tips for fuel economy

bvencil

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Mikemars and allana13 - I have an English friend here in the States who talks about buying fuel in litres while in the UK. So what's the common measure, litre or Imperial gallons?

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MikeMars

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It's litres for fuel, miles for distance, and MPG for economy.

The UK is really not very good at moving to metric. It's taken more than 40 years and only partly done.
 

tditom

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formerly: 2001 Golf GL, '97 Passat (RIP) '98 NB, '05 B5 sedan
...

Tires: ...Going from the proper 91 load to an 89 (an all too common mistake) can take a 1 MPG hit all by itself over the same model tire. ...
While I agree that you should always use tires of the correct load rating, I don't understand how load rating impacts mpg. Can you elaborate? thanks.
 

oilhammer

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outside St Louis, MO
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There are just too many to list....
The tire is more apt to deform and cause more rolling resistance if it is not of the proper load rating.
 

hardtimes

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<quote> Brakes: many Volkswagens, especially A4 platform cars, have chronic stuck parking brake cables. This causes the rear calipers to drag, </quote>

I had this in my 2007 GLI. Went through the rear pads in 40k while the fronts had plenty of meat on them. Who knows how much gas I chewed because of it.

I now have a 2010 TDI JSW. Any suggestions on how to ensure this isn't happening or checking it?
 

kurtisk

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Petrol vehicles have a very inefficient idle, but it's much more efficient on a diesel (one of the main reasons why real-world MPG is much better on diesels than petrols).
I was not aware of this. I know that I have been told by several people (referencing Car & Driver Mag) that it is better to keep a car in gear/under load when stopped at a traffic light because the fuel economy is better. Similarly, they say coasting in gear is better for MPG than coasting in neutral for the same reasons: cars are tuned so that a slight load on the during idle produces the best MPG. Sounds like neither strategy would ever be true with a diesel, but could be true with a gasser.

Another issue for idling is the type of transmission. A traditional automatic does create a noticeable drag on 2000 TDI Beetle when in neutral, My 2006 and now 2012 Jetta with a DSG transmission sounds exactly the same in D or N because neither clutch is engaged until I take my foot off the brake. For a manual transmission, there are two possible states: clutch in while still in gear (which is likely low drag, good MPG, but straining the clutch) or clutch out with no gear engaged (likely minimal drag, no strain on the clutch, some wear to the transmission internals that are spinning, maybe, and additional time/steps to get underway.) Any comments?

[/QUOTE]I can't think of a case where a moving car in neutral & idle would be using more fuel at that moment in time than a car which is under a significant positive load at the same speed (i.e., accelerating). I'm probably misunderstanding your question.[/QUOTE]

Is it possible that a car coasting in neutral & idle would be using more fuel than a car coasting in gear and idle? I don't see how, especially as you note a diesel is more efficient at idle. When I coast in gear with my 2012 Jetta TDI DSG I slow down faster than if I am in neutral, much faster. Being in neutral should make it so I can get more 'free distance' so to speak for the fuel I expended to attain that speed. It is difficult to see how shaving off speed faster would increase economy even if the idle in neutral is less efficient. If being in gear decelerates from 100 to 50 in .25 mi instead of .5 mi in neutral, what kind of inefficient idling could squander the extra .25 mi of coasting?

Also...
I have heard a listener call in to CarTalk (a popular car repair public radio show in the US, hilariously funny BTW) and ask about idling in gear vs neutral (with an traditional automatic presumably) and their take was that it was bad for the car because of wear and tear to the drive-train engaging and disengaging torque to the transmission, final drive and axles.

I wonder how this relates to commercial transport trucks (semi's in the US or lorries in UK) who often leave the engine idling for extended periods. Obviously they don't get better mileage since idling is 0 MPG, but maybe it keeps them warm. Never have heard a good explanation for this.

NOTE: I did find a interesting post of fuel economy here that explains some of the things discussed on this post: http://en.wikipedia.org/wiki/Fuel_economy-maximizing_behaviors
 

kurtisk

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Another question: If I am driving a 40 in my 2012 Jetta TDI DSG, it is turning about 1200 RPM. If I go 38, it has to shift down and turns 1500+ RPM. Which should produce the best economy? Should I pick a speed that keeps my RPM's lowest? (obviously this question only applies in the lower gears)
 

MikeMars

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I was not aware of this. I know that I have been told by several people (referencing Car & Driver Mag) that it is better to keep a car in gear/under load when stopped at a traffic light because the fuel economy is better.
That just doesn't make any sense at all to me (regardless of whether it's diesel or petrol). Certainly not for a manual box (which is what I am talking about) since it'd stall!!

