You might as well give up now: There’s no way you’ll achieve the EPA’s mileage ratings that are so prominently touted on the window stickers of new cars in dealerships’ showrooms.

New car buyers place a lot of faith in the Environmental Protection Agency’s fuel economy numbers on the window sticker only to find out later they can’t achieve those numbers.

That’s the conflict behind the disclaimer found in the fine print on every window sticker that says “Actual results will vary for many reasons, including driving conditions and how you drive and maintain your vehicle.”

It’s not because we’re bad drivers, although driving habits certainly can improve or hurt gas mileage. There are actually a few reasons beyond our control regarding why we can’t achieve those results, according to AAA car expert Jim Prueter.

First, EPA testing does not mirror real-world driving. For example, the EPA’s city driving tests assume an average speed of 21.2 mph. The highway test procedure is calculated using a top speed of 60 mph and an average speed of just 48.3 mph in “free-flowing traffic.”

“These numbers are unattainable by regular motorists because the testers are driving exactly 21.2 miles per hour on a perfectly flat surface,” Prueter said.

That’s only one thing manufacturers do to arrive at their models’ inflated numbers. Since mileage is a top consideration when buying a car, it makes sense. Among consumers, mileage often is considered more important than a vehicle’s quality, value or safety. As a result, the average mileage of new cars continues to creep up.

The gasoline the EPA uses to test vehicles in its labs contains no ethanol. Gasoline at most fuel pumps nationwide contains as much as 10 percent ethanol, and that can lower fuel economy by about 4 percent, according to the EPA.

There are more than 260 makes and models from all automakers in 2013, but the EPA tests just 10 to 15 percent of them. The agency relies on automakers to test their own vehicles and submit data for review. Also, automakers usually bundle one set of EPA city and highway figures for all the variants of a particular model.

Also, manufacturers are permitted to use the same engines in different models — and use the same statistics. Under the EPA’s test procedures, all car configurations with the same engine, transmission and weight class are grouped together. That is true even if they have different sheet metal such as the Ford Focus sedan and Ford Focus hatchback, or they are entirely different nameplates, such as the Fusion and the C-Max.

An automaker can test the best-selling car in the group and use the results for all configurations, a concession the EPA made long ago to spare car makers the expense of redundant testing.

Automakers are quick to boast about their EPA ratings. But sometimes they get caught.

A couple of years ago, Honda was sued by drivers who said their Civic hybrids weren’t getting anywhere near the mileage stated. The company ended up paying Civic hybrid owners who filed a claim about $100 for each hybrid.

In 2012, Hyundai and Kia inflated their models’ gas mileage ratings until the EPA demanded they show their data. When they were unable to do so, the companies were fined and forced to reduce their mpg numbers.

Ford did the same with its C-Max hybrid. After receiving complaints, Ford lowered its mpg ratings in August.

So, the next time you’re in a showroom and see a vehicle’s mileage sticker, Prueter has some advice: “Take 10 to 20 percent off that number, and you should be pretty close,” he said.

Valerie Vinyard is a public affairs specialist for AAA Arizona. Contact her at