V_88:
An auto gear-head would have better answers for you.
Maybe start a thread on the Sensors forum
I don't think the car I am going to work with (haven't even seen it yet), has an ODB-II interface. By Sensors forum do you mean here or in another site like this? I was following this http://forum.arduino.cc/index.php/topic,92624.0.html
But I don't want to use a mechanical switch, I believe an IR sensor pair would be better.
Yes, I mean the sensor forum here on the Arduino site at: Sensors - Arduino Forum
I agree about conceptually wanting to avoid a mechanical switch. A stationary (relative to the suspension) hall effect sensor with a magnet mounted to the wheel or axle should work. Potentially an IR sensor pair with a white/reflective spot on the wheel or axle should also work, but might have issues if the white/reflective spot picks up dirt and/or road grime. Maybe look at the RPM of the drive shaft between the transmission and the differential (assuming rear-wheel drive and sensing desired on the drive wheels instead of the slave wheels...) since there should be less motion trying to knock your sensor arrangement out of whack and tugging on the rest of your wiring. Again, another assumption that this whole sensor bit is being (semi?) permanently mounted to the vehicle frame and using the XBees as a virtual umbilical. Maybe tapping off of the speedometer sensor wires near the computer or dashboard would be cleaner. Even if the speedometer is mechanical (like in my old '72 bug), the 'cleanest' (i.e. most resistant to dirt and grime) solution would be to put the rotational sensor inside the speedometer, sensing where the rotating cable linked to the wheel comes into the speedometer.
You can see that it can be hard to give advice or make final design decisions without very detailed information about the expected environment. I feel for you since you don't even know exactly what car will be used so you can't finalize some of your own design elements yet (like exactly how and where things will be mounted, how long sensor cables need to be, etc.). Location and type of speedometer sensor to exploit also can't be known until year, make, and model of the vehicle is known...
Wait... No ODB-II? How old do you expect this vehicle to be? All cars sold in the US since '96 must have ODB-II, and if you are in Europe, WikiPedia (yeah, I know...) says: "The EOBD (European On Board Diagnostics) regulations are the European equivalent of OBD-II, and apply to all passenger cars of category M1 (with no more than 8 passenger seats and a Gross Vehicle Weight rating of 2500 kg or less) first registered within EU member states since January 1, 2001 for petrol (gasoline) engined cars and since January 1, 2004 for diesel engined cars."
So, if the car you get to work on doesn't have some variant of ODB-II, the youngest would be a European diesel that is over 10 years old...