the internal ratio is 2.5; that is: (diff gear) divided by (small bevel gear)=2.5
your spur /pinion ratio is : (Spur-104 teeth) divided by (pinion, say 36 teeth)=2.88
then: 2.5X2.88= 7.22
ET VOILĄ !
for a 17.5 I would start with a FDR of 5.2ish for a medium sized track... so :
5.2 divided by 2.5= 2.08
104 divided by 2.08= 50, so you are looking for a pinion of 50 teeth... probably a bit more or less, of course, it depens on a lot of things.
Remember that a SPX requires a higher FDR ( smaller pinion) than regular escs when it is used in program "turbo" 7 or 8.
Ask at your track, and don't let anything become too hot.
have fun, the tc4 is a really good car.