Debunking the debunkers of fuel economy bunk

[AZhitman]

Fun little article we whipped up this week:

http://www.nicoclub.com/archives/debunk ... myths.html

[MinisterofDOOM]

The problem with the original article is the same problem nearly all "[x] ways to do [x] better" articles suffer from: It needlessly oversimplifies. It tells you WHAT to do, but not WHY it matters. And because of that, it ends up missing its own points and even being blatantly wrong in some areas. And that's ignoring the whole "teach a man to fish..." aspect. Instead of listing what people should do, the real goal should be to help people understand what aspects of their driving affect their fuel economy in which ways.

There is one case where I can see where they might have been trying to go, but their shot fell far wide for the above reasons.

A dirty air filter hinders fuel efficiency. Older engines pulled air through the filter into the carburetor. That’s not the case today, so air filters don’t affect fuel use.

I think what they're getting at here is that modern fuel-metering methods adapt fuel use to intake air volume, unlike carburetors which dump the same amount of fuel into the intake airstream regardless of filter efficiency.
However, in reality, this doesn't simplify or eliminate the problem; it makes it more complicated. Fuel metering isn't actually very smart. It's essentially 1-dimensional and easy to confuse or fool. If you end up with a severely clogged air filter, you can end up running the car EXTREMELY lean as the car tries to compensate for abnormal operating conditions. While this is technically reducing fuel use, it's not optimal.
Additionally, MAP and MAF systems work very differently, so the impact of your filter will vary accordingly.

I can't count the number of times I've corrected people on this one:

Shifting into neutral at stops saves fuel.

Another variant of this one I hear all the time is that shifting into neutral when coasting downhill saves fuel. It actually does exactly the opposite. An idling engine needs fuel to continue idling. An engine being driven by vehicle momentum needs no fuel, and the engine management system knows that. So it CUTS FUEL. Depending on the situation, you may actually be using ZERO fuel while going downhill, and you're virtually guaranteed to be using less fuel than you would idling in neutral.

[PapaSmurf2k3]

^I can agree with some of that.
If you're decelerating, sure, you're better off leaving it in gear and riding that bad boy down, but if you're coasting, you're actually better off shifting into neutral. You'll decelerate less, which means you'll have to accelerate less (and burn less fuel) after coming out of neutral.
I'm not sure about shifting to neutral while in park though. It would seem to make sense, seeing as how you're technically spinning less mass... but the difference is probably pretty negligible.

I agree with Chris about the air filter thing. Any losses on fuel injected vehicles due to dirty air filters are most likely due to pumping efficiency losses by the engine, as opposed to running richer (like a carb would).


They are on to something with the cruise control, although, again it can be debated. Older cars with vacuum operated cruise seem to be a lot less smooth than current electronic throttle cars. Even still, as a human, I can look ahead and see a hill. I can regulate throttle rather than speed in order to save fuel. Sure my speed fluctuations will be higher, but my fuel consumption will be less. I've actually done this experiment many times, traveling the same route under similar conditions using cruise vs human throttle input. If I'm paying attention (meaning I don't stay on the throttle too long after a hill and end up doing 90+), human throttle seems to win every time.

Manual transmissions don't really get better fuel economy anymore because of a few reasons:
Autos have become more efficient at transferring energy
Autos have more gear ranges. Manuals are typically reserved for sports cars now, which have shorter gearing

Sedans can sometimes get better highway fuel economy due to their longer size, which enables them to control aerodynamics better. This is on a case by case basis though, so I definitely wouldn't call it a rule of thumb. Typically the gains they see on the highway are negated by the losses in stop and go (due to heavier weight compared to compacts).

[Ace2cool]

I'm not sure about shifting to neutral while in park though. It would seem to make sense, seeing as how you're technically spinning less mass... but the difference is probably pretty negligible.

I would think that shifting into neutral would actually be about the same. Sure, more rotational mass when engaged, but the revs usually jump up a hundred RPMs or so in neutral. At least in every vehicle I've ever owned.

And all the deceleration talk would also need to take the transmission type into consideration. Manuals operate very differently from Autos in both takeoff and decel.

[AZhitman]

Modern auto trans cars, in neutral, are still firing the injectors (idling). In drive, coasting, no load, they're shut down. But, like James said, there's driveline drag.

Still, putting an automatic in neutral while coasting downhill is a bad idea, since the fluid isn't circulating.

Manual trans, I'm still going into neutral for a long downhill run.

