What is optimal gear for level terrain on 21 speed mountain bike? I generally use 2 + 7.?

It depends on your legs, and what speed you want to maintain. Level ground is not why you have gears: its for hills, and for bad terrain.

it also depends on the cadenece (rpm of the crank) that is best for you. a 70-90 cadence is recommended for most beginers and as you get more experienced you should be able to run higher cadences.
Hi- You will want to spin your pedals at approx 80 rpms, I believe. As your conditioning and strength improve you will need a taller gear. I don't know what your speed is, but if you want to cover more ground you may want to use tires that provide traction at higher pressures or other ways to reduce resistance. You may reduce a small amount of friction by finding a gear comparable on the large chainwheel so your chain is not so angled by being all the way over. (rear 7th ring) Something to think about...
17th gear, definitely 17th gear. rubber, rubber on the road.

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