no idea what the driving trends were for each of these vehicles. i.e., city vs. highway driving.

Here's another batch of 10 drivers. Similar results as before (see my original post for background), though one guy managed a huge 13 mpg gain. Must be an outlier. The other either ganed modestly (2-3 mpg), held steady or declined quite a bit (4 mpg).

http://im1.shutterfly.com/media/47a2...D720/ry%3D480/

Great idea! Also, I'm sorry my winter tires are messing up your statistics! haha

http://farm8.staticflickr.com/7096/7...d0a9e980_b.jpg

Jesus... there seems to be no rhyme or reason to my gas mileage trends... it'd be a fun roller coaster, though. My worst mileage was 28.11mpg - and that's because my mom left my car running for an hour one morning so that it could "warm up" >_>

EDIT: I also didn't track my mileage when I first got my car... first time I tracked it on fuelly it was already at 2,912 miles...

You would have to assume that they trend one way or another throughout the graph though (city vs. highway). What if, for one the drivers, he drove the first half of his fillups on the highway, and on the last half of his fillups he drove in the city? ...or some other combination. Would that be good data for your study?

It doesn't matter what their driving habits are, as long as each stays relatively consistent. Yes, I'm assuming that someone didn't do mostly highway driving for the first 20 tanks and then completely switch to city driving for the next 20 tanks, which would artificially trend their MPG down. These graphs are intended to give a glimpse into the perfomance of each car assuming they're driven by the same person in relatively consistent driving condtions (meaning mostly city, mostly highway, or mostly mixed). Even a couple of tanks of pure highway when the driver otherwise drives all mixed is not going to have a significant "pull" effect on the trendline.

that graph is really hard to read.

I looked at my fuelly, and my first tank fuelly tank, which was after a 1000 miles on the car was 29.73, my 2nd tank was 32.80, my 3rd 29.45 my 4th 31.18. The up and down trend continued but never varied much and i am at 11,500 miles now. So mine didnt improve much except from the first tank when I had only 24 and the second tank of 27.

You are correct. I am assuming those drivers are few and far between and won't have a huge effect on the overall gist of the graphs.

For example, that driver in my second graph who increased 13 mpg over time, that's probably someone you're talking about. But clearly he/she is an outlier.