I'd have thought you'd want to have an FDR of around 6.5-8 depending on your track layout and the characteristics of the motor itself.
The final drive ratio allows you to compare your gearing with other drivers like for like even though the internal gearing on each car is slightly different. This is much more accurate than just comparing pinions and spurs with other people. Take into account that not all motors have the same power characteristics and you may find that someone with a Speed Passion might gear totally differently from someone with an LRP for example. Add speedo boost and turbo into the mix and it can be quite a minefield.
I've got a feeling the biggest pinion you will manage with that spur is approx 26 though (FDR 8.24). Just to put things into perspective, that's roughly where you would gear a Speed Passion 7.5 or 8.5. With a 10.5, you're probably going to need to get a smaller spur gear (71 or 76) to get the ratio you need.
You can work out the Final Drive Ratio as follows:
Spur / Pinion X Internal ratio (2.58 for the SV2)
71 / 24 X 2.58 = FDR 7.63
I've not run a 10.5 myself, but looked at a few set ups on Petit for gearings. By all means put the biggest pinion on your have and see what happens!
HTH
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