Bigger pinion for higher top end speed, but less torque out of corners.
Track size, layout, surface, etc all have a factor in gearing. Is the track pretty large? I was running 96/48 on a small medium sized carpet track (about 96 ft straight I think.)
Also, corner speed is a huge deal in VTA. Your driving line is going to get you faster than any motor/ESC/pinion/car upgrades. If you can run your car without hitting a board with consistent laps, then look at other factors.