speed passion problem ????

[rutt]

Morning all
After a 20 year break I attended my first meeting the other week.
Im running an sx3 with turnigy 9.5 and speed passion gt2.0
I've run the car previously on grass to get a little practice.
At the race meeting my first run went OK although the car did seem slower than I remembered. During the next two runs it got slower and slower and the motor came off hotter and hotter the car lacking in top end so much so I thought I'd cooked the motor.
So ive purchased a new turning 9.5 and it still seems the same no real top end
Now my question is could the speedo be at fault I've reset end points .tried another sensor lead.running sensorless the motor seems the same which is worrying
I also reverse connected the speedo just for a second at one stage so could that have pooped a phase.
Immore than ready to buy a new speedo just need that confirming that I've killed it
Thanks in advance.
Paul