Well, means you are at 75/24=3.125
you could go with a 96/30=3.20 or a 96/31=3.096 or to get right on the money it would be a 100/32=3.125

To figure that out all you have to do is to take the known ratio, 3.125 and pick a 64 pitch spur gear that you want to run and divide it, so for the 100 tooth, it would be 100/3.125= new 64 pitched pinion gear.

The better way to look at gearing a car is to figure out the roll out you need. That way when the tires wear you can still be geared correctly.

Wow is it just me or does 96/31 seam really big? You must race with some REALLY small tires or have just a huge open track. (I think you might be a little bit over geared)