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Q: Speedometer stops on 40 mph until driving for 20 minutes and then regains correct speed on the speedometer with no other noticeable issues.

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replaced speedo cluster and problem is identical with replacement. local repair shop used computer to diagnose and recommended replcaing cluster. The problem does not come back until cold start. and goes away again after about 20 minutes of driving over 40. Should I replace trans speed sensor? No shifting problems or any other noted problems.
This is a class c motor home. It recently has had new brakes

My car has 72500 miles.
My car has an automatic transmission.

Hi there. The issue with your speedometer not working properly is the transmission output shaft speed sensor has a faulty pickup or broken wire inside it. The speed sensor has a magnet that reads between teeth on a gear within the tail shaft of the transmission. If the magnet has metal on it from the transmission, then the magnet will become weak and not work right. Remove the speed sensor from the tail shaft of the transmission and clean it then put it back in. See if the problem continues. If the problem continues, then I recommend replacing the speed sensor. If you need further assistance troubleshooting your vehicle, then seek out a technician, such as one from Your Mechanic, that can assist you with your speedometer reading up to 40 mph, then after 20 minutes it goes up to the normal speed.

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