arduino for shifting separatly controled overdrive in project car

ok heres setup.  70 chevelle new fuel injected engine.  hooked old automatic turbo400 transmission, behind 3 speed transmission there gearvendors overdrive.  now, heres problem.  gearvendors sends controller unit controlling overdrive.  there 2 types gearvendors send , depending on whether or not transmission automatic or manual.    controller unit has manual mode both automatic , manual, , automatic mode kicks in after 45 mph in automatic controller.  ordered overdrive long ago gearvendors not going me out doing exchange of kind.  want use 1 of arduinos this, plus add added functionality gearvendors controller doesnt offer.
heres how gear vendors controller works:
signal generator (two wires) transmission go controller tell how fast vehicle going.
now, have 2 types of inputs:  manual mode , automatic mode, , on , off button.
in manual mode,  when car @ full stop overdrive off until hit on , off button.  accelerate goes, depending  on speed first, first overdrive, second, second overdrive, third, third overdrive.  when come stop again, have re-engage  on , off button.
on automatic mode unit engages after 45 mph.
my question need , how setup 2 signal generator wires arduino? 
also can point me in way of tutorial might scenario can attempt code it?  have gotten far having led flash on arduino.

do want work manual setup 1st, 1st o/d, 2nd, 2nd o/d
or want switch on @ 45mph?

i'm not clear if manual mode can used manual transmission.

in case seems first task figure out how arduino detect vehicle speed reliably. don't bother thinking controlling o/d until can that.

do know "comes out" of 2 wires transmission in order provide speed signal? might varying voltage or might pulses varying frequency - or maybe else entirely.

...r

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