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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 January 10th, 2011, 15:26   #1
JasonCzerak
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Default High speed MPG - (for thoes mostly near an autobahn...)

So who has calculated MPG at "high speeds" (as compared to the slowness of the USA)

MPG @ 70 for me is 53mpg Summer fuel, round trip north and south 120miles, same pump 8 hours apart for fueling.... That was before I started to modify things. heh.

I'll get MPG up to about 80 maybe 85mph this summer for the trip I just mentioned.

However, what I'm really looking for is MPG at 100mph and faster.
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Old January 10th, 2011, 15:48   #2
shizzler
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I'd be amazed if anyone has data for you (beyond simple scan gauge / MFD readings). Sustaining 100 mph for an entire tank? Difficult - even on the autobahn.

I'm sure 85_305 will chime in shortly with his 50mpg at 90 mph claim, but I personally don't buy it (nothing personal dude).

Aeromods are your friend for mpg's at high speeds. They become exponentially more valuable as speed is raised. Which is primarily why I did mine - my hwy driving is always over 70. I have done entire tanks of 80+ mph cruising (sure, with some % of city on both ends of the trips / tank) and still gotten ~50. But the drag at 90 mph requires almost 50% more power to overcome than @ 80.... and from 90 to 100 is yet another 50+ % power requirement...
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Old January 10th, 2011, 16:57   #3
Bob_Fout
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90 MPH gets you 30 to 35 MPG. Full tilt is about 25 MPG.
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Old January 10th, 2011, 17:03   #4
IndigoBlueWagon
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Full tilt for me on the track, running to 110 in 4th on straights, shifting at 5200+ RPM would typically get me 16-18 MPG. Obviously I'm doing a lot of hard braking, too, which uses up energy. I'd go with Bob's 25 MPG estimate.
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Old January 10th, 2011, 17:22   #5
ruking
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Quote:
Originally Posted by shizzler View Post
I'd be amazed if anyone has data for you (beyond simple scan gauge / MFD readings). Sustaining 100 mph for an entire tank? Difficult - even on the autobahn.

I'm sure 85_305 will chime in shortly with his 50mpg at 90 mph claim, but I personally don't buy it (nothing personal dude).

Aeromods are your friend for mpg's at high speeds. They become exponentially more valuable as speed is raised. Which is primarily why I did mine - my hwy driving is always over 70. I have done entire tanks of 80+ mph cruising (sure, with some % of city on both ends of the trips / tank) and still gotten ~50. But the drag at 90 mph requires almost 50% more power to overcome than @ 80.... and from 90 to 100 is yet another 50+ % power requirement...
Indeed hard to get. So how about 6.25 hours, 584 miles, 12.1 gals or 48.26 mpg?
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Old January 10th, 2011, 19:02   #6
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How about it? Are you completely serious - a 93 mph average for a full 584 miles ?!? I don't know whether I believe your speed or your mpg less.

Some people claim to have these magical TDIs, but really, no one is above the laws of physics. You can quantify the amount of fuel required to sustain 93 mph with basic data - diesel fuel energy density, and vehicle frontal area and coefficient of drag. And it's more than 1.9 gal/hr! Either you have a gross speedo / trip meter error, or are filling up at different pumps (hence bad fuel measurement). Or maybe you drove from the Johnson tunnel west of Denver (el. 11200 ft) down to sea level....?
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Old January 10th, 2011, 21:20   #7
ruking
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Quote:
Originally Posted by shizzler View Post
How about it? Are you completely serious - a 93 mph average for a full 584 miles ?!? I don't know whether I believe your speed or your mpg less.

Some people claim to have these magical TDIs, but really, no one is above the laws of physics. You can quantify the amount of fuel required to sustain 93 mph with basic data - diesel fuel energy density, and vehicle frontal area and coefficient of drag. And it's more than 1.9 gal/hr! Either you have a gross speedo / trip meter error, or are filling up at different pumps (hence bad fuel measurement). Or maybe you drove from the Johnson tunnel west of Denver (el. 11200 ft) down to sea level....?
Durango, Co to Las Vegas NV. One fill outside of Las Vegas in the middle of no where.

