You can use some UI plugins that show fuel consumption as you drive (I think TrackMapPlugin does that). Also Log Analyzer can show you your average fuel consumption in percent of full tank capacity for any completed session. So you`ll need to multiply that percent by fuel tank capacity for your car to know litres.
As stated trackmap plugin does it well, either in laps remaining or time remaining. need to run a few laps for calibration. and its position and size on your screen is adjustable.
Plus trackmap has a lot more great uses. Lap analyzer is a great tool also but is more designed for post race results and stats as far as I know.
