Hi, I'm Terry from MRT. Hopefully I can help to sort out your problem.
There are lots of reasons why you get interference one of the most common is the installation, ie the location of the receiver speed controller and routing of wiring etc in the car. The other common cause is broken, damaged or a lack of capacitors on your motor. It's important you have one capacitor connected from each motor tab (+ve and -ve) to the case and one between the motor positive and motor negative tabs. If you check your motor and it's all ok maybe you could try moving your receiver and/or speedo, try moving them further apart etc and see if the interference stops.
If you can't fix the problem please contact us by phone (Mon to Fri 9-5) if you're in the UK and we will try and help you. Our number is 0870 1624955.