I took my 2013 E350 Sport, which I bought as a CPO in March 2018, on its longest trip last month: 2600 miles. As most know, this car has the 302 hp, direct inject V6 engine.

One day that was almost entirely interstate driving of over 500 miles, I averaged 36.4 mpg. For the entire trip, I averaged 32 mpg. These numbers are much better than I expected and I'm very pleased.

The bad news is that I had lower back pain after two hours of driving no matter what positions I put the seat in. I had to stop at several rest areas to do stretching exercises. Of course, the problem is my back, and I may have had the same pains in just about any car. However, I don't get pain in the captain's chair of my 1997 V8 Dodge van conversion. In that tripping vehicle, however, I get a wallet-busting 14 mpg.

Just some info.

Does your 2013 have a lumbar support adjustment for the driver's seat? Mine does, as does our GLK -- yet the wife didn't realize it was there. She is much happier since she has it adjusted to her liking. . .

Yes, I have lumbar adjustment and played a lot with it to no avail. The problem is really my deteriorating back, so I'm not really faulting the car. In general, I do prefer plushier and cushier seats than the "supportive" seats and bolsters in German cars, but all the other benefits of a Benz far outweigh this preference.

Those of you with back problems can't really blame the seat in the car. I recently drove 1350 miles overnight (20 hours total) nonstop (other than for gas/snacks). Also got 33mpg... but no back complaints. YMMV, but really, there are too many complaints here about the seat design, when in reality, it's how your back jives with good seat designed to please the majority who do not have back problems. Sorry.

Make sure to remove the wallet from your rear for long drives. It kind of shift your pelvis due to the uneven level. At work I remove my wallet too before sitting down in those ergo chair as I also have some lower back problem too.
You can try those memory foam/gel stuff they sell for amazon. Of course your mileage may varies on those products.

Your results echo mine from a 6-7 hour trip in my '11 E350, incredible fuel economy but a sore back to contend with. The worst part is that I was supremely comfortable during the whole drive, the back issues did not arise until the next morning which I just chalked up to the guest bed I was sleeping in. Well even after the return drive of 6.5 hours and an overnight of sleep my back was even worse. Never had this issue with my '09 C300 after doing some 10-11 hour jaunts behind the wheel.

I am about to hop behind the wheel for a 17 hour/way trip, going to Google and YouTube search proper seating position in a car and hope I might just be sat in a position more condusive to shorter trips. If not I am going to be in for a world of hurt over xmas and after xmas. .

x3 on removing wallet and other stuff from rear pockets.
I drive long distances all the time and have 2 memories for my seats.
1 comfortable with recliner-like setting
2 front of the seat max raise and back adjusted to kitchen chair-like position.
Even the 2nd position doesn't feel comfortable at first, it helps with legs cramps I am getting after long hr behind the wheel.
Surprisingly the high chairs in my truck make better comfort on long run. They have high seats to start with and are wider, so I can change my position in wider range.
The only thing hard to deal is neck pain. You don't feel it bad while you drive, but next morning you feel it getting up from bed.