I am trying to push myself a little as I gain fitness and experience. Have been riding almost entirely on the center chain ring. This past week I started using the big chain ring. Made me feel like such a whimp! One thing for sure, it's faster. I just could not stay in the bigger gears long at all. I did show improvement, however, so I will stay at it.

Question is, what, in terms of gear inches, do most of you run on flat terrain in the course of your commute?

I commute on a fixed gear and as I have gotten fitter I have actually dropped my GI from 69 down to my current 60. The key is to spin faster rather than mash harder gears at lower RPM. I can hit the same top speed with the lower GI while putting less pressure on the knees and taking hills faster for an overall faster speed. I'd say don't worry about pushing the big gears until you can average 100 rpm or better in the smaller ones.

I giver when I'm commuting and I average from the mid-70s to the low 80s on flat terrain with no net wind. The key is, having the gears available that you need the most. So, if your current powertrain doesn't give you a lot of choices for the cadence you like to run, consider changing the cassette, or one or more of the chainrings, to get you a selection of 3 or more gears near your sweet spot for flat terrain.

my primary commuter is set up as a 1x9, have also toured with this bike - lots of hills here. overtime i've carefully observed which ratios i use the most. those ratios are, around 37 gear in and 65 gear in, plus a middle step of around 49. this gear range is almost identical to that of a three speed IGH.

I rode a 74" fixie in the hills of Vermont and it was self-flagellation. Torture.
In Kentucky, a few hills here too, 67 works out OK on my single speed commuter bike

That's pretty close to what I've been using. Makes me feel a little better! On the high end I've been topping out in the upper 50's. I'm trying to get that into the mid 60's for a longer period of time. I can get there, I just can't stay there. I try to spin at 80 rpm and change gears when I drop much below 70.

With a 3 x 8 set up, I really have plenty of gears. I'm just trying to increase my speed and fitness. It is amazing how quickly you start to notice improvement with some increased effort. By the end of the week, I could already tell a difference in how long I could stay in the larger gears. Nowhere near what I want, but definately a difference.