Best MPG, fixed speed or throttle?

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Cruise control is a nice option, it makes driving a set speed easy. However, I hear people refer to it as a device that offers good mpg. Aside from the fact that the unit on our CRV automatic causes unneeded downshifts, I don't think CC offers the best mpg. I think the best mpg would be from a steady throttle setting, with uphill speeds being reduced and downhill speed increased. This is the stragety I employ, going uphill slower to reduce drag when there is also the penalty of ascention and go faster downhill when there is a sort of "excess" potential energy being released, easily overcoming increased drag. Is this correct? I don't think the varied throttle of cruise control is efficient.
 
Yup. But for the average driver, cruise control offers a steadier throttle than their big lead foot.

Out in the hills I'll accelerate before a big incline that I know will cause a downshift. That usually gets me over without cruise control throwing a hissy fit.
 
Many years ago, late 60s-early 70s, you could get a 'throttle lock'; similar to what some put on their cycles.
It would lock the throttle where you set it; would dis-engage with brake use or lever; it was a mechanical devise.
Fell out of favor, if it was popular at all, with the more wide spread availablity of cruise control on new cars and the wide selection of add-on cruise units.
IIRC, it was about $30 and no easier to install than the CC were are familiar with.
 
The best mpg is with a manual transmission rolling downhill in neutral.

The rolling resistance of your tires would play more of a role than setting the cruise versus not setting it, unless you are going up agressive steep hills with the cruise on, and its mashing the throttle for you.
 
On a similar note... am I better off driving 40mph in 4th gear at slightly higher rpms, or taking it 5mph quicker and shifting into 5th gear at lowest possible RPM for best economy?
 
Using your highest gear should always result in best economy, as the engine will be turning less for the same distance traveled. (exceptions for hills/lugging)
 
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The best mpg is with a manual transmission rolling downhill in neutral.




No. Rolling at idle consumes fuel to keep the engine running. My car has a fuel shut-off at the injectors when coasting in gear.
 
I've found my best fuel economy is at 70 km/h, where the transmission has just shifted to the highest gear and the engine is at 1400 RPM. For hills I'll kick it up to 90 km/h where the RPMs are closer to 1900-2000 RPM, or else I have no torque at all and will quickly drop to 60 km/h before the crest of the hill.
 
Quote:


Quote:


The best mpg is with a manual transmission rolling downhill in neutral.




No. Rolling at idle consumes fuel to keep the engine running. My car has a fuel shut-off at the injectors when coasting in gear.




Where does the energy come from to keep the engine turning, if the fuel is shut off?
 
?

If you're rolling in neutral, your engine is idling consuming fuel. If you're off-throttle in gear the DME cuts off the fuel supply consuming no fuel. The motion of the car obviously keeps the engine turning.
 
Quote:


Cruise control is a nice option, it makes driving a set speed easy. However, I hear people refer to it as a device that offers good mpg. Aside from the fact that the unit on our CRV automatic causes unneeded downshifts, I don't think CC offers the best mpg. I think the best mpg would be from a steady throttle setting, with uphill speeds being reduced and downhill speed increased. This is the stragety I employ, going uphill slower to reduce drag when there is also the penalty of ascention and go faster downhill when there is a sort of "excess" potential energy being released, easily overcoming increased drag. Is this correct? I don't think the varied throttle of cruise control is efficient.




What about manual driven cars? It seems they get better gas mileage for obvious reasons than automatics.
However, do your principles still apply?
 
I was thinking about your theory more, and while it looks good on paper, I don't think it'll work in some situations. Throttle position isn't the only thing that affects engine RPM - the vehicle's transmission and speed affect it as well. If you just keep it at, say, 20% throttle, you might maintain 100 km/h on a flat road. Once you get to a hill, your vehicle speed slows down and pulls your engine speed down with it. Once you leave the power band of your engine, speed drops drastically and then your transmission will downshift regardless of throttle position.

I think the best thing to do is to accelerate before a hill to get your engine operating at a reasonable point in your torque curve. 2000 RPM will get my Duratec 30 up most hills, 2500 RPM for steeper ones. If I let cruise control do its thing, then the transmission will downshift and shoot me up at 3000-4500 RPM.
 
"cruise control" is reactive to a change in speed.

Hit a hill, and you'll slow, it tries to accelerate the engine, and then you are in a screaming downshift to maintain speed.

A driver who looks a few hundred metres ahead will see a hill coming up, and sneak a bit of throttle in to get a start at it. They will also see as they crest the rise that there is a corner coming up, and button off...

The CC doesn't see it, but reacts to it.

A networked neural network of learned systems may deliver better economy than a reactive system. Hook that up to a GPS, and every Prius realises where it is, and reacts accordingly.
 
It's been done, manually, on an Insight and the driver was able to boost his mpg by 20%, by knowing where the hills are, and managing the engine and battery accordingly.
 
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