Similarly, they say coasting in gear is better for MPG than coasting in neutral for the same reasons: cars are tuned so that a slight load on the during idle produces the best MPG. Sounds like neither strategy would ever be true with a diesel, but could be true with a gasser.
I don't really know about petrol cars, but people tell me that their idle & low load driving wastes energy via pumping losses.

Another issue for idling is the type of transmission. A traditional automatic does create a noticeable drag on 2000 TDI Beetle when in neutral, My 2006 and now 2012 Jetta with a DSG transmission sounds exactly the same in D or N because neither clutch is engaged until I take my foot off the brake. For a manual transmission, there are two possible states: clutch in while still in gear (which is likely low drag, good MPG, but straining the clutch) or clutch out with no gear engaged (likely minimal drag, no strain on the clutch, some wear to the transmission internals that are spinning, maybe, and additional time/steps to get underway.) Any comments?
Everything in my post is referring to manuals (rather than DSG or auto).

Regarding manual, I put it into neutral & release the clutch. The traffic lights here always have a brief intermediate red+amber stage (before green) which is when everyone takes the handbrake back off & back into gear. If I recall correctly your lights go straight to green without any warning.

Regarding DSG... I think the DSGs in different cars work in different ways for freewheeling. For example some Passats automatically freewheel when you take your foot off the accelerator, and engine braking only works if you touch the brake. I've never driven one myself. But the theory should still hold (whether it's mechanically a good idea or not I'm not sure).

Regarding automatic... I know nothing about automatics. Never owned one in my life & disliked it intensely when I used a car with an auto box for a couple of weeks (you stomp your foot down & it clanks away to itself for a while before moving ... horrible). I don't see why anyone would want to buy a torque-converter automatic unless they are physically disabled & therefore have trouble with the clutch or the gearstick.

Is it possible that a car coasting in neutral & idle would be using more fuel than a car coasting in gear and idle? I don't see how, especially as you note a diesel is more efficient at idle. When I coast in gear with my 2012 Jetta TDI DSG I slow down faster than if I am in neutral, much faster. Being in neutral should make it so I can get more 'free distance' so to speak for the fuel I expended to attain that speed. It is difficult to see how shaving off speed faster would increase economy even if the idle in neutral is less efficient. If being in gear decelerates from 100 to 50 in .25 mi instead of .5 mi in neutral, what kind of inefficient idling could squander the extra .25 mi of coasting?
The situation where the coasting-in-gear is more efficient is when you would otherwise need to use the brakes to slow down (for example, when there is a junction or speed camera at the bottom of a hill).

Also...
I have heard a listener call in to CarTalk (a popular car repair public radio show in the US, hilariously funny BTW) and ask about idling in gear vs neutral (with an traditional automatic presumably) and their take was that it was bad for the car because of wear and tear to the drive-train engaging and disengaging torque to the transmission, final drive and axles.
There will always be some wear & tear... I don't think it will be particularly significant though.

I wonder how this relates to commercial transport trucks (semi's in the US or lorries in UK) who often leave the engine idling for extended periods. Obviously they don't get better mileage since idling is 0 MPG, but maybe it keeps them warm. Never have heard a good explanation for this.
Its so that they can run their air con, light bar, & other electrical things (such as a tv at night). Trucks here will usually turn their engines off & not idle for long (often installing a deep discharge battery or a generator so they can run the electrics with the main engine turned off).

NOTE: I did find a interesting post of fuel economy here that explains some of the things discussed on this post: http://en.wikipedia.org/wiki/Fuel_economy-maximizing_behaviors
Also take a look at the VW leaflet on fuel efficiency (link in my signature).
 

MikeMars

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Another question: If I am driving a 40 in my 2012 Jetta TDI DSG, it is turning about 1200 RPM. If I go 38, it has to shift down and turns 1500+ RPM. Which should produce the best economy? Should I pick a speed that keeps my RPM's lowest? (obviously this question only applies in the lower gears)
My guess would be that in this scenario the 40 would be fractionally more efficient than the 38mph.

