Gas-saving tips put to the test

Edmunds.com tests common fuel-saving wisdom and finds some changes make little difference.

NEW YORK (CNN/Money) - Common gas-saving tips were put to the test recently by Edmunds.com, CNN.com's automotive content partner.

While most, it turned out, actually did save gas, some of the fuel-saving advice you commonly hear didn't make much difference at all.

Drivers took two different vehicles on the same 56-mile route eight times under various conditions. The vehicles, both Ford Motor Co. products, were a 2005 Ford Mustang GT and a Land Rover LR3 SE. Both have V-8 engines. The Mustang's EPA-estimated mileage is 15 in the city and 25 on the highway while the Land Rover's is 14 in the city and 18 on the highway. In ordinary driving, most vehicles usually get about 15 percent lower gas mileage than EPA estimates indicate.

Using cruise control

Drivers are often advised to use cruise control during long highway drives on level roads. The idea is that it prevents "speed creep" -- the tendency to gradually start going faster the longer you drive -- and cuts down on other unnecessary speed changes which can eat up gas.

In Edmunds.com's test, the Land Rover got almost 14 percent better mileage using cruise control set at 70 miles per hour rather than cruising at driver-controlled speeds between 65 and 75 miles per hour.

The Mustang got 4.5 percent better mileage with cruise control.

Verdict: It works

Roll up windows, use A/C

When driving on the highway, most of a vehicle's power is used simply to overcome aerodynamic drag. On the other hand, air conditioning also uses a lot of power.

Drivers are often told that keeping the windows rolled up, which significantly reduces drag, and using air conditioning actually results in better fuel economy than driving at high speeds with the windows open. In fact, CNN/Money has offered the same advice in a story we've run on our site.

In Edmunds.com's test, conducted at a steady 65 miles per hour, "windows down" or "A/C on" made virtually no difference in mileage.

The Mustang got 4.1 percent better mileage -- 30.7 mpg vs. 29.5 mpg -- with the windows down. The Land Rover got 1.6 percent better mileage with the windows down, a difference of just 0.3 miles per gallon.

Basically, the extra fuel used by the air conditioner is made up for in improved aerodynamics at high speeds. Your not really saving any fuel but, at least on the highway, the A/C isn't costing you appreciably either.

Verdict: No difference

Avoid hard acceleration

You've probably been told before that rushing up to stop lights and racing away from them wastes gas. Turns out it wastes a whole lot of gas, according to Edmunds.com's test.

Slowing zero-to-sixty times to 20 seconds instead of 10 to 15 seconds makes a big difference in mileage over the long term. Braking slowly and gently helps, too.

Driving the Land Rover gently resulted in a 35.4 percent increase in gas mileage while feather-footing the Mustang garnered a 27.1 percent mileage increase.

Verdict: Big difference

Properly inflate tires

Not having enough air in the tires can cause a number of problems, including a potentially dangerous blow-out. But it also reduces gas mileage.

In Edmunds.com's test, the mileage improvement with properly inflated tires, compared to slightly under-inflated ones, was slight but noticeable in the Land Rover but non-existent in the Mustang. Still, for safety's sake, keep enough air in your tires.

Verdict: Small impact on mileage

Full story from Edmunds.com