How Much Does Driving Style Affect MPG?

Your personal driving style can negatively affect your fuel economy — by up to 33%, according to government estimates. You probably knew that but didn’t pay much attention, because it’s not related to your driving.

But today, technology allows us to measure everything. And if you can measure it, you can improve it. So in order to get our heads out of the sand, we decided to run a test on three vehicles using a data logger from FleetCarma, a matchbox-size device that plugs into a vehicle’s OBDII port.

We plugged in the data loggers while resetting each vehicle’s odometer and fuel economy meters on three vehicles: a 2014 Lexus CT200h, 2014 BMW 328i and 2014 Nissan Pathfinder Hybrid 4wd. We had no idea what we’d find, but in analyzing the data, we were hoping some patterns would emerge.

The data logger measures when the vehicle was in operation, duration of trips and fuel use. This information is then translated into reports that deliver average fuel economy, average speed, total greenhouse gas emissions, idle event fuel use and percentage of hard braking and hard acceleration. This data is translated into an Eco Driving Score.

Here’s how the fuel economy tested:

2014 Lexus CT200h
Miles tested: 319
EPA-rated fuel economy (combined city/highway): 42 mpg
Data logger average fuel economy: 43.5 mpg
Vehicle trip computer average fuel economy: 42.9 mpg

2014 BMW 328i
Miles tested: 366
EPA: 27 mpg
Data logger: 17.4 mpg
Vehicle trip computer: 17.1 mpg

2014 Nissan Pathfinder Hybrid 4WD
Miles tested: 884
EPA: 26 mpg
Data logger: 23.3 mpg
Vehicle trip computer: 25.6 mpg

This prompted some interesting investigations. For the BMW, the difference in the EPA ratings compared to the fuel economy as measured in this test was close to 10 mpg. That’s huge.

Could the BMW driver have driven predominantly in the city, which negatively affects fuel economy? The BMW did have the highest percentage of city driving, but not by much. The BMW’s average speed was 20 mph; the Lexus averaged 21 mph while the Pathfinder averaged 25 mph.  

Could the BMW driver’s driving style have that much influence on fuel economy?

We then looked at hard acceleration and hard braking. For the Pathfinder driver, the percentage of braking and acceleration deemed “hard” was 14% and 13%, respectively, while the Lexus driver scored 14% on both metrics. The BMW driver, on the other hand, performed hard braking and acceleration 32% and 31% of the time, respectively. That’s more than double the percentage of incidents than the other two vehicles.

By the BMW driver’s admission, his driving style correlates with the data. Out of the three available drive modes (comfort, sport, eco) on the BMW, he said he stays in sport mode, which wouldn’t help fuel economy. “I tend to drive a bit faster than most people,” he said, adding that he turned off the start-stop feature that shuts off the engine when stopped. “I found it annoying.”

Well, then — we’ve unearthed some clear evidence of how driving style correlates with fuel economy. Indeed, the Eco Driving Scores back this up. The scores for the Pathfinder and Lexus drivers were 68 and 66 respectively, while the BMW driver scored 46.

Idling was another interesting metric. The logger measures two data points based on the percentage of time spent idling. The first measures intermittent idling, or the amount of time the vehicle is stopped less than 60 seconds. The second measures idling events, or fuel used when idling for greater than one minute.

The percentage these vehicles spent idling was eye opening. The Lexus idled the most (32%) compared to the Pathfinder (23%) and the BMW (26%). That’s a lot of wasted fuel!

Most of the time, you can’t prevent idling — but you can take advantage of technology to save fuel while idling. The BMW driver decided to turn off the start/stop function, which would have turned off his engine at a stop. The FleetCarma report showed that he would have saved 3 gallons over 366 miles of driving. This isn’t chump change.

Another interesting supposition concerned the fuel economy returned by the vehicles’ trip computers versus the fuel economy as measured by the data logger.

The Lexus and BMW models were within an acceptable range of difference. However, the difference in the case of the Pathfinder Hybrid was enough to warrant a second test, with these results:

2014 Nissan Pathfinder Hybrid 4WD
Test two
Miles tested: 1,116
EPA: 26 mpg
Data logger: 21.9 mpg
Car trip computer: 23.7 mpg

The second test revealed a discrepancy of about the same size. Why? It’s hard to say, exactly.

Without getting too technical, the FleetCarma data logger and the vehicle’s computer may measure fuel economy in different ways. (There is no SAE standard to measure fuel economy.) We do know the data logger measures a signal pulled from the OBDII port called MAF, or mass air flow.

We asked Nissan for insight into how its computer measures fuel economy. Understandably, Nissan wasn’t into discussing that process externally. At this point, we can’t say for sure if the data logger or vehicle computer is giving the more accurate reading.

Another measure of fuel economy is the old-fashioned way: writing down the mileage at each fill-up and the gallons of fuel put in the tank. This, too, may not be the most accurate method, but it would be one more piece of the puzzle in our new quest to measure everything.

I’ll try that on the Pathfinder. Stay tuned.

Originally posted on Business Fleet