Iowa drivers paid an average of $4.07 per gallon for mid-premium gas earlier this month. At the same time, Michigan drivers paid $4.34. Gas in Wisconsin cost $4.16 and in Illinois the average cost was $4.26 per gallon.

Ways to save money on gas are as varied as the per gallon price at two gas stations at the same intersection.

One theory involves passenger weight. Author Robert Parker wondered about weight versus travel while he was investigating airline policies that penalized overweight passengers. Airlines subscribe to the theory that overweight passengers add to rising fuel costs. Parker wondered about the effect overweight people had on the cost of gas in cars and trucks.

In his book, "57 Ways to Save Money on the High Cost of Fuel in Your Car or Truck," Parker discovered "some Americans are paying dearly at the pump for being overweight. One overweight family could spend an additional $100 to $400 per year for driving exactly the same miles as the thin family in the next lane."

But according to Chicago AAA Motor Club spokeswoman Nicole Niemi, one of the most proven theories is to simply slow down.

"People need to drive slower," she said. "Anything over 60 miles per hour and your fuel costs rise. To give you an example, if you drive 70 miles per hour, it's like paying 40 cents more a gallon for gas."

More people are cutting back on car travel, especially if there are plans for a vacation.

"We've found that a lot of people will plan a year in advance if they want to take a vacation," she said. "Throughout the year, they will use coupons at the grocery story and just try to cut back on day-to-day expenses."

While there doesn't seem to be an end in sight to the rising cost of gasoline, there are several ways drivers can save money at the gas pump. One of the biggest ways is to change driving habits.

By changing driving habits, people can save up to 37 percent in gas costs. In 2005 an Internet site, www.edmunds.com, conducted several tests that all proved driving at lower speeds save gas.

Philip Reed, senior consumer advice editor for the site, and Mike Hudson, news editor, said their results were based on two separate fuel-testing sessions.

"On each occasion we took two cars from the Edmunds.com long-term fleet and drove on a 56-mile test loop. Our route circled Owens Lake near Lone Pine, Calif., at the foot of Mount Whitney, the tallest mountain in the lower 48 states. We chose the route because it was so deserted we could vary our speed and driving style without interfering with the flow of traffic. The only other cars we saw on the route were a caravan of test vehicles from Mercedes-Benz. We drove the loops back-to-back to ensure we were comparing similar wind and temperatures. We logged our results and later put them on a spreadsheet where the results were averaged."

Their results show moderate driving, as opposed to aggressive driving, could reduce gas usage anywhere from 31 to 37 percent, a major savings. Driving at lower speeds also can save an average of 12 to 14 percent at the gas pump.

In addition, the Car Care Council found aggressive driving lowers gas mileage by up to 22 percent on the highway and 5 percent on city streets. Driving aggressively means you will pay up to 49 cents more per gallon.

Another test found that using cruise control saves anywhere from 7 to 14 percent. It smoothes acceleration and prevents gas pedal surging. It also helps a driver look at the entire road length rather that react to traffic changes around them.

It's a hot summer and you want to turn on your car's air conditioner. But you've heard using it will use more gas. Well, turn that air conditioner on because one of the test results shows there wasn't any noticeable difference between opening windows or using air.

You wouldn't think leaving your car running while you run into the convenience store for coffee would use much gas. You would be wrong. Another test result shows excessive idling can cost drivers almost 19 percent more gas than just turning the car off.

But just the way you drive is not the only way to save on gasoline.

Check your tires and wheels. Under-inflated tires or wheels that are out of alignment make your car's engine work harder. Under-inflated tires could cost drivers two miles per gallon in reduced fuel economy, so drivers should check their tire pressure once a month.

Remove unnecessary items from your car and trunk because less weight means better gas mileage.

You will get more miles to the gallon by keeping your engine maintained. A single spark plug that's missing or a dirty air filter can all add up at the gas pump. The air filter acts like a car's lungs. A clogged air filter can improve fuel economy as much as 10 percent. When you change your oil, it's a good idea to get into the habit of checking your air filter.

More maintenance advice from the Car Care Council includes something most car owners never think about: the gas cap. Almost 150 million gallons of gasoline vaporize every year because about 17 percent of vehicles on the road have gas caps that are missing or loose.

Buying a car is the second-largest investment a person will make in their lifetime. With today's rising fuel costs, it only makes sense to try to save money keeping your car repaired and thinking about the way you drive.