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My vss is also 4.7-4.8 ... not quite 5.0v. Good point from bobski about skipping the resistors and just taking the raw 0/5v for your vss input. One thing about that though, is you never know for sure what the speedo is going to give you. I have 2 civics, one gives me 5v (like the service manual dictates), and the other gives me 12v on the vss. Hopefully the 5.1v zener diode will prevent >5v from hitting your duino, but that's something i'd rather risk with a $20 arduino than a $50 mega.
The other thing about skipping resistors: when i was trying different values, the real low one (1k) made my speedo act funny. It was good at low speeds, but got real jumpy over 25mph. My interpretation is the mpguino was sucking too much current from the vss circuit. My present value is 3.3k, and my speedo has been just fine with that, with one exception -- one morning my odo quit turning. 18 hours & 200 miles later, it was working again, and has been ever since. My point being, you want your mpguino be invisible to the rest of the car. If it can screw with the speedo or odo, maybe it will affect (or even fry) your cruise control or ECU.
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