There are many ideas floating around the internet which promise to save you money on gas. Cosumer Reports says the following three tips are myths.
A common tip is to buy gasoline in the morning, when the air is cool, rather than in the heat of the day. The theory is that the cooler gasoline will be denser, so you will get more for your money. But the temperature of the gasoline coming out of the fuel nozzle changes very little, if at all, during any 24-hour stretch. Any extra gas you get will be negligible.
Air conditioning vs. opening windows
Some people advise you not to run the air conditioner because it puts more of a load on the engine, which can decrease fuel economy. But others say that opening the windows at highway speeds can affect gas mileage even more by disrupting the vehicle's aerodynamics. In our tests in a Honda Accord, using air conditioning while driving at 65 mph reduced the vehicle's gas mileage by over 3 mpg. The effect of opening the windows at 65 mph was not measurable.
A dirty air filter
Our tests show that driving with a dirty air filter no longer has any impact on fuel economy, as it did with older engines. That's because modern engines use computers to precisely control the air/fuel ratio, depending on the amount of air coming in through the filter. Reducing airflow causes the engine to automatically reduce the amount of fuel being used. Fuel economy didn't change, but the Camry accelerated much more slowly with a dirty filter.
Consumer Reports offers eight ways to see actual gas savings.
Drive at a moderate speed
This is the biggest factor. You may have to be a little patient, but driving at 55 mph instead of 65 or 75 will save you money. When we increased the Camry's highway cruising speed from 55 mph to 65, the car's fuel economy dropped from 40 mpg to 35. Speeding up to 75 mph cost the car another 5 mpg. One reason is that aerodynamic drag increases exponentially the faster you drive; it simply takes more fuel to power the car through the air.
Avoid hard acceleration and braking whenever possible. In our tests, frequent bursts of acceleration and braking reduced the Camry's mileage by 2 to 3 mpg. Once up to speed on the highway, maintain a steady pace in top gear. Smooth acceleration, cornering, and braking also extend the life of the engine, transmission, brakes, and tires.
Reduce unnecessary drag
At highway speeds, more than 50 percent of engine power goes to overcoming aerodynamic drag. So don't carry things on top of your vehicle when you don't have to. Installing a large Thule Cascade 1700 car-top carrier on our Camry dropped its gas mileage from 35 mpg to 29 at 65 mph. Even driving with empty racks on the car reduces its fuel economy.
Don't use premium fuel if you don't have to
If your car specifies regular fuel, don't buy premium under the mistaken belief that your engine will run better. The only difference you'll see is about 20 cents more per gallon. Most cars are designed to run just fine on regular gasoline. Even many cars for which premium is recommended will run well on regular. We have found that the differences are imperceptible during normal driving. Check your owner's manual to find out if your engine really requires premium or if you can run on other grades.
Minimize driving with a cold engine
Engines run most efficiently when they're warm. In our city-driving tests, making multiple short trips and starting the engine from cold each time reduced fuel economy by almost 4 mpg. Engines also produce more pollution and wear faster when they're cold. When possible, combine several short trips into one so that the engine stays warm.
Keep tires properly inflated
The Camry experienced a 1.3 mpg loss in highway fuel economy when the tires were underinflated by 10 psi. More important, underinflated tires compromise handling and braking, and wear faster. And they run much hotter, which can lead to tire failure. Check the pressure of your vehicle's tires at least once a month with a tire gauge. The owner's manual explains how to do it.
Buy tires with lower rolling resistance
A tire's rolling resistance can add or detract another 1 or 2 mpg. In our tire ratings, look for high-rated tires with low rolling resistance. They generally won't cost more, and replacing a worn tire could save you more than $100 a year in fuel.
Avoid idling for long periods
Think of it this way: When you're idling, your car is getting zero miles per gallon. When we let a Buick Lucerne, with a V8, idle for 10 minutes while warming up, it burned about an eighth of a gallon of gas. A smaller engine would probably burn less, but idling still adds up over time. As a rule, turn off your engine if you expect to sit for more than about 30 seconds. An engine warms up faster as it's driven anyway.