As others have said, the ratios that you should be using are the ones most comfortable to you. This will vary with speed, incline, surface type, riding position, etc. Pick a trail/ride where you have a mostly flat or a slight incline. Get up into the gear that you find comfortable. Can you go into a higher gear and maintain a similar cadence? If so, this will increase your speed. A computer can be helpful, so you can see your speed and/or cadence. If you can go into a higher gear and maintain your cadence, that's good. Can you go into an even higher gear? At some point, you'll find a gear where you can just maintain your speed (and your cadence will slow). Even higher, and you won't be able to maintain your speed OR your cadence. I typically try to keep my cadence in a range that is comfortable to me, and I allow my speed to vary based on that. Going up hills, I'll gear down, and slow down, but I'll continue spinning at my cadence. I don't measure my cadence...it's just whatever feels right.