in2pro

For those that notice that the DIC's calculation of MPG and that of hand calculation MPG is different, I have found that if you clear your DIC info at every fill up (I do) and take notice of the "gallons used" and put that amount back into your tank at fill up, your DIC and hand calculations should always be the same...

I noticed that I get about a gallon more fuel in the tank when I let the pump run to cut off than what the DIC shows as used...
This is the difference you see in the 2 calculations.
If you are one to "top off" going past the first pump cut off, and continuing fill (very bad idea) your two MPG calculations will be even greater in difference...

I suspect that when the algorithm was written for the DIC MPG calculation is was based on the tank capacity for that given vehicle... since a bit more is put into the filler neck, the algorithm can't account for that extra amount....

I just filled up a partial tank to verify my hypothesis and only put the 10.6 gallons in that the DIC said I used, and both MPG's add up and the fuel gauge does show a full tank...

For the sake of calculations I only went to the tenths digit for calculations, as some pumps will report out to three decimal places.

I would be interested to hear feed back from others that might be willing to try this experiment and the results, opinions....

I got 17.5 miles a gallon out of that 10.6 gallons of gas going 187.1 miles

I couple of extra notes, I just did a "tune up" of plugs and wires and have 92,500 on the clock as of this writing,also have the 3.42 rear end and run M1 full syn since day one...

73shark

If you always fill to the first click-off and at the same pump (I do) and calculate the mpg, then it doesn't matter what the DIC says as it always understates the gallons used. At least the four trucks I had w/ DICs did. That's why I think if you continue to only replace what the DIC says you used, you'll eventually run out of gas or close to it.

It's like the contest to kiss the beautiful girl. A business student and an engineer were standing 20' from this drop dead gorgeous coed and were told that every 30 sec. they could advance towards her half the distance and when they got there, they could kiss her. The engineer said that you'll never get there. The business student said yeah but you'd get close enuf.

(Being an engineer, it always pains me to tell this joke.)

Joe In NY

what rpm's does the motor turn when cruising at 75 mph with the 3.42 rear end? We have 3.73's and our turns around 2,100 rpm at 75 mph...

73shark

Its proportional so you could calculated pretty close. About 1,925.

in2pro

Off the top of my head I couldn't say but sharks answer is pretty much on the spot, my tach is under the 2k mark...
We will hit the road tomorrow, if I can talk the wife into letting drive with my code reader/data collection tool hooked up I'll be able to tell you exactly what it is....

SabrToothSqrl

the amount of fuel in the tank is (or should be) irrelevant.

the vehicle is measuring fuel consumed and distance.

the tank size does not come in to play.

the ECM should know, down to the .0000001 of a gallon how much fuel it's consumed.

73shark

If true, then why is the DIC reading always 0.3 to 1.0 mpg higher than calculated?

Maybe the odometer is only accurate to +/- 5%.

in2pro

After about 4 tank fill ups the past week from our road trip I only put in what the DIC said was used each time. At each calculation my numbers and the DIC numbers matched up every time... Also the fuel gauge showed full after each fill up....
Again the difference is calculations is due to more fuel being put into the tank then what the DIC shows as used... if the pump cut off at the same amount that was reported by the DIC your numbers will always match... since many fill to or past the first pump shut off, you get a higher number of gallons "used" when calculating than what the DIC used to do its calculation, therefore getting an incorrect or different value of MPG...

SabrToothSqrl

you trust the Tach, right?

You trust the speedometer?

You trust the fuel guage?

but not a calculation based upon two of the inputs?

I'm not saying you are wrong. In my own experience, the MPG reading on my Jeep and Tahoe are both different than my calculations at the pump.

So, I've given up.

Unless you GPS measure how far you've driven, and personally measure out each gallon of fuel, and control the climate as to the fuel amounts (fuel expands when hot)...

I'm not sure what any of it matters...

While it's nice to know when you go from say 18 MPG down to 10, without good reasoning, and it's something to look into....

I just fill the tank.

I don't even look at the gallons or price. If I can't afford to fill it, then either it's the wrong vehicle for me, or I need to re-evaluate my career path...

So yea... I gave up trying to reconcile them, and just go with the gauge...

as these gauges become more common on cars, the old method will disappear...

try convincing a 16 year old to calculate MPG or get them to calculate cost per mile in fuel... vs. reading a gauge on the dash...

you aren't watching your engine spin round and round with a stop watch are you?

I'd lose track over 60 RPM let alone 6,000 RPM... (I'm still amazed these things move that fast).

I always assumed the ECM knew how much fuel it was spraying into the cylinders... pressure x duration = volume of fuel. of course temp would play into that too, but it knows the air temp... does it know fuel temp?

Anyway, back to dreaming of my Model X... as I stood freezing my **** off in 30 degree wind I thought how nice it would be to cleanly plug in my car in my home garage for the night..

in2pro

I completely concur that any exact measurement is purely speculative...
My intent is to dispel the notion that the DIC is somehow intentionally wrong with its calculation... its not.