Ahh gearing.. I think the only other slot car topic besides runing in motors and tire truing that will cause no end of (good) discussion.
Gearing can help with parity, like you said, the FF motors are very high RPM.
They can also make a car more drivable or reach a higher top speed.
If you google you should be able to find all kinds of info on it.
Like so many things though it really depends on your set up and what you are driving.
If you are using stock tires on Sport track then the standard gearing should be OK on a mildly twisty flat track.
If you have lots of long straights and mildly banked curves you can go for higher speeds.
Swapping out for grippier tires may also make a difference and require different gearing.
I always remind people of a bicycle... the pinion being the crank gear and the crown being the spindle in the back. Big in front, snall in mack: High top speed but it will take a while to get there.
Let us know what you are racing on and maybe we can come up with some better ideas.