Speed/Odometer Inputs

[Vanguard]

About 2 weeks ago, my cruise control quit. Then I noticed my mileage as calculated by the OBC keeps falling. I reset Mileage 1 and it has since remained at 0.0 mpg. I filled up the tank on Easter, drove about 40 miles to my sister-in-law's for dinner and filled up the tank when I got back.

I calculated my mileage based on the trip odometer reading (dash) divided by the gas used and got 20.9 mpg, but the OBC keeps falling. Under 16.2 and dropping this morning. The car's running fine so I'm suspecting the diff speed sensor, but am curious since the odometer seems to work fine. Does the odometer use a separate sensor/mechanical input?

Searching on cruise control both here and at bimmerforums.com, seems to indicate that the brake light switch and the diff speed sensor are the likely culprits and the falling OBC mileage is pointing me to the diff speed sensor.

[Traian]

I'm not sure if I'm reading this correctly, but I think a 40 mile sample is too low to accurately figure out your mileage. I mean you should probably do it over a whole tank or so, and see how much it differs from the OBC then. I find that the OBC is very slow to update its values, and usually as once I start riding on the highway the economy only goes up after about 4-5 min. With a bigger sample you can see more precisely what kind of correction you need and correct the OBC's values so that it gives true mileage.

[Vanguard]

I've done some more research. The OBC is not reading the speed. I unlocked it, reset and it recalculated the range, but shows a current speed of 0.0.

The speedometer in the cluster is fine and the odometer in the cluster works as well.