DIC Mileage readout computation inaccurate

[Zorb]

The fuel rates computed (Instant and average) on my car are substantially inaccurate. I'm curious about how accurate most members' cars are, and am almost wondering if anyone even cares (except me).

Basically, it can tell me I did 21.5 MPG, but then I can drive 380 miles and put in 20 gallons of fuel. This is, in fact what I was seeing on my drive home when I bought the car, since I went for about a 3000 mile road trip at that point. My best tank according to the computer was 22.8 MPG, but I then put in 20.2 gallons of gasoline at the next station from when the low fuel message came up. Average speed was 74 MPH, distance driven was 406 miles.

This almost has me suspect an unmetered fuel source, but I would think that would show up on fuel trim (negative trim), but I haven't noticed it.

I had this happen on my old Cadillac and it turned out to be getting fuel into an evap line somehow at a bad in-tank assembly, which obviously would be dumped straight into the intake manifold just after the throttle body.

[P1054]

Zorb, you've pretty much perfectly described what mine does as well. It always shows about 2-3 mpg better than what I'm
actually getting.

[Turbowned]

I stopped paying any attention to mine a long time ago. I know I have about 280-300mi to a tank and that's it. No matter how I drive it I get about 15-16mpg average. The best is when I start the car up and it says 35mi til empty, then the next time I start it (after driving 3-5mi) it says 0. Piggy piggy!

[DHall1]

There is a an adjustment in vag

Mine is within 1 mpg all the time

[P1054]

Dave, do you have some more information on the "adjustment in vag"? I would be very interested in adjusting it. I'm with Turbowned in that I don't even pay attention to it anymore and just know how many miles I get to a tank, but if I could get it calibrated it would be much more useful.

[lswing]

http://ross-tech.com/

Should be able to search or go through the manual to find the setting.

[Bigglezworth]

Check page 148 of your owners manual guys. The item in question is an estimate based upon driving habit from previous logging session.

[Zorb]

Yes but computers do not estimate. It's calculated by injector pulse width @ fuel line pressure.

It can be therefore adjusted for accuracy. I actually found a few things online about adjusting it. I will do it and advise on the process and success.

[Bigglezworth]

Yes but computers do not estimate. It's calculated by injector pulse width @ fuel line pressure.

It can be therefore adjusted for accuracy. I actually found a few things online about adjusting it. I will do it and advise on the process and success.'estimate' wasn't meant to confuse what I was stating. Clearly the ECU keeps calculating over whatever consumption you allow between resets. It certainly stands to reason that the computer knows how much fuel it sends to the engine, along with the actual distance the vehicle moves, and is able to provide you a value based upon the most recent data logged.

You could get a job with Car and Driver. :P

http://blog.caranddriver.com/why-your-trip-computer-isnt-giving-accurate-mpg-readings-and-how-to-fix-it/

[Zorb]

Well DTE/fuel range takes average speed and average instant fuel rate, both over the last however many miles (on BMW it's 12, VW/Audi doesn't say), and divides that by current tank level. I mean average fuel rate is inaccurate in my case, but then so is range, by about the same percentage.

[hahnmgh63]

For mine I'll have to agree with DHall1. I have checked mine about once a year an found it to be within 1/2mpg. Rarely on my commute, mostly highway but fast and a little city (no stop & Go) to be much above 17mpg but then I usually hit triple digits on a stretch on the way home for a while.