There are a few people that have swap in a 32 pitch spur gear on an Evader ST/EXT. IMO not needed.

You are looking to get a metal 48 pitch 87T spur gear? Again IMO, not needed. I've ran Duratrax spur gears on my ST for months before they needed replacing.

Just the slightest bit of movement between the spur and pinion gear...and you're golden . Some people like to use notebook paper as a gauge to get the right amount of space needed between the pinion and spur.

When you decrease tooth count on the spur gear, or you increase tooh count on pinion gear, you will have more top end speed. If you want more low end "umph", lower the tooth count on your pinion gear.

If you indeed have an Evader ST, and running a ball diff, the spur gear is not what you need to worry about . It's the idler and diff gear that will give you trouble with "big" power .

Just a lil' FYI... a 90T spur gear is too big to fit inside the Evader's gear cover. An 88T spur gear is about as big as you're gonna get on the top shaft.