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Going slower will increase mileage. That is a separate calculation.
Driving in hills is also a separate scenario.
On a dyno when you increase the load on an engine it will increase the power far in excess of the additional fuel required. 20 HP using 1 unit of fuel, while 50 HP uses 1.5 units of fuel. The additional 30 HP only cost half as much fuel as the first 20, so you are getting much more power for only half as much fuel. This overwhelms the aero drag calculation by a significant amount.This is due to the fact that the lower manifold vacuum under higher loads allows up to double the effective compression in the cylinder and much more power produced for the equivalent amount of fuel.
This is the essence of pulse and glide. My Fiesta will glide down to below 20 MPH and still get over 100 MPG.
Hills offer a chance to minimise the speed variations as long as the hill is not so steep that you can not maintain a safe, or legal speed on the downhill portion. The grade of the hill is crucial to the strategy of climbing and descending grades.
If your downhill coast can be maintained within a legal speed then you have the better choice. Lowest speed at the crest of the hill, highest speed at the bottom of the hill.
Climbing the hill allows you to store energy by increasing altitude. DWL alllows you to pick a load that minimises your mileage hit while climbing with maximum benefit in the downhill coast where you mileage can soar to hundreds of MPG. If you climb at 20 MPG and coast downhill at 200 MPG then it is easy to see that you can average excellent mileage overall.
If your peak speed downhill exceeds safety or legality then just use engine braking instead of brakes, since engine braking consumes no fuel, compared to coasting in neutral, but it is better to coast over engine braking in most cases, again depending on the grade of the hill.
While aero drag does increase as the square of speed, it does not increase to the point where the increased engine efficiency in the pulse overwhelms the gain in economy if you use the pulse and glide technique, as long as you do not exceed safe speeds and or legal speeds. It also does not mean you have to average a lower speed as long as you keep the variations in the range of your target speed.
Using this technique, refined over many miles of experience, I can average over 50 MPG in my Fiesta on the Interstate, while maintaining the average speed of traffic, in a car rated at 38 MPG highway, but the hills here are not steep enough to coast at 65 MPH. To achieve that mileage at that average speed requires focus, work, and concentration, and some drafting also helps.
regards
Mech
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