hills & cruise control, also rpm question

[lostatlantis]

I searched before the post but couldn't find any solid threads. Basically I am interested in how our cars work in terms of efficiency.

I've always wondered if it's better to have cruise control on on hilly roads. Currently I keep it on unless the hills are so steep that cruise will kick the car down a gear and go WOT. Is it better to turn off cruise control with hills in general? My intuition tells me momentum is wasted when the car goes downhill and the cruise is on. It seems to me the car should go kind of like a yo-yo, letting gravity speed it up going down the hill and slow down going up.

Also, when going for fuel efficiency, is it smart/practical when accelerating from standstill to keep the RPM low in the lower gears, or use moderate RPM and shift into higher gears ASAP? I read an article on engine efficiency, and it mentioned that it's not efficient to have the rpm too low. It basically advocated WOT (or at least toward that end of the spectrum) as being more efficient. I don't think that makes much sense though. What do you all think?

[SpeedingFirebird]

I think you will get a lot of opinions on this one.

I find cruise control to be less fuel efficient than handling the throttle myself. This makes sense because on a steep hill, the cruise will really gas it to maintain the exact speed. A foot can be more gentle.

Higher RPMs means less throttle which means less fuel. The 'instantaneuos fuel cosumption' gauge on my BMW confirms this.

[black98V6]

Originally posted by SpeedingFirebird:

Higher RPMs means less throttle which means less fuel. The 'instantaneuos fuel cosumption' gauge on my BMW confirms this.

??? don't you mean lower rpms?


well I have a 5-speed which means no kickdown in cruise, honestly if your car constantly switching gears back and forth you need to just drop out of OD and run in 3rd in a very hilly area. I do know that large trucking companies recommend their drivers use the cruise because in the long run it saves them on fuel.

I admit though if you were to really take the time to control your car on the ups and downs and are willing to loose speed when going up a hill (read: do not increase throttle while going up hill to maintain speed) then yeah you'd get better mileage due to no increased throttle which means no increased fuel consumption. It's a toss up honestly though. I mean think about it, how much better mileage are we talking about here? maybe 1 or 2 mpg tops? I'd rather let the cruise handle the throttle becuase it's more comfortable of a drive for me if the computer is handling the speed control.

[SpeedingFirebird]

Originally posted by black98V6:
</font><blockquote>quote:</font><hr />Originally posted by SpeedingFirebird:

Higher RPMs means less throttle which means less fuel. The 'instantaneuos fuel cosumption' gauge on my BMW confirms this.

??? don't you mean lower rpms?

</font>[/QUOTE]no. higher RPMS require less throttle to keep the car moving. Lower RPMS require a higher amount of fuel for the same effect.