FWIW, topping off is BAD. Stop at first click off.

I agree that monitoring mpg is a good tool to catch problems early which is one of the reasons I do it by hand. Always have.

 

How do you know 10 gallons at a pump... is 10 gallons?

At this point it's more philosophical, but I imagine you aren't standing there with a few gallon milk jugs

And, it's not that I don't care. I do care about my MPG, I just figure the DIC is 'good enough' if not the more accurate device.

 

No, I didn't measure it with a 10 gallon jug. But the guy from the state that put the sticker on the pumps did.

 

which state, how long ago?

lol.

are you sure it was a .. ohh crap, i can't remember the car... from was it my cousin Vinny?

in the courtroom



where the old lady says it's them, and can't see from 10' away...

anyway, it's all moot, in 90 years none of this will matter.. (to me).

 

In NH the state has to certify a fuel meter every year. It's not just a form, the guy comes out and physically checks the meter. We've had it done yearly at our business.


I don't know enough about the technology to say whether a DIC can be totally accurate or not. I do know that in more than a half dozen vehicles that I've checked, none were accurate. So maybe it's the fill level as in2pro has found, but I'm pretty confident that when I burn 2-3 tanks of fuel and do the math that I'm getting the right figure for mpg. Having said that, I do believe in verifying the odometer against a gps. For example, in my Jeep Liberty the odometer is off 5% with stock tires on it. When I calculate mpg I take that into account.

 

If your Liberty is off 5%, you should get that checked out... unless the miles work to your advantage at resale



So your theory is it uses the tank size / fuel level sensor to calculate the MPG vs the TPS / RPM / fuel injectors?

(am I correct)?

Anyway, while laying awake in bed at 3AM and reading Wikipedia about AC induction motors, then some how skipping back to this... I thought...

what if you disconnect your fuel level sensor?

then the DIC wouldn't know how much fuel you've consumed.

and you'd know it was based on the fuel level sensor vs. the ECM system.

right? both the MPG display and the fuel burned should show very wrongly...

of course, now that I think about this, if the fuel level sensor was involved, how would it do 'instant' MPG? that has to be coming from the ECM... so why tap two systems to do the same thing? instant MPG and average MPG are the same with different values...

so... yeah... lol

 

If the level indicator in the tank is used to tell how much is used, that could be way off depending on the incline you are parked on... so that must not be it...
but as long as I always put it what it says was used I always get the same calculated numbers... so the question is where and how is that "used" calculation number coming from...

 

I've not seen any indication of a fuel flowmeter in the schematics. IMO the fuel used must be a calculation by the ECM counting the injector pulses to the engine. As SabrToothSqrl said "down to the .0000001 of a gallon"
The tank level sensor appears to be used to show the fuel range (miles to empty).

 

Sabr, funny you were thinking about all this - I drove 400 plus miles this weekend, and was doing the same! I was thinking about in2pro's experiment, and same thing dawned on me, for the fill level to matter it has to be reading the fuel level, but for instant mpg it has to be using engine input. I find it very hard to believe the fuel gauge could be accurate enough to do more than give the distance to empty.


If using engine input - throttle position, injection events, speed, rpm, etc. - then it may be fairly accurate, but in my opinion nowhere near as accurate as hand calculation over multiple tank fills.


On my Liberty - they are all that way with stock tires. Look at a Liberty forum and you'll see that the odometer reads 5% too low, the speedometer reads 5% too high. This is true on the CRD diesel models '05-'06, not sure if the gas models had the same situation, and of course there were tons more gas Liberty's, only a handful of diesels made. I know, seems crazy.

 

doing it by hand will go the way of the dodo. who's going to go forth the effort?

I'm putting down a $5k deposit on a Model X here, hopefully by winter 2014.

all this silly gasness will go away too!