People buy green cars for many reasons: to save money and/or to save the planet.

But fuel savings don't automatically translate into cash savings.

Diesel vehicles typically cost more than comparable gasoline models, and their fuel costs more per gallon than gasoline in most North American locations.

RELATED: Fewer Gas Stations, But More Diesel Pumps, As Diesel Sales Rise

Those facts can cancel out any savings from superior fuel economy, but potential buyers have to do the math to find out if that's the case.

Research firm Vincentric conducts an annual survey on vehicle cost of ownership.

This year it found that around a third of diesels on the market--11 out of 35 models surveyed--will save their owners money on the cost of car plus fuel during five years of ownership.

2015 Mercedes-Benz GL-Class

2015 Mercedes-Benz GL-Class

Researchers found that the average diesel vehicle yields $855 in fuel savings over its gasoline counterpart, but its five-year cost of ownership is $2,754 more.

Overall oil prices have fallen dramatically over the past few months, but the price of gasoline has dropped more quickly than that of diesel--curtailing some of the financial benefit owners might expect to see from more efficient diesels.

It's worth noting, though, that diesels tend to more consistently match their EPA-rated fuel economy in real-world driving. There are many anecdotes of drivers exceeding highway mileage ratings.

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And even with abundant cheap gasoline, Vincentric found several diesels that cost less to own over five years than their gasoline counterparts. Reflecting the current state of diesel in the U.S., most of these vehicles were luxury models.

The top savers were the Mercedes-Benz GL 350 BlueTec SUV and the E 250 BlueTec sedan, with estimated five-year savings or $7,789 and $3,860, respectively.

2015 Ram ProMaster

2015 Ram ProMaster

Other models that showed a cost advantage over a gasoline counterpart included the diesel versions of the Audi A6, A7 and Q5; BMW 3 Series, 5 Series, and X5; and Mercedes GLK-Class (soon to be renamed GLC) and M-Class (soon to become GLE).

The only non-luxury vehicle anticipated to save its owner money was the Ram ProMaster 1500 delivery van--not something that makes it on many non-commercial buyers' short lists.

MORE: Mercedes-Benz GLK 250 BlueTEC Diesel: Luxury Crossover Fuel Economy

The proportion of diesels that save money over gas counterparts shrank compared to last year. Vincentric found that 46 percent of diesels on sale in 2013 could save owners money, against just 31 percent this year.

Vincentric recently conducted a similar analysis of hybrid ownership costs, and found similar results.

It found 32 percent of hybrids would cost owners less than comparable gasoline models, and that the number of hybrids that would be cheaper to own had also shrunk compared to 2013.


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