I bought two of this for my cars. In one i use the 58t with a 23 pinion gear and the other i use 55t with 25 pinion gear and both go very well. Actually the stock tt-01 gears meaning 61t and 19 pinion gear are very well on acceleration and first power but the top speed its not too fast. If you use a bigger than 19t pinion gear and a smaller spur gear that 61t (stock) you will increase the top speed of your car and you will descrease the accelaration. Pitty they dont exist other than this spur gears 55 and 58 so we can make more combinations with pinion gears. I have all pinion gears from 19 to 25 and unfortunately only 3 spur gears 55 58 and 61. But even with those you can make excellent combinations for your car depending of the use you need it. When assemble this spur gear and pinion gears make sure not to be too tight because ur car will run stifer and will destroy your gears. One simple and costless method to make sure that the gears fit is after you place them take a piece of A4 paper and try to pass it through the gears. If the piece of paper can pass between the gears then you are ok but if the piece of paper doesnt passes then you probably will have problem. Overall nice product from 3racing and we are looking forward for more great gears. Thank you 3racing and Rcmart for great product and lowest prices ever. You are my favorites!