The causes of a sudden decrease in gas mileage

Written by ricky andromeda
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The causes of a sudden decrease in gas mileage
There are several causes for sudden decreases in gas mileage. (petrol station image by Mat Hayward from

Gas is expensive, even if you drive a fuel-efficient car. It's important to maintain your vehicle with regular tune-ups and home checks to make sure you're getting the most out of your automobile. If you notice a sudden drop in miles per gallon (mpg), don't run straight to a mechanic. Instead, check on these common reasons for a sudden mileage decrease that you may be able to fix yourself.

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Inefficient Driving

Inefficient driving can drastically affect your vehicle's gas mileage. Follow the speed limit and avoid going over 60 miles per hour for long amounts of time, as most vehicles become less efficient over that speed, according to the U.S. Department of Energy. If you take a long trip on interstate highways and you constantly drive more than 60 miles per hour, you will notice a decrease in gas mileage.


Before an oil change, always make sure that you get the right viscosity for your vehicle. Most vehicle owner's manuals state exactly what type of oil to use for optimum mileage, with names like 10W-30 or 5W-30. If you get the wrong oil you may notice a drop in mpg and overall poor performance, especially in extreme heat or cold.

Heavy Loads

If you have been driving with a lot of extra weight in your vehicle, you may be decreasing your gas mileage more than you would imagine. For every 90.7 Kilogram of cargo and passengers in your vehicle, you lower your mileage by 4 per cent, according to the Federal Trade Commission. Eliminate excess weight by emptying your boot, back seats or cargo areas.

Tire Condition

Tires can drastically affect your vehicle's mpg. Check your tires regularly and keep their pressure at the manufacturer's recommended pounds per square inch (psi) for optimum gas mileage. The amount is usually printed right on the tires themselves. Also, tires that are old and have begun losing tread can lower your mileage. If your tires are balding or in poor condition, have them repaired or replaced.

Thermostat Problems

In newer model cars, your engine coolant is regulated by a computer-run thermostat. If the thermostat malfunctions or breaks, your vehicle may be cooling itself all the time, even when it doesn't need to, resulting in bad performance and terrible gas mileage. Take your vehicle to a mechanic and have them run a diagnostic test on it if you believe this is the problem.

Blocked Air Filter

For older vehicles with carburettors, a clogged or dirty air filter results in poor air intake and a gas-rich fuel mixture. Using more fuel than necessary lowers your mileage and is bad for your engine. If your vehicle doesn't have a carburettor, a blocked air filter can cause bad acceleration, but not bad mileage.

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