Hi Tom,
Do you know what pinion and spur you are running on your car? The higher the gear ratio (smaller pinion or larger spur) the more acceleration and less top speed the car will have. So to get more top speed you need to run a lower gear ratio (larger pinion or smaller spur).
Because we currently run on a surface with so little grip running a larger pinion might also help the car accelerate. The reduced overall acceleration from the large pinion will make the car less prone to break traction and might increase the acceleration as a result. Just be careful of motor temps when increasing the size of the pinion though
Also you might be able to increase the power of your speed controller or transmitter depending on which makes you have.
I hope this helps