hii having sum problems with my nrg engine in the zip

hii every1 i could do with sum help i think ...well lets start i have put a nrg engine in my little zip and it is running owkay tbh but seems to over rev fairly easily i was thinking maybi it has something to do with the gearing as it has a ten inch wheel on the nrg engine wich is suposed to have a 13 inch wheel does ani1 think the gearing could affect this as its ment for a bigger wheeel ani help would b gr8 chears lads