On DSGs, can you control the current gear via the paddle shift? I.e., try going into the higher gear at 38 & see if the DSG is happy with it.
 

xreyuk

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I don't know how accurate this is, but I was told coasting in gear is better than coasting in idle, because when in gear the car doesn't use fuel unless the throttle is pressed. However, when in idle, the car will use fuel to maintain tickover.

My MFD shows no fuel consumption when coasting in gear, but will show slight consumption if in neutral.
 

MikeMars

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Yeah, but because of the engine resistance, you will lose more energy in gear than in idle (your speed will drop off quicker in gear). The fuel needed to keep the engine spinning at 850rpm is less than the kinetic energy lost via friction when the engine is spinning at 2000 rpm.

The longer your gears the less difference there will be (i.e., because your RPM is lower).

I vaguely seem to recall that in a bluemotion/ecomotive/etc it freewheels automatically if you lift your foot off the accelerator anyway? (i.e., does the RPM drop back to 850 when you take your foot off?) At least Vekke's 3L does this.
 
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xreyuk

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Yeah, but because of the engine resistance, you will lose more energy in gear than in idle (your speed will drop off quicker in gear). The fuel needed to keep the engine spinning at 850rpm is less than the kinetic energy lost via friction when the engine is spinning at 2000 rpm.

I vaguely seem to recall that in a bluemotion/ecomotive/etc it freewheels ausomatically if you lift your foot off the accelerator anyway? (i.e., does the RPM drop back to 850 when you take your foot off?)
It doesn't drop back to 850RPM, but it certainly has less engine braking than a normal car, so will coast longer whilst in gear.

I know my Ecomotive also has some sort of energy recovery system that is uses somehow.
 

EST

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Santa Fe, NM
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2012 Golf TDI DSG
My MPG

I get somewheres between 35 and 37 mpg, but I drive like I'm in a hot turbo charged tdi golf. Which I am.

I prefer to know if all hell breaks loose I can squeeze more mpg if I have too but until then pressing down hard on the pedal is an absolute joy.

Driving a 2012 Golf TDI w/6500 miles on it and only use Chevron here in New Mexico. Lots of hill driving too.

est

ps: that story of folks getting 80+ mpg sounds ridicilous
 

MikeMars

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...
ps: that story of folks getting 80+ mpg sounds ridicilous
Seems plausible enough to me. They're already world record holders for other trips. 14h of steady driving averaging 40mph with the wind, and in the 2012 Passat which everyone knows here does better than the other 2012 cars in the US.
 
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Vekke

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Lupo 3L freewheels in when you release the gas pedal.

Lupo 3L gets 2 l/100km or 117 MPG US at 65 km/h 40 MPH speed on stock condition

On EU highway cycle passat bluemotion does 3,6 l/100km lupo 3L 2.7 l/100km

Calculate the difference passat bluemotion at 40 MPH=(3,6*2)/2,7=2,66 is totally plausible. Do consider that that passat has pretty tall gear ratios which makes it to drive fuel efficiently at low speeds.
 

MikeMars

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I seem to recall that the Passat also freewheels with the DSG gearbox (or at least until the recent update of the ECU which stopped it).
 

berserkerx

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So does the 2011 jetta dsg freewheel when you let off the accelerator? I seriously have not managed a better tank than 46 roughly on the highway on mine. But I don't do anything other than get up to speed and leave it. Then coast in D when going downhill. But hey blame it on the speed limit being 80 on I-15 here in Utah.
 

psd1

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So does the 2011 jetta dsg freewheel when you let off the accelerator? I seriously have not managed a better tank than 46 roughly on the highway on mine. But I don't do anything other than get up to speed and leave it. Then coast in D when going downhill. But hey blame it on the speed limit being 80 on I-15 here in Utah.
Please tell me you arent complaining about 46 MPG at 80 MPH...:confused:
 

berserkerx

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no 46 was going about 65 going from Houston to Dallas and babying it.
When I drive to Vegas going 80 I get about 36-38. I am not at all complaining. considering my other car gets like 24 being nice to it.

