MPG seems "backwards"?

[Intermodal]

Hello! So after a few months of driving my beloved 2003 Jetta Wagon (ALH, 5 speed) it would seem that my fuel economy is "backwards" compared to the published specs; on medium and long trips at 60 to 70mph on the highway I average in the low 30's and around town I can get as high as an average of low 50's across 10-20 miles of driving on surface streets. Has anyone experienced this before? What should I look at to start trouble shooting this?
Note: my economy measurements are based on a Scan Gauge and cross referenced with some pretty loose arithmetic. I've filled up and dialed in the scan gauge each time, but it seems to always need at least a slight correction at each fill up.
Let me know if any more information is needed!

 

[Nero Morg]

I would suggest starting off by going through Oilhammer's guide on fuel economy tips, sometimes the simplest things catch us off guard.

tips for fuel economy

I've put this together in an effort to help some folks diagnose what may be a loss of fuel economy (MPGs) with their TDIs. Keep in mind, many of these items can work against your car in concert to add up to a sizeable increase in fuel consumption over time, so please don't dismiss any one item...

forums.tdiclub.com

Secondly, do you have an auto or a manual? These cars weren't designed for fuel economy at 60-70mph, something a lot of people have done is put in a taller 5th gear for highway cruises, and that's when they get the good fuel eco.
Back when I had my wagon, I had better fuel economy when I was driving about 55mph, keeping my rpm's low.

 

[Vince Waldon]

Thirdly, as a fellow multiple-Scangauge owner myself, I find tracking the old fashioned way (fill from the same pump, exactly to the same spot on the neck, waiting for air in the vent lineto dissipate) is generally much more consistant (and perhaps therefore accurate overall?) than the Scangauge, which to your point always needs a bit of correction each fill, even when following the above.

Doesn't explain if you're getting better mileage in the city than on the highway... but if you're trying small changes to make improvements having a consistant measure of overall economy is probably important.

One other hint: IIRC there's a setting on the Scangauge (is it "Diesel A" and "Diesel B", perhaps) that will reverse the direction of fuel consumption... you've said you see the same pattern with rough math so maybe not a factor, but might be worth re-checking?

 

[Intermodal]

I would suggest starting off by going through Oilhammer's guide on fuel economy tips, sometimes the simplest things catch us off guard.

tips for fuel economy

I've put this together in an effort to help some folks diagnose what may be a loss of fuel economy (MPGs) with their TDIs. Keep in mind, many of these items can work against your car in concert to add up to a sizeable increase in fuel consumption over time, so please don't dismiss any one item...

forums.tdiclub.com

Secondly, do you have an auto or a manual? These cars weren't designed for fuel economy at 60-70mph, something a lot of people have done is put in a taller 5th gear for highway cruises, and that's when they get the good fuel eco.
Back when I had my wagon, I had better fuel economy when I was driving about 55mph, keeping my rpm's low.

Thank you for the link to that guide, what a great resource!
I've got a 5 speed manual. I've heard about the taller fifth gear and it sounds like something I could really get into.
From what I've been hearing a lot of people with the stock gearing seem to get at least into the 40's at higher speeds, so my measly 30mpg has gotten me wondering if something might be amiss.

 

[Nero Morg]

I'd give it a good once over, and also seriously consider that taller 5th gear, especially if you're driving 60+ a lot.

 

[saCusa]

Has the car always done this, or is this a recent development? I just went through a bout of fuel economy issues that had everything to do with my wheel and tire setup. What wheels and rubber do you have mounted? Dead serious, I went from 38 mpg back up to 47 mpg with the change of my wheels and tires.

 

[Intermodal]

I'd give it a good once over, and also seriously consider that taller 5th gear, especially if you're driving 60+ a lot.

Just replaced the water temp sensor (was throwing code p0128) and I will be doing a once over of everything else in that guide. Thanks again for the info!

 

[Intermodal]

Has the car always done this, or is this a recent development? I just went through a bout of fuel economy issues that had everything to do with my wheel and tire setup. What wheels and rubber do you have mounted? Dead serious, I went from 38 mpg back up to 47 mpg with the change of my wheels and tires.

I've only had the car a few months so I don't have a ton of time with it. I recently replaced the stock 15" alloys with some wolfsburg BBS 16"s with 205/55/r16 tires. I don't recall much of a change in economy but am considering throwing the 15's back on in the name of science.
What wheel/tire changes did you make?

