Thinking about speedo readings, effectively the speedo measures output propshaft rpm, as on traditional rear drive cars.
So it will read whatever is the average speed of rotation of the three turning wheels. Given that one wheel on the rear axle is locked and dragging tyres along the road, this must mean the vehicle isn’t going as fast over the road as it would (for those gearbox output rpm) as it would if all wheels turned freely, which supports the idea the speedo will show a higher speed than actually being achieved? (I think!)