Here's some interesting results I found testing with and without an air dam:
Motortrend magazine that was doing some long-term testing of the new Chevy Colorado diesel and it's 30-mpg-highway claim and they measured the mpgs with and without the air dam at city and highway speeds. Their results with the air dam off: worse fuel economy on the highway, but much better fuel economy in the city!
2016 Chevrolet Colorado Z71 Diesel Review - Long-Term Update 5
Quote:
Its first test would be completely stock as delivered from the factory with the air dam affixed. Under our Real MPG cycle, our stock diesel Colorado Z71 4×4 achieved an impressive 21.5/31.9/25.2 Real MPG city/highway/combined. Compared to our truck’s 20/28/23 mpg EPA rating, the Real MPG results detail a 7.5 percent increase in the city, 13.9 percent increase on the highway, and an overall 9.5 percent increase on the combined cycle.
Not too shabby.
With the air dam and side steps removed, I handed the keys back to the Emissions Analytics team and eagerly awaited their results. Their testing shows that without the air dam, our Colorado Duramax 4×4 achieved 23.6/30.6/26.3 Real MPG city/highway/combined. Compared to the same truck with the air dam on the Real MPG cycle, that’s a 9.7 percent improvement in the city, a 4.0 percent drop on the highway, and a 4.3 percent improvement overall. Compared to the EPA, our truck—without the air dam—improves 18.0 percent in the city, 9.3 percent on the highway, and 14.3 percent on the combined cycle.
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Now that's just one vehicle, doesn't speak for all. For example, here's another Chevy Colorado owner's experience, he has the gasoline engine and he experience a 18% negative difference with the air dam off:
http://coloradofans.com/forums/169-2...-mpg-wise.html