5th and 6th Wheel have no friction

So I’m using the SimpleWheeledVehicleComponent for a 6 wheeled vehicle, but the 5th and 6th wheels spin as if they have no friction when any torque is applied to them through ‘set drive torque’. They also stop spinning as speed of the vehicle increases above 360-400 units per second. (This is shown in the picture)

I’ve gone through all the paramaters and I can’t see any indication that ‘something’ should happen at this speed.
There is nothing special about these wheels other than they are wheels 5 & 6, but it is always these two nomater how many times I have re-rigged and and imported the skeleton.

I would be extremely greatful for any suggestions.