I usually use 52/15~52/19 on the flats. I think the problem with using a max gear ratio like 53/12 (is that your max) is that you've no more room to accelerate other than to put more force on the pedals to get the cadence up. It's much easier to spool up your cadence when your cadence is already high and you're changing to higher gear ratios as you gain speed. If you stay in that gear, most other cyclists will be passing you very quickly as you fatigued out your legs. It's like you're driving your car in 5th gear when you're driving at slow speeds-->lugging the engine...and similarly it'll be bad for your knees as others have said.