Did DIC lie to me?

[2tough2park]

I recently had a reasonable trip to take that would let me highway mileage on my LBZ. 184 miles@10.2 gallons. This info is by DIC, not me. BUT, DIC also says I only AVERAGED 17.1mpg. Is this a case of split personality or what? I'm actually ecstatic about the 18mpg average as my 94 only managed 16.5 on a 400 mile trip(grill guard may have had impact). So if I used 10.2 gallons over 184 miles, how did I average 17.1mpg? Off the subject, I think I could pick up a touch more had the trip been longer.

[rob from bc canada]

I have found that the DIC seems to provide a figure somewhere between you actual average since last reset, and your current condition over the last few miles.

The less miles you have since the reset, the more the figure shown seems to be slanted towards current condition.

I know this, because I have sometimes compared litres used over short distances and it affects the average more than it should.

However, I recently let it ride for a couple of thousand miles without resetting, and it almost seemed stuck at the Hgh mpg from the trip, and even after two tanls of local driving the average dispalyed had not gone up.

Go figure.

[BozDMAX]

From my post in the LB7 Fuel economy poll:

"I first noticed the difference between the DIC and my actual mileage in March '05 and have been tracking the difference since then. After throwing out some obvious outliers I ended up with 106 data points and using a least-squares/best fit methodology can pretty much estimate my actual mileage by the following formula:

Actual MPG = .86*DIC MPG + .74
R-squared (if you remember your statistics course) is .86 - not terrible given the wide range of conditions (towing vs. non-towing, hills vs. flats, changing altitudes, etc).

so, 20 MPG on the DIC would work out to about 17.9 on my slide rule (anyone else remember those?) with the actual data. "

The DIC seems to work off what the engine computers THHINK it should have taken to perform as it did, rather than dividing by an actual fuel flow counter. A full tank fill for me is often 2-3+ gallons MORE than the DIC shows I have burned.

[jbplock]

"....After throwing out some obvious outliers I ended up with 106 data points and using a least-squares/best fit methodology can pretty much estimate my actual mileage by the following formula:

Actual MPG = .86*DIC MPG + .74 ...

so, 20 MPG on the DIC would work out to about 17.9 on my slide rule (anyone else remember those?) with the actual data. "

A full tank fill for me is often 2-3+ gallons MORE than the DIC shows I have burned.


Boz,

[FONT=Times New Roman][SIZE=3]I missed your post in the fuel economy poll thread

[2tough2park]

My next trip-down to my mom's house and back. Less than 100 miles, but worked out to 18.28 mpg. 18.28 MPG!!!!!!!! Dam!!!!!! And that's by DIC telling me fuel used versus Trip A odo. Average fuel economy display? Still at 17.1. Make me wonder why it's even there.

[DmaxMaverick]

My next trip-down to my mom's house and back. Less than 100 miles, but worked out to 18.28 mpg. 18.28 MPG!!!!!!!! Dam!!!!!! And that's by DIC telling me fuel used versus Trip A odo. Average fuel economy display? Still at 17.1. Make me wonder why it's even there.

Your average will be an overall average until you reset it. The more miles you have in that period, the slower it will change. Even so, it will become more and more accurate as you rack up the miles in that average. Instant mileage, and tank by tank mileage is usually a much less accurate representation of your overall mileage. An average over several tanks will have a more representative account of all the conditions you drive. The best way to calculate your mileage would be over a long period, say a year or so. Keep track of every oz. of fuel you put in, and the miles you drive during that period. There will be a margin for error every time you calculate, so more info over a longer period will reduce that margin. Averaging your tank by tank mileage will be less accurate than a wholistic average. The computer will do this for you, but you have to wait for it. Most people don't have the patients for this. Instant results....."I want it NOW!!" kind of attitude. It's the day and age we live in. That's why the stock market is so screwy, and oil/fuel prices are what they are.

Also, if you are using any type of electronic modifyer (power module or programmer), you can just toss out anything the DIC tells you. The numbers will be absolutely useless for anything but trends. There are some exceptions, but there is no consistency or explanation for them.

[2tough2park]

I'll try that and see if DIC concurs. Hopefully, I'll get lucky. I'll put more miles on this tank before I try it.

[WILLYD-MAX]

If I reset my DIC at every tank and fill to the neck on my tank then my DIC is usually within .5 of a MPG hand calculated. I add 2% on my trip mileage for my 265's. If I don't reset at every fillup then it varies a lot due to my driving habits as in some times I pull 24k and sometimes empty.

[JohnC]

Fill the tank, reset the average mpg and fuel used, and go for that long drive. Anything else and you won't get an accurate picture of that trip. Like DMM said, ther longer the mpg goes without being reset, the less sensitive it is to current conditions.