IDI engines: fuel economy comparison

[HarryMann]

I'm going to make myself unpopular now...  

I see so many definitive figures above for mpg, here we have 48, there we 53 now we have 59...

All as good as meaningless I'm afraid, and I know your Replys will all be screaming from the rooftops that you 'know' for sure it 'did' 45, now it 'does' 53, or 'was' 51 now 'is' 55... and you drive the same journey every day and fillup at the same pump at the same station, and the tank brims and the nozzle cuts out at exactly the same point and you drive the same and the weather's the same day in day out ....

Don't bother, because without one of these, which shows what really happens from day to day, from season to season and from Mod to Mod, over several thousand miles, I'm not listening...



As you can see, a definitive figure always has a quite considerable band of variance, even for the same journey... manufacturers figures are quoted for 'very, very specific and accurately controlled conditions' - we cannot emulate that.

Note the above graph covers 7,000 miles or so... now we can slowly see what sort of true mileage we are getting (currently I'd quote 23.5 +/- 1.5), and how varied fill-to-fillup (accurately brimmed) mpgs can be.

Blue = fillup to fillup
Yellow = Average of last 3 fillups
Pink = MPG to date (total miles/total fuel to date)

A picture's worth a thousand words, so lets have them... or stop quoting mpgs to within 1 mg and start saying was about 53, now about 55 (+/- 2).. it's easy for the error band to overlap.

Mr Un Popular  

PS. UK Statute miles, UK Imperial gallons (proper ones), odometer mileages corrected by GPS factor (to account for tyres and odo errors)

[gldgti]

yeah, ok. but what if i say that in 12 months i covered 55,000km, and over that whole perieod, i see a consistent trend of my economy increasing (i.e getting better) as i make modifications to the car. for the first 2 months i owned my car (before any mods) i was getting about 6l/100km, and now i'm getting about 5-5.5l/100km, 2 years and many modifications later (none of which were particularly economy oriented, everyhting i have been going for was for increased performance :-) )

:-D

[macka]

Harry,

  You make a very valid point about consistancy. I fill up to the litre when I fill up at the pump, but my results will be skewed somewhat. I changed tires a few times, had a slow leak issue in 2 tires, and I have loaned my car out to my brother a few times. The other thing is, I don't really care how good of fuel mileage I am getting, as long as it isn't as bad as my jeep was (something like 14-16l/100km).

[rabbitman]

Figured I'd pull this back up......and brag a little

Best this summer was 46.65mpg on July 17-18. 525 mile round trip with 4 people and lots of gear, I filled up at 420 miles and it took only 9 gallons filled to the brim!!!!

It helped that I was following a '84 vanagon waterboxer and didn't have to keep it floored, speed on the flat was good at 55-65mph, hills slowed us to 45-50.

Earlier this summer I made a 200 mile round trip with a group of other cars......me being the slowest . I was running full power without holding the egt's over 1100F on the hills and cruising at 70ish on the flats.
That trip was only half the tank and the for the rest I drove normal. That tank still got 45.5mpg

Normal driving for me is to accelerate shifting at 2500-3000rpm depending what gear it's in and staying mostly out of the smoke range and letting it slow a little going up hills. Cruise is 55-65.

[rabbitman]

Ok so my last fill up was when I got 46.65mpg in my last post and I got to fill up with good ol' #2 diesel .

So today I filled up again after 400something miles (I forgot exactly) and calculated 47.2mpg or 4.98L/100km......or 56.68 imp mpg. I'm not bragging .

I wish #2 was easier to get

[ldeikis]

Wow, this one's dug up from the depths.

Thought I'd contribute because I remember reading this thread years ago before getting my rabbit.

Car is a 81 1.6NA with a FF 5 speed, 4 door, aftermarket sunroof, AC but been disconnected for a couple years (though the weight is still there).  I drive around town, then 300 miles round trip to the city to work once or sometimes twice a week.  I do about 12-1500 miles a month, always fill up at the exact same place cause it's a hassle to track down diesel in the city, so I fill up on my way down, and usually drive out the balance around town, then refill on the way down again.  Synthetic in the case and tranny.  I drive 60-65 everywhere that isn't a school zone, live in eastern NY so it's hilly but not mountainous.  I'm not aggressive, but on hills I have it all the way down very frequently, and occasionally have to pull 4th.

I run my mileage at every tank just to be alert to if something changes drastically, but don't tally it long term.  The car was getting 42-44 mpg (every tank, never more or less so long as the AC was disconnected except for one crazy trip back from Michigan with a huge storm at my back for 600 miles).  Last winter I did the headgasket and timing belt, and retimed from somewhere in the mid 80's to .95.  Car now gets 47-49 every single tank.  Doesn't seem any faster, slightly clackier at idle when cold, but every single tank I'm seeing about 5mpg better.  I've owned this car for 4 years and about 50k now.  Hope it helps.

Luke

[Wayland]

Static timing does make a huge difference. My Passat Eco when from 47 to 53 miles per imperial gallon by advancing the timing from .85 (I think, whatever the factory eco setting is) to 1.05.