The onboard calculating, and real time mpg meter on my 06 E320 CDI, was a real eye-opener as per the when of certain mpg the car gets.

I'm really impressed with it's multiple capabilities, including an hourmeter and average mph readings.

I don't know how the car (06 CDI) does the data gathering, but I like it!

Hope you get the concept for the system you're considering turned into a reality.

__________________
'06 E320 CDI
'17 Corvette Stingray Vert