if you do want to tack on to your speed signal, do it through a high impedence input such as an op amp, and use a 12K liftup to accessory on the speed signal, as this will work for all hall effect senders, if yours uses an inductive, then you would have a slightly harder time, but they are rare in modern cars,
it wouldnt matter about actually measuring the pulses, just reacting when there are none, e.g. make an interupt to a delay off of it, or something similar,