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Old 08-19-2012, 06:16 PM   #1
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I want to straighten out some contradictory mpg tips...

I drive a 2012 Ford Focus, 2.0 liter, automatic...if that's helpful.

1) What is generally a good mph "sweet spot" for optimum gas mileage? I've heard 40, 45, 50, 55 and 60.

2) Should I build up speed gradually (an old classic) or get up to desired speed fairly quickly?

3) On flat or minimal grade roads, is a skillful pedal foot preferrable to cruise control? The old, classic viewpoint was that cruise was better, but I've been hearing differently. Thanks in advance for your help.
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Old 08-19-2012, 06:36 PM   #2
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1) It depends on the car and engine. If the engine is relatively efficient throughout the rpm range, then the lowest speed in the highest gear is often the best, due to aerodynamic drag (which increases quickly with speed). If the engine is particularly inefficient at low load, then a slightly higher speed would be better. There is no single speed which is good for all vehicles & engines.

2) I personally build up speed in the middle of the load range, keeping away from 90%+ of full throttle. In theory on my engine, the load range 85-90% would be best for economy under acceleration. But I also try to avoid stressing the transmission, and the closer you go to max torque, the more stress you are putting into the clutch & gearbox.

3) It depends on the driver. If you have a very steady right foot (keeping load not speed steady), it will beat the cruise.
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Old 08-19-2012, 06:39 PM   #3
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Of course, in an automatic car you are not in control of the gearbox, so you have to drive in a way that encourages the car to be in the highest gear possible (i.e., gentle acceleration).

Be aware that most MPG advice relates to manual cars only (since torque-converter automatic gearboxes are horribly inefficient).
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Old 08-20-2012, 06:29 AM   #4
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1. Certainly no significant improvement below 50mph, above about 60mph aerodynamic drag increases much more quickly, depending on how aerodynamic the car is, gearing, & at what revs the majority of the torque is produced by your engine (see if you can find a power & torque graph for your cars engine).

Here's a couple of graphs showing aerodynamic load vs speed giving you a rough idea. http://www.speed-wiz.com/graphs/aero-drag-graph.htm http://www.kasravi.com/cmu/tec452/ae...ehicleAero.htm

And TBH anything below 55mph is impractical/inconsiderate on any BUSY motorway/highway as trucks would have to pass you causing ques & slowing down other traffic, at least on 2 & 3 lane ones anyway.

Anyway, basically you want top gear with the lowest revs that is still IN the main torque 'curve'.

2.Gradual build up for diesels (they have no throttle plate), short low rev 'bursts' of about 3/4 throttle for petrols with a conventional throttle plate (electric or mechanical).

Although be aware some petrols don't have a throttle plate either so those might be better with a gradual build up (although they still produce a vacuum in the cylinders, that's what wastes power).

Also bear in mind theirs no point quickly building up speed if you've gotta slow down several seconds latter!

3. I hate people who slow down & speed up on motorways! Grrr lol. Letting your speed drop by not giving more throttle going uphills will improve MPG vs steady speed (whether manual or cruise control) but it's a PITA to everyone else!

For steady speed, cruise control is better as it naturally won't let you overspeed downhill. Although with any auto you may well have to manual select top gear to stop the stupid box downshifting unnessacarily (as my 325 did!). Obviously don't let the revs drop too low going off the torque curve (re earlier comment).

MMUK

Don't forget on most autos you can manually select gears. Although if you did that all the time it would kinda defeat the point .

Btw most stress occurs on the clutch during pulling away as its (naturally) slipping, or in rough gear changes. Once the clutch is fully engaged it makes no difference how much throttle you give it (assuming it's not slipping due to being oil contaminated, worn out or its max torque limit being exceeded by large torque increases from remapping excessively etc).

I really wouldn't worry about the torque going through your gearbox, lol. Unless you know it's on its last legs, is an inherently weak box or again the engine has had a large torque increase then it won't make any significant difference to box life avoiding peak torque.
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Old 08-20-2012, 06:53 AM   #5
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> ... or again the engine has had a large torque increase ...

Well, a +30% remap and also a 6th gear added (+28% higher than my original top gear 5th). So in combination +66% extra torque going through the box if I floored it in the new top gear.

I don't remember being able to select specific gears (other than the towing gears) in the two autos I've driven myself (both short-term hires). Is this a common ability in torque-converter autos?
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Old 08-20-2012, 06:56 AM   #6
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& I can pretty much guarantee that the oil is good in mine :-) Changed 3 times this year for one reason & another.
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Old 08-20-2012, 10:00 AM   #7
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My transmission is actually a 6-speed, dual-clutch automatic w/manual mode. I've heard it also described as an automatic manual; not your standard automatic transmission, if that extra information might help. Thanks again, everyone.
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Old 08-20-2012, 05:57 PM   #8
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In that case ... keep it in either manual or 'eco' mode, and try to keep your RPMs down. You can use either the gentle acceleration or quick acceleration technique since the gearbox should support both.
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Old 08-20-2012, 11:44 PM   #9
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Unfortunately, I don't know how to drive a manual.
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Old 08-21-2012, 12:00 AM   #10
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>Unfortunately, I don't know how to drive a manual.

In that case, you're back to the 'gentle acceleration to keep it in a higher gear' technique.

If you learn to use it in manual mode, you will probably enjoy driving the car more since it is more responsive that way.
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