Cadillac Repair: transaxle, speed sensor, transmission shifts

Question
1994 deville concour
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-----Question-----
driving at 40mpg all of a sudden seems like tranny drops out motor is running but it feels like its in neutral p 94 fault could the speed sensor cause this?thanks
-----Answer-----
What model, i know in my 90 deville automatically after 40mpH the transmission shifts into overdrive, if its a 80s or early 90s model you may need to replace either [yes] the speed sensor, or a vacuum that goes from the throttle body to the (rear right [if its a longitudal mount]) of the transmission.

Answer
First i would check the vacuum going into the transmission from the throttle body, and its up to you whether or not you want to replace the speed sensor but i think you have  a internal gear issue, take it to a transmission shop and have them diagnose it, or dump some lucas transmission fix in there.