I had just the same question when i first got my Tenere.

I like many others thought that when F-trip started flashing that i was almost out of fuel so off i went and filled up straight away...and in went 14 ltrs..i could not get any more in.

Then after asking on here i found out that almost 1/3 of the tenere tank is used for reserve...meaning F-trip.

I get 250 miles until f-trip and then another 50 easily. I then get about 20ltrs in.

Try doing about 50 miles on F-trip..then 60..then 70 and see how much you get in each time.

But trust us..your tank is 23 ltrs.

Mike