[lne937s]

A dirty air filter only makes a difference at wide open throttle (which affects performance). If your foot isn' t planted to the floorboard, the sensors and computers in a modern car adjust airflow via the throttlebody to compensate for how much oxygen is entering the engine. The EPA actually tested the effect of clogged air filters and found no significant difference in fuel economy (sometimes the clogged filter performed slightly better in fuel economy):

Results show that clogging the air filter has no significant effect on the fuel economy of the newer vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control systems were able to maintain the desired AFR regardless of intake restrictions, and therefore fuel consumption was not increased. The carbureted engine did show a decrease in fuel economy with increasing restriction. However, the level of restriction required to cause a substantial (10–15%) decrease in fuel economy (such as that cited in the literature3,4) was so severe that the vehicle was almost undrivable. Acceleration performance on all vehicles was improved with a clean air filter.

http://www.fueleconomy.gov/feg/pdfs/Air ... 6_2009.pdf


And manual cars no longer outperform automatics in EPA testing-- see page 76 on the bottom (81 on the PDF).
http://epa.gov/fueleconomy/fetrends/197 ... r14023.pdf
If you look at page numbered 31 at the bottom (36 on the document), it also gives some credence to their last point, even if their top 10 is off...
EDIT, looking at the AAA post again, they exclude "Electric and Plug-in Hybrids"... not all hybrids. In this case, the top 10 (as listed by the EPA) do include some large cars and wagons:

1. 2015 Toyota Prius 2015 Toyota Prius Combined 50 City 51/Highway 48
Hybrid, 4 cyl, 1.8 L, Auto (AV), Regular
2015 Toyota Prius c 2015 Toyota Prius c Combined 50 City 53/Highway 46
Hybrid, 4 cyl, 1.5 L, Auto (AV), Regular
2. 2015 Honda Accord Hybrid 2015 Honda Accord Hybrid Combined 47 City 50/Highway 45
Hybrid, 4 cyl, 2.0 L, Auto (AV), Regular
3. 2015 Honda Civic Hybrid 2015 Honda Civic Hybrid Combined 45 City 44/Highway 47
Hybrid, 4 cyl, 1.5 L, Auto (AV), Regular
2015 Volkswagen Jetta Hybrid 2015 Volkswagen Jetta Hybrid Combined 45 City 42/Highway 48
Hybrid, 4 cyl, 1.4 L, Auto (AM-S7), Premium
4. 2015 Ford Fusion Hybrid 2015 Ford Fusion Hybrid Combined 42 City 44/Highway 41
Hybrid, 4 cyl, 2.0 L, Auto (AV), Regular
2015 Lexus CT 200h 2015 Lexus CT 200h Combined 42 City 43/Highway 40
Hybrid, 4 cyl, 1.8 L, Auto (AV), Regular
2015 Toyota Prius V 2015 Toyota Prius c Combined 42 City 44/Highway 40
Hybrid, 4 cyl, 1.8 L, Auto (AV), Regular
5. 2015 Toyota Camry Hybrid LE 2014 Camry Hybrid LE Combined 41 City 43/Highway 39
Hybrid, 4 cyl, 2.5 L, Auto (AV), Regular
6. 2015 Ford C-Max Hybrid 2015 Ford C-Max Hybrid Combined 40 City 42/Highway 37
Hybrid, 4 cyl, 2.0 L, Auto (AV), Regular
2015 Lexus ES 300h 2015 Lexus ES 300h Combined 40 City 40/Highway 39
Hybrid, 4 cyl, 2.5 L, Auto (AV-S6), Regular
2015 Lincoln MKZ Hybrid 2015 Lincoln MKZ Hybrid Combined 40 City 41/Highway 39
Hybrid, 4 cyl, 2.0 L, Auto (AV), Regular
2015 Mitsubishi Mirage 2015 Mitsubishi Mirage Combined 40 City 37/Highway 44
Hybrid, 3 cyl, 1.2 L, Auto (AV), Regular
2015 Toyota Avalon Hybrid 2015 Toyota Avalon Hybrid Combined 40 City 40/Highway 39
Hybrid, 4 cyl, 2.5 L, Auto (AV-S6), Regular
2015 Toyota Camry Hybrid XLE/SE 2015 Toyota Camry Hybrid XLE/SE Combined 40 City 40/Highway 38
Hybrid, 4 cyl, 2.5 L, Auto (AV), Regular

http://www.fueleconomy.gov/feg/topten.jsp

Regarding cruise control, it all kind of depends on who is driving... if you let the car slow slightly when going up a hill and speed up slightly when going down, it will increase fuel economy. And, since most modern cars use fuel cutoff when coasting (where there is no fuel going to the engine when you lift off the throttle and forward momentum spins the engine), pulse and glide can improve efficiency versus a steady speed... but again, it depends on who is driving (and how smart the cruise control is).