MPG vs Speed

[Fingaluna]

Hi All,

I have been in pursuit of mileage since I bought my 1994 TD Jetta about 2 years ago.

Have always thought it was a little on the crappy side... Usually got around 650km per tank on 100% Biodiesel. So, I:

Increased tyre pressure to 40PSI.
Set timing to .93
Rebuilt the injectors with Smog's nozzles.
Cut the pre-filter screen off the air intake box (was BADLY corroded, not much free space left on the screen).
Went to synthetic oil.

All this made a pretty big difference. Went up to around 750km per tank, sometimes a little more.

Then, recently I had a work trip to Utica, NY & back so I figured I would try slowing down from my usual 115-120kmph to 100kmph for the whole trip. I tried to make all the gear shifts at below 2200rpm and tried to take it real easy. No jackrabbit starts.

Well, I got 900km on that tank. It was 50% Biodiesel. Cool. An extra 100km or so for the tank.

Then a weeks or so ago, I had a work trip to Montreal, so I figured lets try it at 90kmph the whole way. Well this time I got a little more, 920km for the whole tank - but - this tank was once again 100% Biodiesel, and I had around 10% In-Town driving as well. I figure I would have been up at 1,000km had I been using regular diesel & had just kept to highway driving.

So folks, the verdict is:

Driving style and speed made MUCH more of an impact (to MY car, YMMV) than the minor mechanical/maintenance improvements that I had done.

I thought that inquiring minds might like to know.

Cheers!

[rabbitman]

yeah, I did a test a while back with my rabbit. With a one gallon jug sitting on the floor for a tank and marked every quart I could get 12.5 miles per quart going 45mph and if I went 55-65 I got 10 miles per quart. So 50MPG going slow and 40MPG going fast. :shock:

[vanagonturbo]

I concur. aerodynamics can help with top speed and mileage, but drag is drag no matter how you look at it. cruise everywhere in 5th gear at 35 mph and you will get rediculous mileage..

[DonGTI]

5th at 35 mph... and very low rpms... most mechanics i know dislike such driving due to increased carbon build up, how do you people look at it?

I went overland from italy to germany cruising at 70 mph on a totaly unmodified aaz (apart from home made cruise control) with a high diff on a mk2 golf and got 55-60 mpg all the way... on the way back in order not to miss the ferry back i had to floor it and got 36 mpg's! so revs and acceleration are critical for mpg's

[jtanguay]

5th at 35 mph... and very low rpms... most mechanics i know dislike such driving due to increased carbon build up, how do you people look at it?

I went overland from italy to germany cruising at 70 mph on a totaly unmodified aaz (apart from home made cruise control) with a high diff on a mk2 golf and got 55-60 mpg all the way... on the way back in order not to miss the ferry back i had to floor it and got 36 mpg's! so revs and acceleration are critical for mpg's

that carbon build up is mostly for gassers.  you can foul up the plugs by running too low rpm's and 'lugging' the engine.  of course with our diesels you can carbon up the motors too by 'lugging'.  i'd say keep it in 5th and accelerate smoothly.  don't mash the pedal expecting the car to just leap to speed.  downshift if you really need a quick spurt.