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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old March 30th, 2012, 04:08   #61
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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Old March 30th, 2012, 06:32   #62
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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.
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Old April 3rd, 2012, 12:26   #63
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Quote:
Originally Posted by oilhammer View Post
...

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.
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Old April 3rd, 2012, 12:28   #64
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The tire is more apt to deform and cause more rolling resistance if it is not of the proper load rating.
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Old May 1st, 2012, 18:34   #65
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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?
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Old May 9th, 2012, 21:05   #66
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Quote:
Originally Posted by MikeMars View Post
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_ec...zing_behaviors
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Old May 9th, 2012, 21:11   #67
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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)
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Old May 10th, 2012, 03:54   #68
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Quote:
Originally Posted by kurtisk View Post
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!!

Quote:
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.

Quote:
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.

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?
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).

Quote:
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.

Quote:
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).

Quote:
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_ec...zing_behaviors
Also take a look at the VW leaflet on fuel efficiency (link in my signature).
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Old May 10th, 2012, 04:01   #69
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Quote:
Originally Posted by kurtisk View Post
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.
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Old May 10th, 2012, 04:22   #70
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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.
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Old May 10th, 2012, 06:26   #71
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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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Old May 10th, 2012, 06:28   #72
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Quote:
Originally Posted by MikeMars View Post
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.
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Old May 13th, 2012, 05:42   #73
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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.

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ps: that story of folks getting 80+ mpg sounds ridicilous
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Old May 13th, 2012, 09:37   #74
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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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Old May 17th, 2012, 13:56   #75
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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.
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