The speedometer face in my 1997 A6 has "K=6703" printed on it. You can see it here at the bottom.
That's the number of VSS pulses per mile expected by the speedo. For the MPGuino, simply double that number and enter it as the VSS pulses parameter in setup. So, mine is 13406 and the distance is dead on. The number must be doubled because the MPGuino counts both the rising edge and the falling edge of each pulse.
I've seen other Audis with a similar, but not identical number, printed on the speedo. You might have to dissemble the instrument cluster to see it. Remember, assembly is the reverse of disassembly! (8'D