good suggestions
I've always wanted a clock. I have padded out the unused pins and am looking at the 328 so there will be room for improvements.
re speed from gears, problem w/that is some (many) of us here do a lot of coasting out of gear, perhaps with the engine off, so you might only be tracking 1/4 of the actual miles.
I like the wii nunchuck idea on the rallyduino
But my focus here of course is efficiency and not best ET. I would use the remaining analogs for things like tps or map/maf (or o2 sensor as you suggest), overtemp can be a very simple standalone led circuit however.
I think traction control probably deserves more dedicated hardware (we are at %50 cpu just watching the speed/injector signals and displaying them)
But even something that seems simple, like a clock, you still have to have some sort of "user interface" for it so that you can set the time, and that eats up precious flash space.