[UK Mini]

Timing strobes are inductive - plenty of volts in plug leads for them to sense. Most programmed ignition setups will output an rpm digital signal somewhere.

I'm certainly interested in an expanded mpguino but I'm a noob to this kind of stuff - I'd be happy to help with/fund some beta testing.

Some ideas:

Vehicle speed by multiplying engine speed by a gear ratio variable, using a few switches on the selector mechanism to sense what gear is selected. You wouldn't need a separate speed transducer; you could gain a gear indicator (7segment led?) & you could build an intelligent shift light, or even a fancy paddle shift/auto selector. A second speed sensor, on an undriven wheel, would give the possibility of traction control. I see people are already using cut switches: which could be automated for TC, and EOC with an additional clutch switch - or those gear selector switches.

A time clock - display default when there's no mpg and/or mileage being counted.

A persistent odometer, in case you do away with the original speedo, so you can still keep an eye on service intervals. You could also count engine hours.

Scrolling display - would make a really cool 'n easier to read electronic speedo.

Analogue inputs used for engine temp, oil pressure, fuel level, manifold pressure etc - user defined labels with a bar graph display. A change in value would put the effected data on the display. You could add a maximum/ minimum reference that triggers a buzzer or warning led. The next level would be to save trip max/min values to revue later

Most AFR sensor kits will put out a 0-5 volt signal too so you could treat that in a similar way. Lambda, manifold pressure (and rpm) are useful as numeric values so a user defined conversion factor would be handy to convert the displayed value in to meaningful units.

Have you also seen Rallyduino? A project ripe for some convergence.