Diesel Vs Small Turbo Gas Engines: Does Real-World MPG Matter?

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2012 Volkswagen Passat TDI Six-Month Road Test

2012 Volkswagen Passat TDI Six-Month Road Test

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As fuel efficiency standards tighten continuously between now and 2025, buyers will have more powertrain options to chose from.

And many have suggested there's a battle coming among smaller gasoline engines, diesels, and hybrids.

If so, we hope that buyers will do the research to look at the real-world fuel savings of each alternative--and NOT just the EPA ratings.

That's hard, because legally, the only figures automakers are allowed to cite in new-car ads are the EPA fuel-economy ratings for the city, highway, and combined test cycles.

In theory, that gives all manufacturers a level playing field and a set of reliable third-party data.

But as numerous sources now show, some vehicles pretty much hit their EPA combined ratings, while others vary considerably--often on the low side.

The EPA's FuelEconomy.gov website aggregates self-reported real-world mileage figures from owners of each vehicle it rates, and other sites--including the popular Fuelly--do the same.

And real-world efficiency is where diesels often shine.

It's widely acknowledged that because some EPA test cycles are so outdated and gentle, they don't reflect how people actually drive. The EPA has added additional test cycles in recent years in an attempt to address this problem.

But diesels are at their most efficient when running hard, meaning that they may thrive under hard use--especially in frequent high-speed highway use.

2012 Volkswagen Passat TDI Six-Month Road Test

2012 Volkswagen Passat TDI Six-Month Road Test

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(Hybrids are often better in stop-and-go low-speed traffic because they shut off their engines when the car stops or even slows.)

Owners of Volkswagen Jetta TDI diesel cars have overachieved on fuel efficiency for years, and often compare real-world mileages of 40 to 50 mpg--against a combined rating of 34 mpg.

Meanwhile, some of the newest hybrids don't come anywhere near their EPA ratings--Ford C-Max Hybrid and Ford Fusion Hybrid, we're looking at you--and there are questions about the real-world efficiency of turbocharged small gasoline engines too.

So is this the "secret sauce" that will lead to diesels quickly surging to prominence in the efficiency stakes?

2014 Ford Fiesta: EcoBoost (European version)

2014 Ford Fiesta: EcoBoost (European version)

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We can argue both sides of that, but clearly diesel makers feel their time is now at hand.

And they also have survey results showing that gasoline drivers might buy diesels, whereas hybrid buyers are very different from diesel drivers.

So if you're one of those buyers who's weighing fuel economy in your purchase, and you're weighing smaller turbocharged gasoline engines versus diesels, do yourself a favor.

By all means start with the EPA data. But then, have a look at the real-world results achieved by owners of each kind of car.

You may be surprised at what actually happens in the real world.


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Comments (10)
  1. This article could be clearer.

    Let's pick up on one point, Jetta is rated at 34 mpg versus owner claims of 40 to 50 mpg?

    Even a quick look at fuelly would have shown the author that these claims are not typical.

    In reality 34 mpg epa and 38.4 mpg Fuelly. So Diesels do better than EPA but by a modest 13%. Bear in mind, it might be possible that Diesel owners smartly travel more highway miles than the EPA predicts.

    A second point, sure Ford's hybrids are having trouble with mpg, but how about Toyota Prius C.

    EPA 50 mpg, fuelly 50.5. So right on.

    Perhaps someone else can fill in the details on small turbo EPA vs Fuelly as both Voelcker and I can't seem to be bothered to do so.

  2. Haven't done an analysis of the fuelly results, but here is the results from the mileage obtained by Consumer Reports for the respective technologies...

    Diesel (8 vehicles)...

    Average = 29.625 mpg (CR) ÷ 28.375 mpg (EPA) = 104.4%

    Turbo-GDI (9 vehicles)...

    Average = 22.11 mpg (CR) ÷ 24.22 mpg (EPA) = 91.3%

    See also - http://news.consumerreports.org/cars/2013/02/consumer-reports-finds-small-turbo-engines-dont-deliver-on-fuel-economy-claims.html



  3. Interesting. The data is much appreciated.

  4. Regarding the Jetta, I would say that the author is about right. I drive a VW Passat TDI with DSG that is EPA rated at 30 city/40 hwy/34 combined. The only time I get the combined number or below is when I do a lot of short trips with little to no highway driving. Diesels perform best on the highway when they are warmed up. Under those conditions, 45 MPG or higher is a typical trip for me. In stop and go traffic, the EPA city figure is pretty much accurate. Just my $0.02...

  5. "I would say that the author is about right"... well if you blindly ignore the FUELLY data and prefer anecdotes, then it is great reporting.

  6. I get 45 mpg city/highway combined in my Golf TDi all the time. On the freeway, 50-60 mpg is easy to achieve. Driving 40-50 mph in mild rush hour, 60-70 mpg is achievable. If I got the numbers listed by the EPA, I would be dropping the car off at the shop because something would be wrong with it. Yes...real world numbers exceed EPA numbers by far with diesels.

  7. Maybe it is time to do a series-hybrid plugin diesel (similar to Volvo's idea) that will combine the efficiency of an EV in stop and go and the efficiency of the diesel on the hwy...

    I think SUVs and Pickup trucks would be the perfect cars for series-hybrid diesel plugins...

  8. It always helps to read user reviews. The Citroen DS5 diesel hybrid (beautiful car BTW) on paper has much better mpg values than a Prius, but in reality it actually does a lot worse than a Prius.

    The Volvo V60 plugin hybrid diesel should do a lot better and will if you plug in regularly and have short routes, but in reality average numbers are equal to or (far) worse than a standar Prius.

    It all just depends on the specific type, on how big the battery is, if it is plug in and so on. The thing I don't see mentioned is that diesels might, potentially, be better regarding fuel economy (and CO2), but they are still far more polluting (although improving) regarding other substances. I say all electric, period.

  9. Hyundai and Kia offer only a naturally aspirated 2.4L 4-cylinder and a turbocharged 2.0L 4-cylinder engine options for their respective midsize sedans.

    Some articles and forum posts I've read say turbocharging doesn't actually increase real world efficiency because you almost always have to drive the engine harder so that the turbo spools up to give you the power that you're looking for, but below that range the engines are too underpowered.

    This seems like the most important aspect to consider when comparing a small turbo gas engine to a small diesel engine.

    I drive a naturally aspirated 4 cylinder 1994 Ford Ranger, so I have no experience with turbocharged engines.

  10. If you drive over 20,000 miles per year (mostly interstate hwy) and plan on keeping the car for at least 7 years, then diesel is the way to go for the higher upfront cost. Other than that......
    And VW is coming out with a whole different engine a year from now so wait for that.

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