I would recommend using an atmel avr and use the codevision c compiler. This setup is very common and a lot of people use them all the time.
Embedded C programming and the Atmel AVR is the book I recommend reading on it.
I have been thinking about making one for a while. The way I figured would be to monitor the injector signal and the vss sensor. Have the controller calibrated by driving and adjusting the speed constant to get it to match the speedo. Then for the gas fill the tank then drive around for a while while the controller adds up the total injector open time. Then it can calculate the flow rate of the injectors when the tank is refilled.
From there it is just a matter of calculating and displaying it on an lcd.
The reason I say use codevision is that 75% of what is needed for this is already included in the libraries it includes. The lcd interface library is on there and it is simply a matter of telling it what to display. The only thing that would need some real consideration is to build a circuit that can work with pretty much any injector signal and any vss signal. Having a few different circuits would make installing it much harder and nowhere near as user friendly.