Diesel vs Hybrids: The Costs and Benefits of Both (And a Shiny Calculator!)

February 11, 2014 | 5:28 pm
Dave Cooke
Senior Vehicles Analyst

If you’re thinking about buying a vehicle, chances are you’ve noticed a number of automakers touting the benefits of new “clean diesel” vehicles with fuel efficiencies of 40 to 45 mpg on the highway. That sounds awfully high – but what does it mean, and how does it compare to the most efficient gasoline-powered vehicles on the road, hybrids?

What the heck is “Clean Diesel”?

Anyone who was around in the 1970s or has ever been behind an old big rig belching black smoke into the air may wonder how in the world a diesel vehicle could be considered “clean.” Well, the good news is that over the past forty years, air quality regulations of both light- and heavy-duty vehicles have driven diesel manufacturers to reduce both particulate and smog-forming emissions. The end result is that today’s diesel vehicles are designed to meet the same air quality standards as a typical gasoline-powered vehicle. For further technical information on how a diesel vehicle works, click here.

Hybrids: How do they work?

Since the first introduction of the Prius to the United States back in 2000, a number of hybrid-electric vehicles have appeared on the market, from the small and sporty to the big and roomy. While they vary in how they get their improved gas mileage, there are two basic principles at work in any hybrid: (1) a big electric motor that works with a combustion engine to make the system more efficient, and (2) regenerative braking, which recaptures energy lost while braking. For more info, see our hybrid page.

Cost to the environment – oil savings, carbon emissions, and air pollution

Diesels and hybrids don’t just vary in the technology that enables their high fuel economy ratings—crucially, they also vary in the fuel that powers these vehicles. This factor leads to significant differences in their environmental impact, even for vehicles with comparable fuel economies.

Much of the reason for diesel vehicle’s high fuel economy has to do with the diesel combustion process; however, some of the increase in fuel economy is due to the simple fact that a gallon of diesel fuel contains more energy than a gallon of gasoline. In order to produce this more energy-dense fuel, it requires more oil. It actually takes 20% more oil to produce a gallon of diesel than a gallon of gasoline. This means that from an energy security and oil consumption perspective, comparing diesel and gasoline vehicle on a miles-per-gallon basis does not tell the whole the story. The same is true for global warming emissions – diesel fuel is more carbon-dense than gasoline, so burning a gallon of diesel fuel actually results in about 20% more emissions than burning a gallon of gasoline (including emissions from production of the fuel).

Another way to think about this is to directly compare the impacts from diesel vehicle to its gasoline counterpart. Using EPA label data, I compared the fuel economy of diesel vehicles to their gasoline counterparts. On average, diesel vehicles today have about 25% better fuel economy than a similar gasoline vehicle. However, this improved fuel economy does not translate directly to oil savings. On average, a diesel vehicle will reduce oil consumption and global warming emissions by just 5% compared to its gasoline counterpart.

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Another way of putting this is that a diesel vehicle with a fuel economy of 35 miles per gallon consumes as much oil and emits as much global warming pollution as a gasoline vehicle with a fuel economy of 29 miles per gallon.

Both diesel and hybrid vehicles are subject to the same smog/air quality regulations, but flexibility in the standards allows vehicles to be certified to different levels. On average, hybrid vehicles tend to be certified to a lower level of smog-forming pollution than diesel vehicles, but the EPA now includes this information on every vehicle label, so you can see for yourself how well a specific vehicle scores compared to other vehicles in its class.

In general, the best source of information on environmental impact can be found at a website hosted by the EPA and Department of Energy, fueleconomy.gov, where you can compare the environmental impacts of any vehicle being sold today as well as its fuel economy. Or, you can also use our Diesels vs. Hybrids Calculator, which I’ve linked to above.

Costs to you, the consumer

Both hybrid and diesel vehicles offer very efficient engines which may be able to save you money. However, there are a few things to consider:

Highway vs. City driving

The most efficient characteristics of a hybrid vehicle make it perfect for stop-and-go driving. However, at constant highway speeds, a driver is taking advantage of neither the idle reduction capability nor the regenerative braking that make a hybrid vehicle so efficient. On average, today’s hybrid vehicles use about 40% less fuel in the city compared to their conventional counterparts but only 20% less on the highway based on my survey of fuel economy data for comparable vehicles.

A diesel-powered vehicle does not have such a disparity, however. A diesel engine does perform best at constant speeds, which makes it most efficient on the highway. In the city, on the other hand, you will continue to waste fuel while sitting idle at a stoplight, and diesel engines tend to be most fuel-inefficient and “sluggish” in bringing the vehicle up from zero to cruising speed, which is why they are usually accompanied by a turbocharger. Additionally, manufacturers recommend frequently driving long distances to avoid clogging the diesel particulate filter, so short city commuting shouldn’t be the sole use of a diesel vehicle. However, in both city and highway driving a diesel uses about 20% less fuel than its gasoline-powered equivalent.

Upfront cost vs. total cost

Hybrid-electric versions of a vehicle tend to come with an increased upfront cost due to the added technological complexity as compared to a conventional version. Diesel vehicles, too, typically cost more upfront compared to a conventional gasoline vehicle. However, over the lifetime of the vehicle, these added costs pay off in fuel savings.

One thing to keep in mind when shopping for vehicles is that in the United States diesel fuel is more expensive than conventional gasoline. This means that even though a diesel vehicle may use 20% less fuel per mile driven, at today’s prices it will only save you 5-10% in fuel costs to drive that mile. Most hybrid vehicles use regular gasoline, which mean their fuel savings translate directly into cost savings for the consumer compared to its gasoline counterpart (i.e. 30% less fuel use means 30% less $).

Over a five-year period and 15,000 miles (55% city, 45% highway), a typical hybrid driver would thus expect to save between $3,000 and $4,000 in fuel compared to a similar conventional gasoline vehicle, while a diesel driver would expect to save only between $800 and $1,800 in fuel over the same time period. More specific estimates of a particular vehicle’s fuel costs are available at fueleconomy.gov.

Conclusion: They’re both good choices, but hybrids are better

Diesel vehicles have certainly come a long way from the days of the black smoke monster emanating from their tailpipe, and the high-torque performance, particularly at high speed, can make them both fun to drive and better for the environment and your wallet compared to a typical gasoline vehicle. And especially for applications where a hybrid vehicle might not be available (for example, SUVs or pick-up trucks), they are certainly worth considering.

However, hybrids are still the undisputed kings of savings when it comes to combustion-engine vehicles. Not only do they achieve roughly comparable efficiency on the highway compared to diesels, but they excel in the stop-and-go driving of a city. Furthermore, all of the efficiency gains go directly to reducing the amount of money you spend at the pump, the amount of oil consumed to fuel your vehicle, and the amount of global warming pollution generated during your travels.

The EPA has a plethora of information and tools available for consumers to help individuals make the most informed purchase choice that is right for them. Hybrids may offer a greater potential for reducing the environmental footprint of our transportation system, but both diesel and hybrid vehicles are providing more options for consumers to significantly reduce global warming emissions and oil consumption from the light-duty fleet, which is a critical part of our Half the Oil Plan.