Does cruise control save gas? And what's the rule of thumb where speed and mpg are concerned?
In some ways, yes it can. The faster you go, the more fuel there is that gets used per mile because the car is fighting the air that it is moving through. We refer to this as resistance from the air as "drag". The faster you go, the greater the drag. The cruise can be very effective at preventing the driver from going faster, and faster, and faster when on the highway. I use my cruise for exactly that reason, although I use it more to prevent me from speeding and to keep from getting a "high speed driving certificate" than because of the fuel mileage issue. :)
As far as the rule of thumb and speed are concerned, every car is different. You will have to be very dilligent to test average fuel useage and correllate that to your own vehicle. Peak fuel mileage will actually occur somewhere between 40mph and 65mph on the majority of cars.
All that stuff that john g said is true. I remember the owners manual or other paperwork that came with my '74 Duster stated it got it's maximum fuel economy at 37 mph. That stat would be different for every car due to variables in weight, gearing, engine hp and displacement, wind resistance due to the shape of the car...all kinds of things.
Saw a show, mythbusters maybe, that studied this. On that program they concluded you got better fuel economy not using it. Can't remember why.
I use cruise control to reduce the chances of getting a ticket too. I've got a commercial drivers license which I'd loose if I got a ticket for going 15 or more over the speed limit. Caught myself exceeding that by 'running with the pack'. Don't want the police to.
What about mountain driving? Do you let it downshift itself to keep the speed up or disconnect, slow down and go up hill manually?
In other words is it more fuel economical to downshift, keep the speed up and get over the hill sooner at low mpg (8 mpg typically) or slow down and take longer up the hill at higher mpg 16 (for example)?
Assume an 8 mile hill, @ 64 mph, 8 mpg. Time to top is 1/8 hr, gas rate is 8 g/hr. Total gas burned is 1 galn.
@ 32 mph, 16 mpg, Time to top is 1/4 hr, gas rate is 2 g/hr. Total gas burned is 1/2 galn. ---But it took 1/8 hour longer.
This assumes constant velocity bottom to top. The truth is somewhere in betweeen, of course.
The energy required to get up a hill reaches a point very quickly that it exceeds the air drag. It simply will take more fuel no matter what. Coming back down the hill, to control speed, you will be faced with touching the brakes, which disengages the cruise anyway, or downshifting, which depending on the vehicle manufacturer will likely cause a cruise disengage as well.
As for your different climb rates I don't have any real reference to know how much fuel can be saved by significantly slowing for the climb. I'd hate there to be hills around here that take seven to fifteen minutes to climb. There are a few really big ones east of here in the Appalachians, never timed the climbs and descents though.
Contrary to some information out there. Mountain braking with the hybrids does not put all of the energy back into the batteries. The batteries are operated between about 40% and 80% charge capacity to improve the battery life. Once the batteries are charged fully (about 80%) the rest of the energy is just wasted.
I've definitely got a lead foot. I find it almost impossible to maintain a steady speed with a conventional accelerator. Thus I find myself going faster, then slowing down over and over again. Cruise control it is then.
It seems I have to expand the topic a little bit. Somewhere we were on the topic of AC's contribution to the subject of gas consumption. As I recall, the loss equals approximately 1 mile of mpg rating. So if I were to ride sans air conditioning, what is the effect of open windows and drag? That seems like it also would be a substantial loss. So what's the best way to drive a car from point A to point B without wasting gas?