If a summer road trip takes you to the Detroit area, you may want to consider filling up the gas tank before driving back to West Michigan.

Learn why Detroit-area summer gas is the most efficient in Michigan.

To understand why Detroit gas is the most efficient, we must first understand a few things about how gas works.

The U.S. Department of Energy explains that winter gas is less efficient than summer gas. They say gas mileage is about 12 percent lower at 20 degrees than it would be at 77 degrees.

The lower winter gas fuel economy is due to a variety of reasons. One reason is that winter gas contains less energy per gallon than summer gas.

Popular Mechanics explains that there are about 20 different blends of gasoline available to meet various state and federal regulations. All blends are below 14.7 PSI or gasoline would become a gas.

In summer, the point at which gasoline becomes a gas is lower due to the change in atmospheric pressure. The EPA requires summer gas to be between 9.0 PSI and 7.8 PSI. AAA says that gives conventional summer-blend gasoline 1.7 percent more energy than winter gas.

Jennifer Holton of the Michigan Department of Agriculture & Rural Development tells us that eight Michigan counties have a summer vapor pressure requirement of 2 PSI less than other Michigan counties (7.0 v. 9.0) due to requirements to help SE Michigan be in compliance with the federal Clean Air Act.

Those requirements, which run from June 1 to September 15, make fuel found in those eight counties more efficient.

The eight Michigan counties required to have more efficient summer fuel are: Lenawee, Livingston, Macomb, Monroe, Oakland, St. Clair, Washtenaw, and Wayne.

The summer-blend gas found across SE Michigan is more efficient, but it can also be more expensive so purchase accordingly.