Do you know the duty cycle of the sensor output ?

My thought is to use the "Port B interrupt on change" feature on something like the 16F88. Looking at the datasheet it seems to indicate that an interrupt will be generated on any change so low-high and high-low would both generate an interrupt.

Create an interrupt routine that generates a short pulse on a pin and feed the speedo with that. Provided the speedo only needs to count the pulses rather than needing an exact mark/space ratio then it should work. You would obviously need to ensure that the pulse you generated was short enough to be completed in about half the time that pulses would be arriving at maximum speed.

The only caveat is that it depends on how it calculates the speed. If it counts the number of pulses in a second then it should be OK, but if it times the interval between pulses then there could be a problem if the output of the sensor is not 50/50 as alternate generated pulses would have different spacings.

Dont know if any of the 8 pin PICs would be able to do it as I haven used any of those yet.