Why a diesel engine won’t automatically save you money

Have you been eyeing a diesel-powered car in the hopes of bringing down the cost of driving?  Have the advertisements promising 45 mpg got you convinced that diesel is the secret to lowering your cost per mile?

As enticing as these ads are, there are costs that cloud the waters when it comes to seeing savings. And, in terms of mileage, there’s a little surprise there as well.

Looking at the hidden costs of a diesel engine

Take the Volkswagen Jetta for example. The Jetta is a compact sedan that offers both diesel and gasoline motors. The Jetta’s price for the gasoline model is $15,545 and that gets you a 2.5 liter 5-cylinder engine. The diesel-powered counterpart comes with 4 cylinders and starts at $21,295.

That’s a big difference in price for two cars that, when viewed from afar, are basically the same.  So, what do the additional dollars buy you?

To start, they buy you fuel economy. The diesel 2014 Jetta (the TDI) is rated by the EPA at 30 mpg city and 42 mpg highway while the gasoline version (the 2.5) is rated at 23 mpg city and 30 mpg on the highway.

Those extra dollars also buy you an engine that produces less in the way of greenhouse gasses.  The carbon dioxide produced by the diesel engine is significantly less than the carbon dioxide produced by the gasoline counterpart.  But, particulates (in this case, think of diesel soot) are much higher than from a gasoline engine and require a very complex system to handle them.  More on this wrinkle later.

Speaking of wrinkles, the biggest hindrance to savings is the $5,750 premium paid for the diesel model over the gasoline model.  That cost means that a whole lot of miles have to be covered before the upfront cost is recovered by less frequent visits to the pump.

MPGs: A closer look at mileage rating numbers

Here’s where things get interesting with the mileage.  While it’s well known that the EPA’s mileage ratings have always been somewhat optimistic when it comes to gasoline-powered vehicles, it has been found in numerous tests that—due to the characteristics of the diesel engine and the EPA’s testing methods—diesel engines frequently get better real world mileage than the EPA estimates.  And, that’s exactly what was revealed in the February review of the Volkswagen Jetta TDI in The Chicago Tribune.

In the Tribune article, reviewer Jill Ciminillo found that she was able to coax 45 mpg out of the TDI model on the highway, but fared worse with the city mileage getting only 26.  It should be noted that this particular test occurred during the winter months when fuel mileage tends to decline due to cold temperatures.  However, in many other reviews, it has been reported that the diesel Jetta can easily return highway figures into the low 50s and city figures in the high 20s to low 30s.

So, with those big numbers, will that price difference be quickly erased with fuel savings?  The answer is, unfortunately no.

Urea: The secret sauce you don’t know about!

While gasoline burns cleanly and efficiently compared to diesel, the diesel engine is, by its very nature, quite dirty.  To combat pollution in gasoline engines, manufacturers have long employed catalytic converters to stay in compliance with federal regulations.  In the case of the diesel, manufacturers have begun using an additive that is sprayed into the exhaust stream to help burn off particulates from the combustion process (soot) in order to meet EPA regulations.

This fluid resides in its own special 5-gallon tank that has to be refilled every 15,000 miles.  And, like diesel, the car won’t run without it.  The price per gallon of the urea fluid (yes, urea!) called Adblue varies from $4.50 to $7.50 a gallon so, while not devastating, it does cut into the savings.  Most manufacturers offer this fluid for free for the first few years.

Then there’s the cost of diesel.  It tends to range 50 to 60 cents above gasoline, so that hurts the potential for savings as well.

Conclusion

The bottom line is, yes, the diesels, for the most part, will get better mileage than their gasoline equivalents but the savings aren’t that large after the higher per gallon cost, extra fluids, and the cost of acquisition are taken into account.

But, if you’re the type who drives a car several hundred thousand miles (particularly at highway speeds), the overall cost of ownership will likely be lower.  Just not by as much as you might have hoped.

Of course, VW has upped the ante with the 2015 model which is expected be rated at 32 city/45 highway which extends the small advantage the diesel already has.

David Lardner is a volunteer at Team Clark Howard’s Consumer Action Center.

For further reading:

• A Compact Electric Car from Mercedes Benz
• Lowering The Cost of Electric Car Chargers