 

[turbodieseldyke]

Scangauge only gets generic OBD data, and can't tell your true mpg. You need a MFA, or some other device that gets realtime fuel consumption data from the ECU.

The only way you're getting 50+ in the city, is if you have long stretches at 45mph with very few stoplights. If you're stopping once a mile, or half mile, forget about it.

 

[burpod]

needless to say, getting 30s mpg on the higway at 70mph something is seriously wrong. i would check physical cam/crank timing, and get VCDS, check timing with that as well as take some logs to show timing (like 011 for boost + 004 for timing and also 007 for coolant/fuel/air temps). and of course other things like brakes, wheel bearings all good, fuel filter, air filter or fuel sender not clogging

 

[saCusa]

I've only had the car a few months so I don't have a ton of time with it. I recently replaced the stock 15" alloys with some wolfsburg BBS 16"s with 205/55/r16 tires. I don't recall much of a change in economy but am considering throwing the 15's back on in the name of science.
What wheel/tire changes did you make?

That setup should be fine, BBS wheels are typically pretty light and 205/55/16 is very close to factory size. I wouldn't imagine the slightly undersized tire would create a discrepancy that large. I was on 225/45/17 with Arietta wheels which had performance tires on them. Wider, heavier, stickier, stiffer, all contributed to my poor fuel economy. If you still have your steel wheels, it never hurts to try an experiment.

Chances are it is something a little more involved than that, but a free easy test is worth doing imo.

 

[jettawreck]

Scangauge only gets generic OBD data, and can't tell your true mpg. You need a MFA, or some other device that gets realtime fuel consumption data from the ECU.

The only way you're getting 50+ in the city, is if you have long stretches at 45mph with very few stoplights. If you're stopping once a mile, or half mile, forget about it.

Scangauge can be very accurate once calibrated and your driving stays within the "average" that you calibrated it around. Deviate from "average" and it will be off. Calibrating it to the changed driving and it will again be off when back to "average".
When you calibrate don't enter in the total difference. Go half way (Ex: Scangauge says 15 gallon and you used 14 enter 14.5).
Any short term mileage estimates are meaningless really. Full tank fill ups over several tanks are the only reliable results.
My lifetime average(s) over three 2001-2003 TDI is a bit over 52 mpg when not towing. Seldom drive them anymore since I'm not commuting to work now. When I commuted the SG was very accurate as there was little change in route and pace for most of the tank duration.

 

[J_dude]

Scangauge only gets generic OBD data, and can't tell your true mpg. You need a MFA, or some other device that gets realtime fuel consumption data from the ECU.

If I remember correctly doesn’t the ecu just basically guess at Liters/Gallons per hour based off of several other measurements? I’m pretty sure I read on the forum here that scangauge does also display exactly that information from the ecu. I’m may be wrong but I don’t think there’s anything that actually measures how much fuel is used.

Also fwiw my scangauge is very accurate, I compared it with my hand calculated mileage using GPS trip and the fuel station pump.

 

[turbodieseldyke]

If I remember correctly doesn’t the ecu just basically guess at Liters/Gallons per hour based off of several other measurements? I’m pretty sure I read on the forum here that scangauge does also display exactly that information from the ecu. I’m may be wrong but I don’t think there’s anything that actually measures how much fuel is used.

Also fwiw my scangauge is very accurate, I compared it with my hand calculated mileage using GPS trip and the fuel station pump.

The ECU knows exactly how much fuel is needed for each stroke. It controls each fuel injection, and sends that info directly to the MFA. That is much more accurate than trying to reverse-guess fuel usage by looking at MAF and Engine Load and whatever else the Scangauge uses.

If it works for you, then cool. I just wouldn't use it to troubleshoot a supposed mpg problem that doesn't make sense in the first place. I've seen someone else say their Scangauge reports using something like "20 gallons per hour" during a hard acceleration. That's just not possible, and makes me question the device overall. It is what it is. You can't build a $99 miracle device that works perfectly for multiple functions across multiple platforms with different fuels.

 

[J_dude]

If it works for you, then cool. I just wouldn't use it to troubleshoot a supposed mpg problem that doesn't make sense in the first place.

Agreed, that’s where you’d want to calculate it by hand.

 

[turbodieseldyke]

Agreed, that’s where you’d want to calculate it by hand.

In OP's case, he thinks he's getting poor mpg at hwy speeds, and great mpg in the city. He either needs to burn 10+ gallons consistently at each type of driving, or use a better tool to diagnose (Vagcom).