What about this option on some cars, a add on hall effect. Put 1-2 magnets on halfshaft close to tranny and mount a hall effect like this one. Only like $3 and would put out consistent readings.
Hall Effect Switch | Low Sensitivity | Manufactured by Melexis
And all you would need to know at that point is tire size to know pulse rate.
155/80-12 927 rev/mile
1 magnet = 927/mile
8 magnets = 7416/mile
You could attach magnets to large hose style clamp and strap to inner cv joint case.