Use the Recommended Grade of Motor Oil You can improve your gas mileage by 1-2 percent by using the manufacturer's recommended grade of motor oil. For example, using 10W-30 motor oil in an engine designed to use 5W-30 can lower your gas mileage by 1-2 percent. Using 5W-30 in an engine designed for 5W-20 can lower your gas mileage by 1-1.5 percent. Also, look for motor oil that says "Energy Conserving" on the API performance symbol to be sure it contains friction-reducing additives.*
Drive Smoothly
The EPA recommends you avoid the three hallmarks of aggressive driving: speeding, rapid acceleration and hard braking. All of these add up to a large decrease in fuel economy. Also, just like in your home, the more you use the air conditioning, the higher the energy bill.
Remove The Excess Weight
All that unwanted weight in your car causes your fuel economy to plummet. The EPA says an extra 100 pounds in your vehicle could reduce your miles per gallon by up to 2 percent. It may not seem like a lot, but it adds up over the long haul.
Monitor Your Vehicle
The EPA suggests you keep on a routine maintenance schedule. That means keeping your car tuned, checking and replacing air filters on a regular basis, keeping the tires inflated to the proper PSI and using the recommended grade of motor oil. These few steps will go a long way to increase your mileage.
*according to fueleconomy.gov