Originally Posted by carleton

There is no exact science about it. It's a mix of factors:

- Crank length
- Rider weight
- Terrain
- Rider strength
- etc...

OP complained about 48/15 feeling big. I think it's OK in perfect conditions, but I think 48/18 provides a great gear for getting up hills and spinning fast (120-130) won't have you going at break-neck speeds (25-27mph).

I think a 48/18 is a very reasonable gear for a beginner which you are not