What front cog on road bike when on flat road?

It depends on several things including your current fitness, wind conditions, and road conditions but in general on flat roads unless I'm pushing a huge headwind I'll be in the big chainring (BTW, cogs are in the rear, we don't call them cogs when they're up front).

Basically you pick gears that feel comfortable and that you can spin relatively smoothly without totally bogging down and grinding but also avoiding gearing that's too small so you're 'spinning out' or bouncing around trying to pedal crazy fast. On flat roads that often translates to the big ring up front and a mid range to smaller cog in the rear but use whatever works for your current fitness and conditions.

-Dave

awesome, thanks!

It also depends on your speed. if you are going like 15 mph or even below you will likely spend most on the time on the small chainring. but if you go 20+ you will be on the big chainring.

you mostly want to stay between 85 and 100 RPM (of course faster when sprinting and slower when climbing) so the gear must be chosen accordingly.

I go to the big ring at about 16 mph but my big is a 50 teeth while most are 53

I do most of my flat rides all in the larger chain ring, shifting to the smaller one at stops.

The point is well made by daveryanwyoming. It's about finding a gear ratio that is comfortable to pedal. Some of those gear ratios may only be available by using the big ring but quite often similar gear ratios are found using the small ring, depending of course on what cassette you've got on your bike and what chainrings you've got. You should always think about what gear ratio you need rather than should you be in the big ring or not.

If you are riding fast, you should be in your big ring. If you are riding slow, you should be in your small ring.

If you find that is not true, you might want to change your cassette or rings to something more appropriate.

Every reply I've seen has good info! I would like to add that as rule I use my small chainring almost excluslively during the first 1,000 miles of each season unless I am in exceptional condition and have been working on my "spin" during the winter.