It's surprising to find a speedometer cable on a vehicle this new. It would also be surprising to find a speedometer cable-driven cruise control servo, but just so we don't overlook something stupid, check if the cable goes into the cruise control servo. I'm 99 percent sure it doesn't, but if it does, it will have flyweights that come out due to centrifugal force, and those can come apart and catch on the housing. That will cause the lower cable to lock up at higher road speeds, and will twist that cable off.
Next, use a piece of the old cable to try to hold the pinion gear in the transmission from spinning. If you can do that with not too much force, either the teeth are worn off or there may be a plastic gear on the tail shaft that is cracked and slipping.
A less-common thing to consider is on most transmissions, the speedometer drive gear is set in a housing that can be rotated to three different positions when it is installed. The position is determined by the diameter of the gear which is a factor of the number of teeth it has. Different numbers of teeth are needed and determined by axle ratio and tire size. Larger gears are installed with the shaft further away from the gear on the tail shaft. If a smaller gear is installed that way, there will be very little tooth contact, and it will be easy for enough to wear away to the point the speedometer gear stops spinning.
On some designs, the speedometer gear slides over the metal part of the housing, and heat deforms that part of it and makes it stick. The gear can get so tight that the teeth get chewed off, and it stops spinning. You'll find that when you try to pop the gear off and you'll have to tug really hard on it.
Wednesday, December 2nd, 2020 AT 8:14 AM