What's the thing near the Zetec odometer

[Katoomba]

There's a readout in the odometer area that is not really explained in the manual.

It's a circle with a diagonal slash through it next to a speedo with the pointer at 10. Next to that is a number which in 6100 ks I've only seen read 48 or 51.

The manual seems to indicate it's an average speed but it doesn't seem to change all that often.