No claim to a magical TDI. Just a plain one. Sounds like you are full of it.
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Old January 10th, 2011, 21:36   #8
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http://en.wikipedia.org/wiki/Durango,_Colorado

http://en.wikipedia.org/wiki/Las_Vegas,_Nevada
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Old January 10th, 2011, 21:37   #9
ruking
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You might think and say that, but all you have to do is map quest or google it to know that is not true, aka false. Indeed this has not been the only trip. The reason why either 12.1 gal or 584 miles is significant is that is when the low fuel lamp goes off. I've done from this area to the Santa Monica turn off @ a steady 90 mph in a driving rain and posted 50 mpg. So yes, slower gets better.

Last edited by ruking; January 10th, 2011 at 21:52.
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Old January 10th, 2011, 21:40   #10
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Newton is turning over in his grave.
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Old January 10th, 2011, 21:47   #11
ruking
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Quote:
Originally Posted by Bob_Fout View Post
90 MPH gets you 30 to 35 MPG. Full tilt is about 25 MPG.
I would believe that is what you probably get if you are saying that is what you get. But for all the mods you list, shouldn't you expect better?

Last edited by ruking; January 10th, 2011 at 21:50.
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Old January 10th, 2011, 21:49   #12
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Quote:
Originally Posted by ruking View Post
I believe that is what you probably get, but for all the mods you list shouldn't you expect better?
I don't drive that fast, those figures are from people who do drive those speeds. There was a post some years ago about it.
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Old January 10th, 2011, 21:53   #13
ruking
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Quote:
Originally Posted by Bob_Fout View Post
I don't drive that fast, those figures are from people who do drive those speeds. There was a post some years ago about it.
Me either ! So you have never done what you profess to know to be true? Figures !!!

Last edited by ruking; January 10th, 2011 at 22:11.
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Old January 10th, 2011, 22:41   #14
Ski in NC
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Bored. Fun with numbers: I don't know top speed on a mk4 stock, but I'll guess 115mph. My mk4 runs 2400 at 70mph, so at 115 it turning about 3950. And assuming it is making full hp at that rpm, which might not be true as above 3750 (rated peak hp) ecu is probably backing off fueling a bit. So call it 90hp anyhow. Using a marine engine rule of thumb, each gallon per hour makes between 17 to 20hp, depending on engine and how close to peak bsfc is the operating point. So since this engine is nowhere near peak bsfc, I'll pick 18hp/gph. 90/18= 5gph. 115mph/5gph=23mpg.
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Old January 11th, 2011, 07:00   #15
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Ruking, your originally posted trip descended over 4000 feet of elevation! Completely irrelevant data point. This is not helpful, as presumably the OP is not asking us "how good can the mileage be at high speeds, when driving downhill for an entire tank, with a tailwind and magic fairy dust in the fuel tank?"

You are not posting normal fuel economy numbers. We have a few geezers who drive at 50mph to pull off their great #s. And a few newbs who don't know how to properly evaluate fuel economy. But then there is you, repeatedly claiming mileage feats that no one else can accomplish. So yeah, either your TDI is magical, or you aren't being completely truthful with yourself and your mpg calculations.

I am an engineer... dedicated to using hard data to find truth. Not interested in exaggeration, manipulation of data, or, on the other hand, knocking people's results when I have no reason to doubt them. But I do have reason to doubt your #s here. Actually, to be honest, an odometer error makes sense for your car, since you also claim 112,000 miles from a set of goodyear tires.

Let's take another look at this situation objectively:

Have you seen an understood the BSFC map for the ALH TDI engine? Search for and read the BSFC thread if not. Below is a calculation of the load required to push a stock Mk4 Jetta down the road in top gear. Let's run some simple numbers for your stated speed of 90 mph.

A fairly basic calculation tells us we need ~42 BHP (31.5 kW) to sustain 90 mph. With stock gearing and tires, this is ~3100rpm, which means we are requesting ~71.6 lbf-ft (97 N-m) of torque. So, that puts our engine efficiency at around 245 g/kW-hr. 245 grams * 31.5 kW means you are burning 7.72 Kg (17 lbs) of fuel per hour at this speed. Divide by the density of diesel fuel (6.9 lbs/gal) and you can see we are burning 2.45 gals/hr. At 90 mph, this is 37 mpg AT BEST. And you claim 50 in the rain? Ridiculous.

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