Why is winter diesel fuel mileage lower than it is in the summer?

It’s well known that gas mileage goes down in winter, partly because they change the composition of winter gas to make it more volatile and burn properly in cold weather conditions. But diesel users also complain about lower mileage in the winter. Could that have anything to do with changes in the diesel fuel?

Sure it can.

Just like with gasoline, refiners have to change the composition of diesel fuel in the winter to help it function better in the cold weather. But this change has much less to do with “volatility” and more to do with both cold gelling performance and sulfur reduction.

Taking the sulfur out of diesel has given us ultra-low sulfur diesel (ULSD) in the winter which has about 3% less energy value than winter low-sulfur diesel used to have. A lower energy value means fewer miles traveled per gallon.

Refiners also seek to mix in additional product “streams” to change the properties of the winter diesel so that its gelling temperatures are reduced. There are lots of options at the refinery level when you consider that you can get 180 different kinds of petroleum products coming out of a barrel of crude oil. Some of these – aromatic chemicals, naphtha, #1 kerosene – can be mixed into the diesel by the refiner to give a little more temperature margin when it comes to how quickly the diesel fuel will gel in cold weather.

In addition to cloud point/gelling concerns, the composition of winter diesel fuel has to be adjusted to make it more volatile. This greater volatility serves the purpose of making the engine easier to start in cold weather.

Most of these additions to the fuel result in less energy value (and lower mileage). Consider that normal #2 diesel fuel might have about 140,000 BTUs of energy per gallon. Kerosene has closer to 130,000 BTUs. The more kerosene you mix in, the lower the total energy value.

Is There Anything The Diesel Driver Can Do?

If fuel was the only influencer of vehicle mileage, you’d have to say “Not really”. Winter diesel is not going anywhere. And it shouldn’t. Most truckers would gladly sacrifice a little bit of mileage to keep from being shut down on the side of the road or in the parking lot because their fuel gelled unexpectedly. But there’s a bigger “mileage leech” that any driver can pay attention to and realize much greater gains than just the small percentage lost because of winter diesel.

Speed is the biggest factor

55 mpg is the optimal combination of speed and fuel mileage. The faster you go above 55, the more your mileage drops. Research shows that the difference is about a 1.6% drop in MPGs for every 1 mph above 55.

This is due to air resistance and rolling tire resistance. When a truck is being propelled down the road, it has to fight both of these. And both of these influence how good or bad the fuel mileage is going to be.

At low speeds, there’s very little air resistance but a lot of tire resistance. As tires make contact with the road, they change shape, forming and unforming as they make and break contact with the road surface while the wheel is turning. This continual shape-changing requires energy, which comes from the fuel. If you carry a heavier load, the energy drain by the tires is even greater at low speeds.

But as you increase speed, tire resistance becomes less of a factor while wind resistance becomes more. Research shows that 45 mph is the point at which the factors flip – now wind resistance is the greater drag on your fuel mileage. And the faster you go over 45 mph (like when you’re on the highway), the greater role wind resistance plays.

At 55 mph, tire resistance is responsible for one-third of the fuel mileage drain, wind resistance is another third, and everything else is the final third. At 75 mph, air resistance now accounts for almost half (46%) while tires now account for much less (24%). Furthermore, for the many trucks that are equipped with high-efficiency tires, you now get less benefit from them at high speeds. Not to mention the fact that driving at higher speeds wears the tires out faster and costs even more money over time.

And the net result? Almost 40% drop in fuel mileage at 75 mph vs 55. Now we begin to see that we shouldn’t be worrying so much about winter diesel fuel as we should about how we’re driving.

Sure, you do have to balance out time saved vs fuel cost when you’re deciding how fast to travel. You’ll take 36% longer to get to your destination at 55, but you’ll get 39% better fuel mileage. The longer the trip, the more each factor represents.

It should be clear that driving habits and driving speed have the biggest potential to impact your wallet.

Another Useful Habit

Here’s another useful habit to save money. Treat the diesel fuel with a regular-use detergent treatment.

Diesel fuel doesn’t come with the same detergents that gasoline does. This means your diesel engine and injectors probably have significant deposit buildup. And engine deposits in these areas can be a substantial drag on performance and efficiency.

Investing in a concentrated diesel fuel treatment and cleaner (like Dee-Zol) can typically return 5 or 10 times the purchase cost in improved efficiency and equipment condition, provided the treatment rates are concentrated enough to make the treatment cost low. Some fuel treatments (whether for diesel or gas) cost six, seven, or eight dollars a bottle, but only treat small amounts of fuel (like a one-tank treatment for a car). That means you’d be spending upwards of fifty cents a gallon or more. The right diesel treatment, like Dee-Zol, will get the job done for ten cents a gallon or even less. And that’s when it becomes financially worthwhile to spend that little bit extra.

Check out these other posts related to winter diesel fuel mileage:

• New rules for diesel cold flow problems: reasons to be concerned
• Diesel Fuel Gelling - Get Prepared for the Coming Cold Weather
• Equipment & Vehicle News: Diesel Fuel Contamination Will Cost You