You can't really cover that range with one spur only. The adjustment of the motor is not that much and some ratios just jump too much changing pinions. You'll find soon that another spur (or a few) are needed. In actual fact, changing spurs allows a finer adjustment than changing pinions.
For our track for instance (large outdoor asphalt) we are in the low 3 to 3.5 FDR.
Here's Yokomo's gear chart that shows clearly you're not going to cover that range with a single spur
http://www.teamyokomo.com/download/m...7_gear_48p.pdf