[JRMichler]

Much of the advice in this thread is unclear to me. So here's my attempt to explain DWL. It's how I drive my truck.

When driving through rolling hills, observe how the cruise control works. Most cruise controls continually overshoot - they let the vehicle slow down uphill, then give it too much gas and top the hill going too fast. A reasonably skilled driver will match the accelerator to the hill and maintain constant speed up and down. Such a driver will normally get better gas mileage driving manually than by using the cruise control.

If that vehicle has a manual transmission and engine similar to mine, he will need full throttle (1-2 inches vacuum) uphill, and back completely off the throttle (20-25 inches vacuum) going downhill.

With some practice, it is found that partial throttle uphill, about 5 inches vacuum, allows getting up the hill while losing only 1 or 2 MPH. Similarly, careful throttle usage downhill allows the vehicle to pick up the speed lost uphill without completely backing off the throttle. The MPG improves some more.

The driver starts by maintaining constant speed by using manifold vacuum over the full range of 1-2 inches to about 25 inches. As the (s)he gets better at DWL, the range of manifold vacuum starts to decrease. First 1 to 25 inches, then 5 to 20 inches, then 7 to 17 inches, then to ????. As the range of manifold vacuum decreases, the speed at the tops of hills decreases, and speed at bottom decreases. DWL is minimizing the range of throttle opening, as shown by minimizing the range of manifold vacuum.

My own driving, after several years of practice, is almost all between 10 and 15 inches vacuum. Most of my driving is 55 MPH in 55 MPH speed limits. I regularly top hills at 50 MPH, and hit 60 MPH at the bottom. This technique is the largest single reason for the gas mileage that I get.