Whether you’re driving a semi-truck for the long haul or looking into purchasing a new Ford F350 or other diesel-fueled pickup truck, it’s essential to understand the weight of diesel fuel and how it will impact the potential legality and performance of your ride. The weight of a full tank of fuel can impact the ability of a semi-truck to pass a Department of Transportation weight inspection for professional drivers, while regular motorists should be aware of how fuel weight can affect performance and even the fuel’s efficiency.
Diesel is a considerably denser fuel than gasoline, meaning that it contains more matter (and therefore weight) within a measure of volume. A gallon of diesel fuel actually weighs almost a pound more than a gallon of gasoline, thanks to its nearly 12% higher density. The weight of a standard unit of diesel fuel can vary a bit by country due to differing fuel standards and units of measurements, but a safe general weight to use for discussion is that diesel in the United States weighs seven pounds per gallon. Gasoline, on the other hand, weighs about six pounds per gallon. While that pound difference may not seem like much at first, consider that most pickup trucks have tank sizes of 34 to 48 gallons and semi-trucks can hold as large as 75-gallon tanks.
Diesel also doesn’t evaporate like gasoline. If a gasoline tank isn’t completely airtight, or even if you park in the sun on a hot day, you will lose a non-negligible amount of fuel weight to evaporation. Diesel, on the other hand, has much stronger molecular bonds and does not evaporate, so you can’t count on a hot day or loose gas cap lightening your load. The amount of diesel you load into your fuel tank will only be impacted by the distance you drive, not heat or other factors like in gasoline-fueled vehicles.
Thanks to the different densities and properties of gasoline and diesel, engines that run off of each fuel actually work quite differently. In gasoline-powered engines, gasoline is mixed with air before being compressed by pistons and sparked into ignition by spark plugs. In a diesel-fueled engine, the air is compressed first with the fuel then being injected. The air heats from being compressed, which ignites the fuel as it is injected.
Due to the unique mechanism of diesel motors and the higher amount of energy in the dense fuel, diesel engines are considerably more fuel-efficient, primarily when used on highways with little stopping and idling. A gallon of diesel fuel, in addition to its pound of extra weight, contains about 30% more potential energy than a gallon of gasoline, which can translate into considerably higher miles per gallon, especially in ideal driving conditions.
In city driving, with its constant stops, diesel loses quite a bit of its efficiency advantages. However, even in a traditional city, driving diesel is slightly more fuel-efficient than gasoline. The higher energy potential in diesel fuel also allows for higher torque, a scientific term for a twisting effect, which, in a combustion engine, translates to faster acceleration ability.
Comparing The Cost Of Diesel Fuel To Gas
Currently, diesel fuel tends to run around 20 cents per gallon higher than gasoline, but the increased fuel mileage of diesel makes that cost negligible or even slightly advantageous. Diesel engines tend to add about $700 US dollars to the cost of a car or truck versus a traditional gasoline engine. The greatest potential price is in finding qualified diesel mechanics, as many shops in large cities exclusively service gasoline-powered engines. This drawback is likely nonexistent for drivers in rural areas or farming communities where diesel engines are prevalent.
Diesel Emissions Compared To Gas
Emissions and environmental impact are a considerably trickier comparison to make between diesel and gasoline-powered engines. Diesel is a more carbon-rich fuel, but it also burns more cleanly leading to overall lower CO2, or carbon dioxide, emissions. However, heating air through compression, which diesel engines rely upon as the first mechanism of action before injecting fuel, leads to the toxic byproduct nitrogen dioxide, NO2, which is mostly nonexistent in emissions from gasoline-powered cars thanks to the processes contained within their catalytic converters.
It is important to note that all of the numbers, performance, and emissions discussed above are based on standard diesel. In many parts of the US, especially the Midwest, corn or soybean-based biodiesel is extremely common, sometimes to the extent that standard diesel is more challenging to find. Biodiesel weighs less than standard diesel and has a lower amount of potential energy. While still higher than gasoline, it is weaker and will affect both performance and fuel efficiency by as much as five percent. Bio-diesel reduces CO2 emissions while simultaneously showing a slight increase in toxic NO2 emissions.
The Real Impact Of The Weight Of Diesel Fuel
With all of the pros and cons of diesel in mind, let’s turn our attention back to the weight difference. A 75-pound difference, in a 75 gallon capacity semi-truck fuel tank, between gasoline and diesel, may not seem like much to a lot of us, but for professional drivers who need to stop at weigh stations that relatively small difference can be the deciding line between an expensive ticket for being over the weight limit or happily passing inspection and continuing on their trip.
Consider that, in the United States, the weight limit for a single semi-truck is 80,000 pounds with a single-axle weight limit of 20,000 pounds. Given that most trucks have 50 to 75-gallon tank capacity, semi-drivers are looking at 350 to 375 pounds of fuel when fully refueled. While this may not hold much of a candle to 80,000 pounds in total, it is almost 2% of the single-axle weight limit. With truck drivers carrying as much cargo as they can for the sake of efficiency and profit, a 2% margin of error due to miscalculated or forgotten fuel weight can spell out significant fines and big headaches.
Anyone driving professionally needs to remember the size of their tank, the general weight of diesel (seven pounds per gallon) and on which axle the weight of their fuel tank rests. While it may be easy to meet the 80,000 general weight limit, a 20,000 single-axle limit can easily be skewed by mismanaged fuel. Remembering the weight of one’s fuel and the impact it can have on weigh stations is just as important as keeping an eye on the fuel gauge itself.
Even for non-commercial drivers, the Environmental Protection Agency (EPA) estimates that every 100 extra pounds on a vehicle decrease the fuel efficiency by one to two percent. An additional 100 pounds of diesel, while more fuel-efficient in and of itself, loses a bit of that overall efficiency due to its increased weight. On driving days with extra wind drag or suboptimal driving conditions that extra weight could lead to even further reduced ability. On the flip side, that little bit of excess weight, especially on a rear-axle, can actually be a benefit for personal pickup trucks in snowy and icy conditions where a lighter rear-axle is a liability for drifting and poor maneuverability.
Diesel may be a great fit for your next vehicle, especially if you put on mostly highway miles. It is, overall, a more fuel-efficient and cost-efficient fuel for your car when maintained well and used for highway driving conditions. If you decide to pursue purchasing a diesel vehicle, make sure you understand the different working mechanisms of the diesel engine and the impact that the increased density and weight of diesel fuel could play in your driving efficiency and enjoyment.