Speedometer Problem 5.0 1990

Arnold Zwiffle

New Member
Dec 30, 2013
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Car- 1990 Convertible, 5.0, 3.27 rear end locking ( data plate, tag on rear end and by measuring tires vs drive shaft revolutions) Automatic tranny - tires 206/60/16 and new, Shiny Green with a perfect white interior. Speedometer needle does not jerk, hesitate or go wild, not even a vibration, mechanic when changing cable gear has slowly move the speedo cable and can feel no hang up or binding , car does not hesitate off the line, or smooth driving or pedal to the metal at any speed, idles maybe a hair fast but very smooth and quiet.



I bought the car Oct 2013, on my way home doing 75mph up the freeway I noticed everyone was passing me including the cops ( several gave me good look and a thumbs up) I checked the Odometer against mile markers and notice I was better then 1.15 miles on odometer vs 1 mile on the road marker. NOW the fun begins, 21 purple gear was in the car at that time


I attempted to buy a New gear for the cable, using the help guides. I made a mistake and bought a 23 tooth black, this reduced the difference to apox 7-10% faster on the odometer then actual. This spring I then used the quick guides and was advised to use a 19 tooth beguge (?color?) based on a 7 output shaft. I then went to 25% fast on Odometer then actual. Changed the shaft size in the guides to a 8 and bought the 20 tooth orange gear. This reduced the difference apox 20% over.


In addition, the 23 tooth black gear when removed after apox 1500 or less miles, was missing apox. Half the teeth half way around the gear on the shaft side-shaved clean off,


So does any one have an idea of when is going on. When I use the gear that the guide says is correct I get the worst reading, and when I the toothed gear closest to correct speed that gets chewed up? Two mechanics have now worked on this, one with a lot of experience on this model of car, and both are out of ideas, Calls to tranny shops also have not produced any help beside, "that does not sound right comments" and the suggestion to do what we already have, ck the inner cable and try a different gear. This is not doing what it should but 180 degrees opposite from the know facts.


I was told that a shift kit was installed by the last owner of the car. The body condition would indicate some took very good car of this car in it's youth, but unknown if a tranny from something from a all together different car was dropped in or some sort of strange modification was done

Thank you in advance for for any help your help

All gear where matched against a GPS

19 tooth 30mph GPS 40 on Odometer

20 tooth 32.4 mph GPS 40 on OD

21 tooth 55mphGPS 64 on OD In car when I bought the car

23 tooth 55mph gps 61 on OD

Used gear guides form both Late Model and Muscle car to determine gear
 
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mikestang63

SN Certified Technician
Aug 27, 2012
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In the garage
Make sure you are buying the correct speedo gear based on your 90 AOD. I believe the output shaft is 7 tooth? The problem with any 23 tooth speedo gear is the matching gear in the trans is 21 teeth and willl always result in the 23 tooth gear wearing prematurely. Your best option at this point is to have your speedo recalibrated by a speed shop.
 

Arnold Zwiffle

New Member
Dec 30, 2013
7
0
2
Make sure you are buying the correct speedo gear based on your 90 AOD. I believe the output shaft is 7 tooth? The problem with any 23 tooth speedo gear is the matching gear in the trans is 21 teeth and willl always result in the 23 tooth gear wearing prematurely. Your best option at this point is to have your speedo recalibrated by a speed shop.
 

Arnold Zwiffle

New Member
Dec 30, 2013
7
0
2
Thank. I will try a speedo shop, only trouble is over 125 miles one way from me, already called them.. be a few weeks.
Looked at the black 23 gear closely and noticed that 1) the clip give is maybe 3 times the size 2) the clip groove is located closer to the end of the gear 3) the teeth of the gear start and stop at different places on the gear, making the teeth short by about 3/32, vs the other two gears that I have.. Yes is a for automatic and not manual