I find that using power to go up hills and letting it coast downhill both gives me the steadyiest possible cruise speed and excellent gas mileage. By excellent gas mileage, I mean 45 to 47 mpg in a 2007 Toyota Yaris 5 speed manual.
The energy used to accelerate cars and climb hills is not wasted energy, it is stored energy. The kinetic energy of speed is not wasted until you throw it away by nailing the brakes. The potential energy of elevation is not wasted unless you have to ride the brakes all the way down the hill. The less you use the brakes, the less stored energy you throw away. Air resistance is another brake that throws away energy at a rate that is exponentially proportional to speed, so high peak speeds of a varying speed also throws away a lot of energy.
Ah, but alas, not quite. You can control it to a better degree than your cruise. CC will just floor it when the speed drops more than X MPH (for me, it’s like 3 or 4) - to include downshifting and higher (sometimes much higher) RPM’s - whereas you can allow your car to slow a tad, without putting the pedal to the metal.
Chassios, mine keeps it constant. It does downshift of necessary, but on a normal fed-spec modern highway it doesn;t need to. It’s just on hilly backroads that it needs to.
That does, however, vary by vehicle.
Whitey, you made an excellent point about the cruise control.
You merely make my point for me. On level ground, cruise is the way to go.
On hills, however (where almost this entire thread was focused), throttle control will yield better results than cruise will. On inclines, depending on the incline, speed set, load, and other conditions, cruise can and will downshift to maintain speed. Sometimes more than one gear, bring your RPM’s from a meager 15-1800 up to as high as 3500. Once it slows enough (varies by vehicle, normally about 10), it will disengage on it’s own.
Chaissos…is that a 4runner I see ? You need a more powerful vehicle if your cruise downshifts more than one gear or “floors” it. But I agree and would not use cruise on hills that required that much throttle. Maybe you need a hemi ?
Whitey December:
"“I find that using power to go up hills and letting it coast downhill both gives me the steadyiest possible cruise speed and excellent gas mileage.”
What a coincidence! That’s how your cruise control does it! "
Yes, but cruise control doesn’t shift into neutral and let the engine idle while coasting down the hills.
I agree with kieth. The only thing is he left out one important fact. All this needs to be second with safety first. Disrupting traffic around you is not safe. If you notice other drivers getting irritated, forget saving one mpg and go with the traffic.
That said, it is not all that difficult to apply the principles presented without disrupting traffic.
I suspect keith does not allow his economy driving to disrupt traffic around him.
Yes, it’s a 'Runner. The only time it downshifted like that that was coming back down to TX, from NH, through the mountains. Uphill, with almost 4K behind it. After that, the cruise was left off for the remaining 2500 miles (or so) - and the wife was left in the passenger seat.
I have the V6 version, and it has plenty of pep for my normal driving. It’s rare for me to tow that much (3 times in 7 years, bought it new). I didn’t get the V8 on purpose, and the hemi motor (IMO) is just a waste of fuel. You can get off the line faster than me, sure, but you’ll run out of gas a heck of a lot faster than me, too.
Thank you Joe, I do not hold up other drivers. In fact, I am very safety conscious and part of that is keeping a low profile. If I pass you, or you pass me, my goal is that if someone asks you 5 minutes later if you remember that blue Saturn, you won’t.
The SameMountainBike, I don’t think you understand my approach. The difference between my speed going down the hill is not that much faster than my speed going up. A total difference of about 10 mph. Most roads I drive on are 4 lane so anyone who wants can go around. On two lane roads, if you want to get by me, I will do what ever I can to help you get by. Chances are I am right on the edge of getting a ticket anyway so I’d rather you get it for me. I have seen more than a couple of cars/trucks that have passed me on the side of the road a short time later with blue lights behind them. I whisper a thank you as I go by.
I get about 38 mpg overall in my Saturn. It was rated at 29/40 under the old system and I have to use AC in the summer. I drive about 60% highway, the rest in town/city. BTW, in the city, I watch as many lights ahead as I can and adjust my speed so that I don’t have to stop. That does have more potential to anger other drivers, but most of the streets are multi lane so they can just go around. Sometimes I end up angering someone, even though I am in the right lane and they have plenty of room to go around, and they are going well above the speed limit. I’m talking about variations of 5 mph and still slightly above the speed limit, but that is enough in most cases to not have to stop. Of course you can’t catch them all green, you have to accept that and I won’t go too far out of my way to try.
Oh one more thing, unfortunately for me, there have also been a few occasions when there wasn’t one of those people who wanted to pass me when I desperately needed one. I tried using that excuse on a cop one time, but he didn’t see the humor.
Keith, I understand the approach but still maintain that speed should be kept as constant as reasonably possible. A a 10 mph variation is, IMHO, too great.
I guess we’ll have to agree to disagree on this one.
Whitey: @B.L.E.: “Yes, but cruise control doesn’t shift into neutral and let the engine idle while coasting down the hills.”
"If you let your car build up speed going downhill, it isn’t “the steadyiest possible cruise speed…” is it? "
What makes you think the car speeds up going down those hills? Do you think I live in Colorado? Have you seen my commute to work? Some of those downhills are so mild that the car actually slows down a little. Also, I don’t treat my target cruising speed as a sacred cow like cruise control does.
By “steadiest possible speed” I meant avoiding extremes in speed and going zero miles per hour at a red light is one extreme that many people who think they drive at a steady speed spend a lot of time doing.
I feel there is a lot of misunderstanding about engines and efficiency. Engines don’t really want to operate a constant rpm, they want to operate at a constant load.
There are a lot of downhills on many roads that have just enough downgrade that gravity is doing the entire job of propelling the car, the engine is making zero horsepower, yet is burning a lot of gas. By opening the throttle a little more, the engine is making some horsepower instead of zero horsepower for just a small amount of extra gas consumption which is offset by the lower fuel used in the next uphill due to the cars increased speed at the beginning of the hill.
"But doesn’t the fuel injection completely shut off when you are coasting downhill?"
No, it doesn’t.
In order to allow the car to coast without slowing down, the engine has the throttle open just enough so there is no engine braking. It is burning the same gallons per hour that it would be if you were parked in neutral and were free revving the engine at the same rpm your car cruises at.
My method of shifting to neutral and letting the engine idle substitutes idle fuel consumption for 2400 rpm free revving fuel consumption. I use less gas, my results prove it.
There are situations where the downgrade is so steep that you actually need engine braking to help keep the car’s speed in check. That’s the condition where the fuel cuts off and when I find myself it such a situation, I actually do keep it in gear.
