Guides
Background reading for anyone comparing vehicles by fuel economy, from how the EPA rating is measured to what to expect when cross shopping a gas car against an EV. Every fuel economy and cost figure elsewhere on this site traces back to the standardized testing process and unit conventions explained in these guides, so they're worth reading before you take any single number at face value.
Why the testing details actually matter
It's tempting to treat a fuel economy rating as a single settled fact, but every number on this site, from a specific vehicle's combined MPG to an annual cost estimate to a side by side comparison, is built on the same underlying lab test methodology. Understanding roughly how that test works, what it does and doesn't simulate, and why an electric vehicle's MPGe isn't directly interchangeable with a gas car's MPG, changes how you should read every other number on this site: not as a promise of what you'll personally experience, but as a fair, standardized basis for comparing one vehicle against another.
This matters most when the numbers are close. A 2 MPG gap between two competing vehicles is real and measured consistently, but it's also small enough that your own driving style could easily overwhelm it in either direction. Knowing what actually moves real-world results, covered in detail in the EPA testing guide, helps you judge when a fuel economy difference between two vehicles is large enough to matter and when it probably isn't.
- How EPA Fuel Economy Ratings Are MeasuredHow the EPA actually tests a car's fuel economy in the lab, why the rating differs from what you will see on the road, and what changed in 2008 to close that gap.Last updated July 24, 2026
- MPG vs MPGe ExplainedWhat MPG and MPGe actually measure, why an electric vehicle's MPGe number looks so much higher than a gas car's MPG, and how to compare the two fairly.Last updated July 24, 2026
Frequently asked questions
The dashboard estimate is calculated by the car itself in real time and commonly reads 5 to 15% more optimistic than a true tank-fill calculation, while the EPA sticker comes from a standardized lab test. Neither one is wrong exactly, they're just measuring under different conditions; see our guide on how EPA ratings are measured for the full breakdown of what the lab test actually covers.
They're on the same numeric scale but measure different things: MPG counts gallons of liquid fuel actually burned, while MPGe converts an EV's electricity use into a gasoline-equivalent figure using a fixed EPA conversion constant. Both are useful for comparing efficiency, but for a true cost comparison you need to convert both to cost per mile using local gasoline and electricity prices; our MPG versus MPGe guide covers this in detail.
Yes. The EPA testing guide applies to every powertrain sold in the US, including diesel and hybrid vehicles, and the MPGe guide specifically addresses how plug-in hybrids get both an MPGe rating for electric-only driving and a separate MPG rating once the gasoline engine takes over.
The window sticker is the EPA rating; dealers don't calculate their own figures. What you should be cautious of instead is treating that lab-derived number as a guarantee of your own results, which is exactly what these guides walk through: the testing process behind the number, and the specific real-world factors that predictably push individual results away from it.
Each guide carries a last-updated date at the top specifically so you can see how current it is, and the content is reviewed against the latest publicly available EPA testing documentation rather than left static indefinitely.
Start with the EPA testing guide, since it explains what the numbers on every vehicle page on this site actually mean and why your own results will differ somewhat. If you're specifically weighing a gas vehicle against a hybrid or EV, read the MPG versus MPGe guide next, since it directly addresses how to compare ratings that aren't on the same scale.
No, though the two are closely related. Fuel economy is a measure of energy efficiency, miles traveled per unit of fuel or energy, while fuel cost also depends entirely on the price you pay for that fuel or electricity, which varies by region and by fuel type independent of how efficient a vehicle is. A very efficient vehicle in an expensive fuel market can still cost more to drive than a less efficient one somewhere fuel is cheap.
Not directly; each calculator page includes its own explanation of how to use it. These guides instead cover the background concepts, what the numbers mean and where they come from, that make the calculators' results meaningful once you have them.