Calculated. In as much painstaking detail as I can get :-)

I use the lifetime odometer reading and petrol receipt to three digits after the decimal as my raw data.
It actually never occured to me to use the screen, since I have always considered it something of an estimate -- just like the gas gauge. I'm now sorry that I didn't track it, so I would know how much it varies by. I'll start up if the question starts to bug me.