Most vehicles, depending on aerodynamics, suffer a dramatic decrease in fuel economy over 60 mph. For every 5 mph over 60, driving costs an estimated additional 26 cents per gallon. This effect is more dramatic on vehicles such as trucks and SUVs with much more frontal area and squared off aft sections. When the rear of the vehicle has a flat area, it causes vortexes -- a swirling of the air causing a partial vacuum -- which increases with speed. This adds to drag considerably. This is used in NASCAR; another car moves in close behind a car and even though it has little aerodynamic drag, it is sufficient at those speeds to pull the rearward car to a small degree. On a large flat area, the drag is quite significant, which reduces fuel economy.
Cruise control maintains vehicle speed, but works the best on flat land. Tests have been done on flat terrain that showed a noticeable increase in fuel economy. It eliminates the surging caused by driving manually, and the sudden accelerations. When using cruise control on steeper hills, the computer increases fuel consumption by opening the accelerator all at once as the vehicle slows down, instead of gradually, to bring the vehicle back up to speed.
Fuel Economy Essentials
Weight and Fuel Economy
To increase the fuel economy dramatically with or without cruise control, start by removing any unnecessary weight in the vehicle. For every 100 pounds, the fuel economy drops 2 percent. Many people carry a lot of non-essential items in their vehicles. Make it a fun mission to see just how high you can get the fuel mileage. Make sure the engine is tuned properly, the oil is changed, the tire pressure is perfect, and the air cleaner element is clean and unrestricted. Check the vehicle body for anything hanging or sticking out to cause drag. Roof racks are excessive drag.
Driving for Fuel Economy
The longer the vehicle is allowed to coast, the better the fuel economy. When approaching stoplights, slow down sooner and allow the vehicle to coast to a stop. When possible, slow down approaching a light on the chance that it will change and it will not become necessary to stop, thus eliminating acceleration from a much slower speed. Slowly accelerate from a stop and whenever possible, maintain a constant speed. Avoid aggressive driving practices. Avoid excessive idling. If you are stopped for a long time, shut off the vehicle. Always use overdrive.
Cruise Control and Terrain
Always turn the cruise control off in hilly terrain. The cruise control has a built-in speed drop limit before it accelerates to regain its intended speed. In hilly terrain, the speed quickly drops, and the cruise control makes abrupt adjustments in acceleration to regain and maintain the intended speed. In many cases, it causes the vehicle to drop down to a lower gear to regain its intended speed. The farther the speed drops, the harder it accelerates; this drops the fuel economy considerably.
Check your cruise control for its drop in speed before it takes corrective action. The speed should drop no more than 5 mph before it takes corrective action. The farther it drops, the harder it accelerates to regain its speed. If the speed drops farther than 5 mph before corrective action, adjust the cruise control box adjustment screw to bring it within limits, and it will result in larger fuel savings.