My trucks not new enough for trip computers, so I just figure it by hand. With the 5er I got 11.3 on the clean economy run I did, I'll know better after my trip next month.
I just fill it up, let the pump click then round to the next dollar (It's an OCD thing of mine.) and let it ride. I've had this truck long enough I know how much it will take to fill it up from most any point on the gauge so if it's close to that number, I'm good.
2001 HO Cummins powered Dodge 2500
2014 Sierra 346RETS
Nights camped in 2014-28