a 16x4 or 20x4 lcd is not that expensive and can be directly connected to the Arduino. I think we should go ahead and settle on the size of the screen so everyone can get one and start working on building the interface part of the system. Also the number and position of the buttons would be helpful.
The calculation part is pretty easy really, just a few numbers to manipulate. The sensor interface is also pretty simple. The only thing that is going to take some time is making sure the variables don't overflow and getting an easy to use interface figured out.
http://priusdiy.fc2web.com/image/SCHEMA05.jpg is the supermid schematic and what I was thinking of is basically the same but he already has the numbers filled in. Look in the top left for the injector and tach input circuit.
Those 2 inputs and the lcd interface it is pretty much ready to code up and make work.
I already have lots of 20x4 displays and buttons and junk so as soon as I get around to ordering the arduino I will have everything I need to start building a menu system for it. But it will probably be after school is out before I get into it really.
Another output option would be to hook up an analog meter to it. A 10k rpm tach could be driven to show 0-100mpg by driving it with pulses from the controller. A vss based speedometer could be driven to show mpg instead of mph
