2011 Volkswagen Jetta TDI: Aerodynamics and MPG
June 16, 2011
I've always thought that the instantaneous fuel economy gauge is pretty useless since the number bounces around so much it's hard to draw any conclusions about fuel consumption. But while driving the 2011 Volkswagen Jetta TDI through the San Joaquin Valley, on the way to the Bay Area, it was so straight and so flat that, when I set the cruise control, the instant mpg leveled out and gave me a nearly constant reading. My father was along for the trip, and he's a huge fan of diesels (he's owned five diesels dating back to the 1980s and currently owns the 2009 Jetta TDI) so we decided to do a rough test of aerodynamics.
Let me quickly underline the phrase "rough test." I know that this isn't conclusive. But it does give an interesting indication of the drop off in fuel economy as the speed builds and the wind resistance increases. Here's what we found for 60, 70, 80 and 90 mph.
First, here is how we did the test. I used the cruise control to set the speed exactly at the different speeds. I then watched the instant fuel economy reading for several miles. It still fluctuated and it is very sensitive to any incline, but after a few miles the average becomes clear. It was 94 degrees Fahrenheit with little perceptible wind. We did the test twice and got nearly identical results.
Here are the results averaged for the two tests:
60 mph = 47 mpg
70 mph = 40 mpg
80 mph = 36 mpg
90 mph = 33 mpg
So it's clear that the higher speeds reduced the fuel economy. But it was a little surprising that the fuel economy didn't drop off more sharply at higher speeds. When my father made a graph of the results it was nearly a straight line, not the exponential result I had expected. However, it's also important to remember that the diesel engine might have different characteristics than a gas engine.
You might be yawning and saying, "So what?" Well, for many people fuel economy is basically about saving money. So I did a few calculations to try to drive my point home, so to speak. If you were driving 500 miles and decided to go 80 mph rather than 70 mph, it would cost you almost $6 more in fuel at $4.30 per gallon of diesel (the current price in California). Driving at 90 mph, common on Interstate 5 in the San Joaquin Valley, it would cost $11.40 more. Are you still saying, "So what?"
One interesting thing I noticed was that the road surface seemed to have an effect of fuel economy. In several cases I drove over smoother concrete surface rather than the pebbled asphalt and the mpgs jumped up. This makes sense because I once heard that hypermilers drove with one tire on the white stripe at the end of the road to reduce friction.
There's one other thing I'd like to mention: this Jetta TDI is one of my all time favorite cars.
Philip Reed, Edmunds senior consumer advice editor @5,174 miles