Rolling hills... whats the best way to drive them?
 

I have 20 miles of rolling hills to drive... just about equal parts up or down with no stoplights.

I can:


1. Coast down each hill in Drive (my auto will decelerate going downhill but will use 0gph), then accelerate up the hills under a load

2. Coast down each hill in Neutral (my auto will not decelerate, but will burn idle fuel at about .25 gph), then accelerate up the hills under a load, but with a better entry speed...though the manual shifting of an automatic does worry me.

3. Accelerate moderately down hills with very little load, then coast up the next one as much as possible.


Thoughts?

depending upon traffic,
I generally accel down the hill to my target speed,
then either coast in neutral,
or keep the power up just slightly to attempt to keep speed if slowing down,
then accel for the next hill to peak at the crest while DWL at my lower limit.
Mine is typically 65mph top speed, and 55mph lower.
This does depend on traffic as I drive at peak commute times.

I've wondered this myself as well. A lot of the driving around here is rolling hills and maybe not equal amounts up and down, but what I usually do is accelerate down them, using gravity to allow me to get a higher speed with less power, then I coast up the next hill. Increasing the speed going down seems to provide higher mpg in the end, however, your speed will fluctuate a good bit so it's recommended to do this when there is A: little traffic on the road or B: a way for cars to go around you without obstructing the flow of traffic. Being behind a person who is fluctuating their speed a lot can be frustrating for drivers who like to use cruise control on such roads.

This might not be the best way, but I use cruise control, the ScanGauge and the up down buttons for speed when in rolling country. For me flat and level highway cruising is done with a LOD in the mid to high 30 range.

When climbing, what I do is I press the minus button very quickly to bleed off speed in very small increments to keep my LOD fairly even to achieve some kind of minimum summit speed - kind of like what you have left over when climbing a hill with your bicycle. If there's a lot of close traffic behind you, you can't have as low of an exit speed. You can also take a note on the grades of various parts of the hill and on the steeper sections is where you'll bleed speed quicker. A moderate hill might have a target LOD of 50, steeper 60 and a long mountain pass 70 if its too long to bleed much speed.

Once summiting, I wait for the LOD to drop below 30 and then I start adding speed back - first to get back to travel speed and again like the grades going uphill, the first steeper spot will be where you can start gaining back the speed quicker. If the LOD drops below 20, I go to neutral. The only exception would be if there was another very long hill coming up and low risk of a speeding ticket, then I'd try and see what kind of speed I could build up with the LOD still well under 30.

Bleed enough speed and your fuel economy will actually go up while climbing but only for so long. It's a tough compromise but my right thumb likes this technique. BTW, in our Accord this doesn't work well at all. It seems to work well with a slushbox / DBW / very small step CC. In the Honda, each button tough is way to much, easier there to try and maintain a constant TPS on the climb that bleeds speed at the desired rate to get you to the top at your desired summit minimum speed.

Option 4: Maintain constant throttle up and down hill - don't adjust for speed

For me, I've found optimal to be accelerating down hill and then pulling back on the throttle going up hill (to avoid going too slow in traffic). No use burning fuel against gravity AND wind :D Let gravity win, and then use your weight to pull you back down... Gravity always wins :P

Of course, YMMV - hill size is a big factor in picking a method...

I drive in mountains that kind of roll here in Vermont, with an automatic, generally. The technique that has yielded the best results are for me to pick a target RPM and speed. I accelerate on the downhill, not exceeding the RPM, till I get the desired speed; I then coast usually not in neutral, unless it's a really long way down. On the up hill I add throttle keeping my target RPM and not caring about how far I drop in speed. Whether this is really the best way, I don't know, but it has worked out the best for me. Constant throttle may keep traffic happier but that's why they have other lanes, so I can get much better mileage.

This is the same method I use. I keep the same rpm up the hills while the speed bleeds off all the while giving more throttle keeping rpm constant. With the automatic I have to keep the speed above 48mph or the transmission will kick down. Once on top keeping rpm constant until I accelerate back to speed then coast.

rolling hills
 

The SAE published two books on mpg years ago.One article dealt with the Chrysler team which entered the Shell Mileage Marathon,and how they dealt with hills.They pre-tested their car for best mpg per road grade.Then,during the test,they used a ball-bearing in the bottom of the ashtray as a crude inclinometer to tell the grade of the road,and then drive according to the pre-determined speed for highest economy They did allow the car to accelerate on downhills and then basically held a constant throttle position as the car lost speed on the uphill.They might let speeds fall to 50,40,30,or 20-mph,depending on grade.They never attempted to gain speed on an uphill reach,avoided moving the throttle as much as possible.I favor letting gravity help accelerate on downhills,although the highway patrol watches that sort of thing.Be careful.I've enjoyed good results driving a constant throttle setting,letting the speed rise and fall as it may.I understand that some automatic transmissions will burn up if coasted,however that may only be true for an "engine-off" scenario,where the pump is not turning.Don't know,be careful.as you know,trannies are very expensive!The other thing is that if there is only one lane in my direction,I'll drive the posted speed limit if there is traffic around.If the highway is empty,I'll take liberties with speed.Impeding following traffic,which is moving at the speed limit wastes the other guy's fuel and makes for hard feelings.I won't do it.