2014 GEM e2 stalls showing 0 MPH and wrench symbol (error 81)

Intermittent stalling of vehicle. Speed guage shows 0 MPH. Take foot off the gas and it will reinstate when you depress the pedal though still O MPF and wrench icon shows. Turn it off the the error will tend to clear. Getting error code 81. Thinking it could be a speed sensor based on other threads I read. The only change we made was a slightly larger wheel that we added. Would appreciate some suggestions including best place to purchase the part.

Which motor do you have ?

Its the original motor made by Advanced Motors and Drives. It says 72 voltage, part number GU3-4002. Produced 12/21/13. Thanks.

Let me know if you need anything else.