It’s been said with reference to certainties in life, only death and taxes are assured, which means your EPA-estimated mpg is definitely not guaranteed.

While in recent years we’ve heard of automakers including Ford and Hyundai/Kia paying out for failing to meet mpg estimates they certified on window stickers, this was because of procedural errors or omissions under the EPA’s honor system of self testing.

The U.S. Environmental Protection Agency which oversees certification protocols otherwise says “your mileage will vary.” But why is that?

SEE ALSO: Study: EPA Understates Hybrid MPG By Up To 33 Percent

Several reasons, and for hybrids the EPA goes a step further and says while hybrids potentially return better fuel economy, they also may vary even more than non hybrids depending on a few factors.

On the positive side, the U.S. EPA is relatively strict, and has improved its testing procedures through the years. It has five tests for city, highway, and combined estimates, and specific additional tests for all-electric cars as well as hybrids. Compared to European and Japanese standards, the U.S. looks downright conservative – and more realistic.

As just a few examples, a 2015 Toyota Prius is rated for 77 mpg under Japan’s JC08 testing whereas the EPA lets it pass with a far-more realistic 50 mpg. Just as optimistic is the first-generation Chevy Volt which on Europe’s NEDC cycle is rated for 52 miles of all-electric range. The EPA calls it at 38.

The whopper is the Nissan Leaf, rated in Europe for 120 miles range and in Japan as high as 141 miles range. They wish! EPA says 84.

This said, you still may get in the mid 30s in your Prius if it’s malfunctioning, freezing out, or you drive it aggressively.

Those are a hint of some of the reasons why you have ideal “lab” estimates and “real world.” Actually, the EPA says its labs try very hard to simulate real world, but they follow specific averaged tests, and the real-real world has many more factors affecting results.

Following are some of the variables accounting for why your mileage may vary.

You’re a Terrible Driver

Just kidding! We’re sure everyone reading this is very careful and capable, but the EPA does say “how and where you drive” tops its list for why mpg will be on the money, or above or below the mark.

Common sense tips include drive sensibly, observe the speed limit, don’t do jack rabbit starts, avoid wind-dragging cargo on your roof, and don’t carry excess weight in the vehicle.

According to the EPA quick acceleration and heavy braking can reduce fuel economy by up to 33 percent on the highway and 5 percent in slower city and suburban driving.

Excess idling also diminishes your fuel economy average.

Pop quiz: How many mpg do you get when you are sitting still with the engine running.

Answer: You guessed it! Zero. (And if you think this is just us being snarky, the EPA actually does state this for those who need to be told).

Cruise control in open roadways may also let the vehicle return better mileage, though there may be exceptions to this rule.

If you have four-wheel drive that can be switched to two-wheel drive, do so when advisable. Four-wheel drive vehicles are tested in two-wheel drive operation.

Car Needs Work/Maintenance

Cars like people need some tender loving care, and if you are in an abusive or neglectful relationship with your vehicle, don’t expect top mpg.

Checking the Tires

Checking the Tires

You know that maintenance light that’s been lit for a few months? Or the Check Engine light you keep meaning to do something about? To Whom it May Concern: That’s your car talking to you saying it needs to go to the doctor – service technician – for a checkup.

Simple things like monitoring your tires’ pressure and keeping them inflated to specification is also a huge help.

Maintenance and repair keep the car running closer to specification. Old worn cars also may diminish their efficiency potential.

These are facts of automotive life.

Different Fuel Quality

In the winter in cold regions fuel blend may be different than in the summer, and regionally, things vary.

Typical conventional summer gasoline contains about 1.7 percent more energy than typical conventional winter gasoline.


This is over and above whether you pressed regular, mid-grade, or premium on the pump.

Other variances included oxygenated fuels or reformulated gasoline (RFG). These can decrease fuel economy by 1–3 percent.

Today’s gas which commonly has up to 10 percent ethanol (alcohol) is itself less efficient .

Gasoline with 10 percent ethanol decreases fuel economy by 3–4 percent.

Cold Weather

Like most people, cars – including ones using propulsion batteries like hybrids and EVs – like it balmy better,

A short city trip may see a conventional gasoline car’s gas mileage drop by 12 percent at 20°F compared to 77°F. If you hop in and drive a few miles before warming the engine to optimum operating temperature, it can diminish by as much as 22 percent.

The effect on hybrids is worse and fuel economy can drop about 31-34 percent under these conditions which means warming up the car is a good idea.

Beyond this, winter driving poses a number of challenges that could incrementally decrease efficiency, including slippery road conditions in snow belt regions, and extra energy spent on keeping you warm inside the vehicle.

Your Car Was Built on Friday

Have you every heard that saying? It’s a joke symbolizing the last day of the week when factory employees want to go home, might have rushed their work, and – kidding aside – the EPA does acknowledge variances between otherwise identical cars in polite terms.


“Small variations in the way vehicles are manufactured and assembled can cause MPG variations among vehicles of the same make and model,” says the government agency. “Usually, differences are small, but a few drivers will see a marked deviation from the EPA estimates.”

Another normal factor is engine break-in, which could take 3-5 thousand miles before optimal efficiency potential is reached.

Tips For Advanced Vehicles

Aside from cold-weather challenges, hybrids, EVs and plug-in hybrids need drivers to understand how to best utilize their capabilities.


A simple first step is to read the owner’s manual for info on operating the vehicle, maintaining it, maximizing fuel economy, driving range, and battery life as the case may be.

Using the Economy mode also reduces energy usage. Avoiding hard breaking maximizes energy recouped from the regenerative brakes.

Obviously plug-in vehicles should be recharged to maximize propulsion battery range, and being mindful of energy use from accessories such as HAVC also makes a difference.