Well, this problem is solved now. I ended up building a little circuit with a comparator to clean up the pulses, and amplify them to 5V, no matter how low the signal voltage. Seems to work well with vref set to 0.7V. At lower voltages, the needle oscillates as the car starts moving (noise in sensor/line I guess), and at higher voltages, once the car starts moving, the speedo doesn't, then there is a big jump to the actual speed (like say 20mph). But 0.7V works well. I just needed to recalibrate the speedo for the lower pulses.
Thanks for the help,
-Neil.