I am a little ashamed to admit that the S Type is my first car with the ability to display MPGs (fuel economy). That said, I am not for certain if the display is functioning normally or flakey. The MPG display has been this way since the day I bought the car.

Situation: Overcast day, 60 degrees F, no wind, flat highway, 65 MPH, cruise on, 6th gear.
MPG: Varies between 37 and 42, not necessarily in a periodic manner, more like a double cyclic, that is, 37 to 39, then up to 42 to 40, then down to 37 to 39, etc

Firstly, I really doubt the car is getting 37 mpg at a minimum. The best I have calculated is 31 MPG, from miles per gallons consumed. Secondly, is the MPGs supposed to vary so much under constant, stable conditions? Or is something amiss and the variance an indicator of such?

Note: The car runs great, the idle is smooth, and when I am at speed, the tachometer and speedometer are rock-steady.

I am thinking that there is a minor air leak somewhere, or a sensor is returning intermittent values. I guess I gotta break out the mongooser and take a look at the fuel trims.

 

Bet you it's the fuel trims.

The ecu manages mixture by "hunting" across the signals on the upstream O2 sensor. So that might be what you are seeing.

After the novelty wears off, the "average" fuel economy display seems to be more useful and less distracting.