Join Date: Jun 2008
Location: Boerne, TX
Vehicle: 2016 SLK55, Mystic Blue, Black Nappa, P3, Performance Pckg, Carbon Trim, Illuminated Doorsils
Other Toys: 1700cc Vulcan Cruiser, all black and chrome
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Thanked 78 Times in 38 Posts
So far the best I have managed is about 24-25mpg (US gallons). Then again that was with my cruise control set at 80-85mph, which means engine revs were staying above 2000rpm (2200-2400 IIRC).
My problem with the cylinder deactivation for improved fuel economy in the R172 55 is that it only works when travelling on level terrain or downhill. As soon as I hit a bit of an incline, it immediately kicks back into V8 mode. And unfortunately in the Texas hill country, smooth/flat roads are few and far between, so long stretches of V4 mode are rare.
Mixed city/highway usage is returning 20-21mpg on my drive to work (about 15 miles each way), which seems good considering my 350 and 43 both only averaged about 21-22mpg each week in the same usage routine.
I dont see how you folks are getting 31-34mpg at 70mph unless its relatively smooth/flat highways with little to no incline, especially on an r171 without cylinder deactivation. I think I might get around 26mpg with the cruise control set at 70mph, but will need to test it since I rarely cruise below 75-80mph on Texas highways. No way I will ever see 30+. Maybe the limited slip differential in the performance package is to blame? I've heard those can negatively impact fuel economy.
As soon as my wife gets her Toyota 4runner, I'll either be using her old C300 as my daily driver or trading it out on something more my speed to use as a daily so I can keep the miles low on the SLK55. After that fuel economy shmuel economy. 😄