LOL, For the price of one
sg you could put
mpguinos in all three cars
The injector monitoring approach is typically more accurate than obd solutions.
I think there is a version of the megasquirt that has obd, but if you can build a megasquirt, you can modify that to display mpg on something (like a $15 serial lcd).
But take a look at the obduino,
OBD MPGuino gauge , it is a
mpguino like self contained computer (atmega and an lcd and some buttons) that can read some obd protocols. You can see what pids it uses to determine mpg, as well as the messaging details.