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I think for the input signals the easiest thing would be run a 10k resistor to a 5v zener. Then maybe use a transistor to isolate the microcontroller if needed. That should get a signal the controller is able to handle.
The VSS is just a voltage pulse every so often. Usually so many pulses per mile. So to measure it, just set an interrupt and use a bit of math to calculate speed by averaging the last few pulses for a steady number.
The injector would need a pair of interrupts. One rising edge and one falling edge. That can give a good way to measure pulse width of the injectors.
Once the basic math is figured out converting to whatever unit is needed would be a simple matter I figure. The avg mpg I was thinking of was 'Last minute avg MPG' That seems useful to make sure you aren't getting high peaks in the instant mpg reading but killing your avg by doing it wrong.
Really once the vss and injectors are going into the controller anything you can do with those numbers would be pretty easy to implement. Writing it in a higher level language like C and not assembly makes stuff like that just a few minutes worth of work.
There are some really simple atmel programming setups. So a programming circuit could be as simple as a single IC and a few wires. I figure there will be a lot of people not willing to build their own and will want to buy one. Having a easy to build system would be nice for whoever wants to make and sell them assembled. Another advantage of the atmega setup is that even if the chip is discontinued it would be a 5 minute job to port the code to a new one assuming there isn't one that is directly compatible.
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