Saab SID Fuel Range

[Gen-X'er]

My 9-5 is the first car that I own that has a fuel range feature. I like it as it seems to be pretty accurate especially when it gets down around the bottom of the tank. One thing I have noticed though is that it changes drastically between shut off and start up. If I finish a drive in the evening and it says I have 125 miles to go before empty, the next morning it might say I have 85 miles to go. Is this it's way of readjusting itself or is my Saab using an extraordinary amount of gas on being started? It may seem like a silly question but I am suspecting that the DIC is getting close to needing to be replaced as it is sometimes hard to start in the mornings. Sometimes I need to punch the gas pedal to get it to ignite. Does anyone else notice a difference in the SID Fuel display between shut off and start up?

[CaptainMediocre]

Yeah I see the same thing. I have a 53 mile drive home from work, and there were times where, according to it, I had more range in the tank when I got home than when I left. I assumed when it started up it was a base estimate and that it adjusted itself based on your gas mileage after that.

[Gen-X'er]

This is a little different. Mine is a 26 mile one way trip and it will adjust while I'm driving depending on how I am driving. I expect it to do that. I can drop from 80 mph to 70 mph and the fuel range will go up, (or vice versa). What I am talking about is directly on start up the next morning. It seems every time I turn the car over, there is a huge difference in the estimation of what I had the night before. As an example. I pulled into the driveway last night and it said I could go 78 miles more before empty. This morning, it was 39 miles on start up. Getting to running speed may increase it by 5 to 10 miles but nothing like the 40 I lost. Since the car travelled 0 mph and used 0 gas, I would assume there wouldn't be that big of a difference. Today's mileage is pretty accurate but I assume if I had kept driving last night, I would have been able to go close to that 78 mile estimate. Other than crazed gremlins syphoning my gas, I'm not sure why there is such a huge difference.

[9-3 Aero]

No crazed gremlins rather than Saab algorithm. The fuel range feature is calculated based on inputs:
- Fuel level;
- Odometer reading;
- Fuel consumed since starting;
- Coolant temperature.

When you start up in the morning there's a relatively high fuel consumption to low (no) mileage, whereas this averages over the trip (at the end of the day).

The calculated value is updated every 16 seconds.

[cynicaldriver]

Easy way to explain this...
The estimate at the end of the day (before switching off) is for if you continued to drive from that point.
The estimate from the morning is as if you had not even had that trip the night before, so it is a default value... As explained accurately by "9-3 Aero."

The DTE reading you are getting is based on instant results, rather than an average over time.
For an average over time, you will want to use the MPG reading instead.

I have purposely tested the computer on this every year.
In the last 6 years, since I bought the car, it has always been very accurate for both DTE and MPG.

You can confirm the results by completely filling your tank to the top, then resetting the MPG readout. Record your odometer at the time of fillup, and then run it until you fill it up next time. Record the odometer again, and the amount of fuel you put back in to top it off again. This will allow you to run the numbers, regardless of how far you have gone, or how much fuel you consumed between the two fillups.

Fillup 1: 100,000
Fillup 2: 100,500 (15 gallons)

100,000 - 100,500 = 500
500 / 15 = 33.3333

MPG: 33.3333

You can measure the overall accuracy of your DTE meter with a similar method:

Fillup 1: 100,000
Fillup 2: 100,500 (15 gallons)
Tank Capacity: 18.5 (varies depending on model)

18.5 - 15 = 3.5 gallons * 33.3333 MPG = 116.66 DTE (Computer showing more than 100 miles and less than 115 miles would be accurate enough to rely on in a pinch)

Hope this helps anyone finding this through a search for DTE (Distance to empty) readings.

*Edit* I am using easy numbers, and an average of 33.3333 MPG is not typical, nor is it the results I get with my car. Your results will vary greatly from the EPA (Or other agency) estimates, due to your unique driving scenario: Length of drives, speed, frequency of stops, weather, etc...