Since the economic scare of 2008 and 2009, automakers have been in a pretty damn good hurry to produce vehicles that are more fuel efficient and "green.

" Some have been more successful than others and the manufacturers have taken a variety of approaches. With new regulations taking hold in the not-too-distant future, the auto builders need to be firing on all cylinders to meet requirements while seducing buyers.

Initially, the German contingent decided to shy away from hybrids and lean hard on diesel powertrains. After all, this is what they had been doing in Europe all along. Obviously this has now changed as we see and hear about plug-in vehicles and hybrids are being delivered to the marketplace. What also has changed is that the big three German luxury automakers are sending their oil burners stateside.

After dipping their feet into the water to test the market out with diesel sport-utility vehicles, it seems that there is a business case for sedans.

This neatly brings us to our subjects, the Audi A7 TDI, the BMW 535d and the Mercedes-Benz E250. Any buyer can't go wrong with either of these autos; however, for our purposes we wanted to test all three, see how they all shook out and determine WHO is the king of the class. 

So, let's get down to the skinny: WHO "won" the fuel efficiency test after one week of driving? The results are below.

1) Mercedes-Benz E250 BlueTEC — 32 mpg**
2) BMW 535d — 31 mpg**
3) Audi A7 TDI — 30 mpg

**Equipped with economy mode — the vehicle had this switched on for the entire duration of testing.

But there's two more important aspects to the story. That's because each vehicle was tuned differently and each, of course, had its pros and cons. Let me give you all a bit more color.

Mercedes-Benz E250 BlueTEC

Fresh off an updated design, the E250 is easily the most conservative choice of the three. The exterior shape and interior design has a whiff of tradition and old-school Mercedes to it. It's not a negative, provided you're into the whole nostalgia thing. The E250 is the least powerful offering with a 2.1-liter 195 horsepower and 369 lb.-ft. torque motor sending power to the back wheels — you can option the E-Class with all-wheel drive but it will cost you a couple more grand. Because of its limited power compared to the competition, you'll constantly find yourself throttling the four-cylinder, twin-turbo engine to get it up to speed. And because its the most chattery engine of the three, you and your passengers will know it. The good news is that in true Mercedes fashion the vehicle is comfortable and refined. It does all the things you'd expect a mid size luxury sedan to do.

What's remarkable is how the motor is optimized for wafting around town or the subrubs. I saw as high as 36 mpg driving on local roads. That's a helluva achievement for a non-hybrid luxury sedan. As expected, traveling at highway speed quickly takes down your best efforts.

The Good:

- It does everything you'd want a mid size luxury sedan to do
- Comfortable and refined
- Old-school Mercedes quality to it — feels as though it'll stand the test of time well

The Bad:

- This motor lets you know it's a diesel — a lot of chatter
- Not being powerful, you'll find yourself having to rev it up a lot
- From a design perspective, it's not going to win any style awards

Diesel Sedan Showdown

BMW 535d

Expected to be the "driver's diesel," I was happy to see that my test 5-Series was equipped with the M Sport package. It's sort of a goofy concept on a diesel but it does look good, I will admit. Like the Mercedes, the 5-Series was just refreshed for the 2014 model year and the subtle changes are nice. I love the all-new LED headlights and foglights. A really nice touch, indeed.

Armed with a 3.0-liter six-cylinder engine, the 535d puts out 255 horsepower and 413 lb.-ft. of torque. When you dial it up to Sport + and give it the beans you'll certainly feel the motor working. To be straightforward, it blows away the E-Class and it's doubtful you'll be left wanting more juice. One of the things I love about BMW diesels is that they're not particularly chattery or loud. If you listen with a good ear you'll note at low RPMs the motor is a bit agricultural; however, once the revs come it switches to a sweet sounding six.

When you put it into Eco Pro mode, you'll notice that this car drives completely differently. I am not sure what geeky stuff BMW did to the transmission but if you find yourself driving on country roads, you can let off the throttle and the car will just coast endlessly. After picking up the vehicle I had a long drive on country roads and quickly saw the vehicle edge up to nearly 35 mpg. Of course the problem is that if you're in Eco Pro mode on the highway it seems counterproductive

The Good:

- You'll never be left wanting more power
- Eco Pro mode is great and you can really rack up mpg, if you live in the suburbs or drive on country roads
- You can blend into the crowd with the M Sport package

The Bad:

- If you do a lot of highway driving, mpgs fall off quickly
- Though a handsome design, it's not the most creative
- If you are hoping for BMW steering of yore, prepare to be disappointed

Diesel Sedan Showdown

Audi A7 TDI

Now I am sure some of you are wondering why I didn't test the A6 as that would be a true apples-to-apples comparison. Well, that's what I asked for and this is what the four rings sent.

In my humble opinion, there's no doubt that the A7 is the best designed vehicle, inside and out. It has a bit of chutzpah to it and I like that. While the BMW and Mercedes interiors are nice, they just aren't all that modern. The A7 feels like you're in a Milan cafe. Granted, I have come to despise the forward-thinking MMI controls, but that's an other story. Like the BMW, the Audi's motor won't have you revving the pants off it to get 'er going. Powered by a 3.0-liter six-cylinder, the A7 TDI makes 240 horsepower and 428 lb.-ft of torque. The motor is nicely refined and you wouldn't know it's a diesel unless you really take note when the vehicle is idling at a light — it has a low-end rumble like a diesel locomotive. When you're motoring on the highway, there's no question this car is a GT. The mix of power and ride quality is optimized and it feels startlingly similar to the Bentley Continental GT. Around town though it definitely doesn't feel like a Bentley as its ride is a bit harsh. 

Of course there's one problem: the A7 TDI doesn't have an economy mode. You just get in and off you go. My test car had other settings to play with if you wanted to dial up the steering, throttle and suspension but nothing in terms of eeking out more mpg. The way Audi tuned this engine, it gets obliterated during around-town driving. It would perform better if you do more highway miles.

The Good:

- Of the three it easily has the best design
- The ride quality is excellent at speed — feels like a baby Bentley
- Refined powerplant that doesn't really let you know it's a diesel

The Bad:

- No economy mode
- The ride quality isn't great if you're in a pothole-laden neighborhood
- Far too expensive when the A6 TDI can be had for approximately $10,000 less

Diesel Sedan Showdown

The Bottom Line:

So, I know what you're thinking: "00R that's great you've provided me with some context here but what would YOU buy if it were your money?"

Now we're talking.

Personally, if it were me, I'd think of it more like this:

1) Mercedes-Benz E250
2) Audi A7 TDI
3) BMW 535d

The Mercedes is a great offering if you don't mind stomping on the throttle. It does everything well and I could sacrifice some "swagger" with its more traditional styling. Oh, and it's also the cheapest.

Sure the A7 has the worst mpg but it is the most stylish, has a great motor and I can deal with a rough ride in the city. I hate MMI but I can get over it. One caveat: the A7 TDI starts at nearly $67,000, however, the A6 prices in at $57,500. Obviously, the A6 is a relative bargain if you want to sacrifice style points.

Last place for the 5'er? Well, yes. It doesn't drive like the BMWs of yesteryear and although Eco Pro works great if you live in the suburbs and drive on country roads, I wouldn't want to drive it in Eco Pro on the highway where it doesn't work so great. So all that hard work goes down the tubes.

Diesel Sedan Showdown

REVIEW: Mid Size Luxury Diesel Sedan Showdown, Who Takes The CROWN? Audi A7 TDI vs. BMW 535d vs. Mercedes-Benz E250

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