The vehicle I was driving was a 2010 Subaru Forester. Some friends have the same vehicle and have been pleased with getting around 27 MPG on the highway. We typically get only 23-24 MPG on the highway and I had been wondering why.
Among the features of this car is an average gas mileage display that's tied to the trip odometer. So, sampling the gas mileage is as simple as setting the cruise control, resetting the trip odometer, driving a set distance and reading out the result. As I was crossing the relatively flat plains of Idaho (speed limit 75) this seemed to be a good opportunity to gather some data.
Over a period of several hours, I took a bunch of samples following the above method and using my GPS to track altitude changes. I abandoned samples where the altitude change was more than a few hundred feet. The result is 27 good samples. I've posted the raw data here in case you want to play with them. Most of the samples are for 20 mile segments but some are as long as 40 and some are as short as 5 miles.
As you can imagine, the lower-speed samples got a bit tedious. But I was curious enough that I even took a side trip on a remote road (off the freeway) to get samples below 45 MPH. There's considerable variability in those results as you can see in the plot below. Halfway through the trip I refilled with fuel. I switched from regular (87 octane) to premium (92 octane) to see how that might affect mileage.
|2010 Subaru Forester Fuel Economy vs. Speed|
Here are a few things I've learned from this:
- My friend with the other Forester drives slower on the highway than I do.
- I had not known how sensitive vehicle gas mileage is to speed.
- Everything I have read led me to expect no benefit from higher octane fuel once the vehicle's requirements have been met. In the case of the Forester, higher octane actually reduced fuel economy. This observation is confirmed by the official EPA ratings.
- I would love to see tables like the one above before purchasing my next car.