But I just see some members that have the CR getting 46 as an average and that was like an all time best for me.
 

berserkerx

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I guess I shouldn't complain since most of my road trips I have the car nice and loaded down since I am gone for two weeks at a time. I got that 46 while the car weighed 4040 with a full tank and all occupants.

EDIT: I just want to clarify that this was done with a lot of coasting on the small hills coming out of Houston and I had a tailwind with me. Then since I was going slow enough I did a little drafting too.
 
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MikeMars

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So does the 2011 jetta dsg freewheel when you let off the accelerator?...
If you have a rev-counter then it should be obvious, if you don't then I don't know. I was under the impression that the Jetta didn't freewheel while the Passat does.

...Then since I was going slow enough I did a little drafting too.
Make sure you are a safe distance back before drafting (2 seconds minimum), particularly if your nearest&dearest are in the car... (however if you are giving a lift to a bunch of tax inspectors, please feel free to draft as near as you like :) )
 
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berserkerx

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Make sure you are a safe distance back before drafting (2 seconds minimum), particularly if your nearest&dearest are in the car... (however if you are giving a lift to a bunch of tax inspectors, please feel free to draft as near as you like :) )
I think my main problem is that heavy right foot. Then my wife's heavy right foot which is actually worse than mine.:eek:


I was about 3 seconds back.

What if I was giving a lift to a bunch of politicians? I would stuff them in the trunk just to get enough of them in there.
 

MikeMars

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...
What if I was giving a lift to a bunch of politicians? I would stuff them in the trunk just to get enough of them in there.
Don't forget the roof rack :)
 

deltatech

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I'm enjoying reading these threads. Question---I have a Jetta TDI BEW engine and an automatic trans. Would it be ok for me to shift into neutral for coasting down hill? Or am I risking my transmission? On a second note-I used to get 45mpg all the time. I hit a bridge expansion joint that was sticking out of the road bed. About 40 other motorists hit it. It blew the side curtain airbags on both sides-seat airbags-destroid both tires on the left side and the front alloy wheel-bent the rear axle. Since then if I get 40mpg I'm lucky. Allignment is ok so where could the problem be?
 

MikeMars

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Are you experiencing uneven tyre wear since that incident? (remember to check the inside 1/3 of the tyre as well as the easy-to-see outside edge).
 

deltatech

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My tire ware has been pretty even-just a little more o the outside of each tire. I rotate about every 10,000 miles.
 

Sfkjeld

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Third fill up since new

Our first tank got us 43mpg in our new 2012 JSW TDI with 6sp man. Was pretty excited. That was mostly freeway, but a lot in 100degrees with AC howling. Was pretty excited. The next two tanks were mix of City and freeway, almost no AC. Mileage dropped down to 33-34mpg. We did mount our Thule rack tho. Some claim that makes a diff. Took it off and will see. Will be bitter sweet if it matters that much. I will dwell on all the MPG's we lost driving all those years in our Passat with the same rack. Ouch.
 

MikeMars

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It has a big effect at highway speeds, no effect at city speeds. However I suspect most of your MPG drop was due to the city driving.
 

TdiNC

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hello,
I just purchased a 2004 4door golf tdi (5speed) the weekend before July 4th. It has 200,000 miles on it and supposedly has had the belt and waterpump changed already. I have been reading all these posts about people getting 50+MPG. I have already put 2000 miles on it and noticed I am only getting 35-40 MPG. 14 gallon tank and im getting around 500 miles. I am going to do an oil change for good measure soon, but would like some advise as to what else I should also look into. The car seems to run fine. One owner before me and they seemed to have used this as a hwy car for business or something. I can still hear the turbo so i know its working. But all these people saying they are getting such and such is making me very envious! I believe I should be getting at least 45 MPG. Any inputs? What else can/should I do?

Thanks!
 

MikeMars

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If I could be permitted to reply with a long series of questions ;)

* What sort of driving do you do (highway or city)?
* How long are your average trips?
* What is your highest highway cruising speed if you do highway driving?
* Have you checked your tyre pressures (& what are they?)
* How wide are your tyres, and how big are your rims?
* Do you have any roof rack / trailer?
* If you put your hand onto the wheelnuts after a long drive, do any of them feel hot? (the fronts should be warm, and the back ones should be cold, and even temperature side-to-side).
 
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