Why is my mileage so much lower than others claim?

[cervelorod]

I have a 1990 jetta IDI that seems to get much lower fuel mileage than I see others reporting.  I consistently get 33-34mpg.  I got that when it was NA, and now that I have added a turbo, I still get 33-34 mpg, although with a better grin factor.  My engine has 195k on it, but it is solid.  Very little blowby, starts well cold or hot, idles good, etc etc.  I have some 17x6 wheels and tires on, but even on stock wheels and tires I get the same.  I have it lowered about 1.5" and no brakes are sticking.  I have max fuel turned up a bit with the turbo, but even straight hiway miles at 70-75 nets the same.  Most of my driving is country 60-65.

Any ideas or is this were it stays??

Thanks,
Rod
1990 Jetta IDI Turbo

[steevz]

I'm pretty sure for max fuel economy the driving speed is 90km/hr, or 55m/hr ish.. Maybe it's because of your speed.

[rabbid79]

Does it get the same fuel economy with the fuel screw set at its normal position?  I noticed a big drop in fuel economy with my fuel turned up a bit, even though my driving habbits were the same.

[cervelorod]

I don't think I was getting any better when I first got it.  I have no idea what the PO had done, but the collar was still on the fuel screw when I attacked it.  I adjusted it while it was still NA, because I was not getting great fuel mileage anyways, and the power difference was very noticeable.

I think driving 55 on the Interstate here would get me a ticket...  If max economy was at 55mph, I would hope that I could come close to that at 60-65, but I'm ~20% off.

I am timed at NA specs, 1.01, I've been to lazy to turn it back for the turbo.

Thanks!

Rod

[the caveman]

all things being equal, the 3 things i can figure will affect mileage is pump timing and internal pump pressure. But recently someone from this site who i know told me that he found some crud at the bottom of his fuel tank  that must have been affecting his pump because once cleaned, his mileage went way up

[cervelorod]

all things being equal, the 3 things i can figure will affect mileage is pump timing and internal pump pressure. But recently someone from this site who i know told me that he found some crud at the bottom of his fuel tank  that must have been affecting his pump because once cleaned, his mileage went way up

Who'da thunk it.  I changed my fuel filter, but never thought to clean the tank.  I'm installing a veg oil conversion over the next few weeks, so I'll drop the OEM tank and see what I can find.

Thanks!

Rod

[Mark(The Miser)UK]

x 2 on the internal pump pressure.
If your mileage is fair in town, but doesn't improve by more than a few mpg on a run, then lack of dynamic advance is a possibility.
General low compression will also contribute...

[larry104]

My 1992 Ecodiesel (http://www.vwdiesel.net/forum/index.php?topic=8978.0) routinely gets 46-48 mpg combined city/hwy and has touched 50 mpg pure highway running 70 mph.

[8v-of-fury]

I can achieve anywhere between 40-50mpg depending on how i am driving and the speed.

100km/h cruising for a tank (2450rpm for me) will net an easy high 40's.. but I rarely get to do that slow on the highway! LMAO 135-145km/h is more the mainstream.. 3300-3600rpm. and even then.. can stay above 40mpg.

[burn_your_money]

Is your odometer slipping?

[vanbcguy]

My best ever recorded mileage with Jezzie is only ~36 MPG - 6.5L/100km.  She's a bit tired though, she burns about 1L of oil per 1200km or so. 

Around town I often get more like 30 MPG - 8L/100km. 

[8v-of-fury]

In reality.. The ones claiming an easy 50mpg are probably not taking their readings right.. Fudging it a bit to look "superior".. And possibly just have a really tight motor in a very flat area, and it never has to work..

If you actually drive the bag off your car like I do.. Anything over 30mpg while having fun is a godsend to me

[wdkingery]

I can pull 50, but it ain't easy, my motor is tired not tight, and it is flat here. My f250 mod set me back 2 mpg, and my odometer reads up to 3% high, which I proclaimed a while back.
Same as before, I essentially try not to hit the gas or the brake lol. And I can squeeze high 40's like this.

[clbanman]

Mileage claims based on one tank can vary substantially.  (How do you know how full it was when you started the run and how do you know you filled to the exact same point?)    I tracked my NA over the course of one year and 16,800 km.   I averaged 42.17 mpg Imperial, and 36.71 mpg U.S. in that time  (corrected for verified speedo error).  Stock engine (as far as I know as it had over 250K on it when I bought the car) with 2.25" exhaust added to modified toilet bowl manifold.   After I changed the exhaust I tended to beat on it.  Note that I'm not sure what tranny I have but it's definitely geared substantially different than most diesel trannies.

[burn_your_money]

In my old 1.6na I regularly got around 5.5l/100kms with my best being 5.0/100 (uncorrected, maybe a 10% error IIRC). That engine was tired. It had less than 350psi in each cylinder and used about 1 liter/1000-1500kms. I wasn't easy on it, but I usually wasn't hard on it either. 91 Golf ftr