There seems to be loads of apps available that can record journeys you have ridden. Whether iPhone or Android the app stores have loads.
What I find most useful is this app. Mpgcalc.
I upgraded to the plus version for a small amount and it keeps track of your miles per gallon. Nothing too flash but if, like me, you like to know, then it calcs in seconds.
The app version I use is the UK version tailored for the larger imperial gallon rather than the smaller US gallon.
The stats page is quite good and shows enough information on how you are riding. I know on days where the right hand has been more active that MPG will be lower.