There is a chance it will not fit. Lipos are more square than a NiMh battery. I am not familar with your car but if it is molded for the round cells of NiMh your choice of lipos that fit will be limited. Also lipos vary quite a bit in size. The mah and C rating all effects the size of the lipos. I have also noticed the less expensive lipos also appear to be on the larger size of similar higher end batteries. It helps to know what size will fit in your car and make sure the battery is the right dimensions, almost all lipos list the LxWxH.
You know the guy that beat you at the last race? It was not because of his car, or motor, or battery, it is because he was a better driver. Now go practice so you can beat him.
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