Adis

I'm currently using a 40/16 ratio and I like going fast on flat. What is the best gear ratio for going fast on flat? My crank arm is 170mm.

I live mostly on flat.

Mikefule

It will vary with age, weight, fitness, pedalling style, the design and fit of the bike and a number of other variables. Assuming you have 700c wheels you're on about 69 gear inches. That's roughly what I ride for general purpose riding, commuting and some easy cross country on a road bike with drops. I'm 57 years old and not as fit as I was, so if you're after speed on the flat as a priority, you can probably increase your ratio a bit from where you are now.

ofajen

One way to think about it... with no wind on a flat route, what is “fast” and what cadence do you want to be doing at that speed?

For example, I run 42/16 on 700x32 which is about 71 gear inches. I will be doing about 17 mph at 80 rpm, 19 mph at 90 rpm and 25 mph at 120 rpm.

This might seem a bit slow using drop bars on the flats, but with the hills around here, it’s about the most I want to do.

I’m trying out touring bars to increase wind drag at higher speeds. That seems to give just the right level of effort on the flats without having to make the hills too painful. And the touring bars are great for long, standing climbs.

Otto

mack_turtle

too subjective. if you find that you're spinning out a lot, add two teeth to your chainring. experiment until you find what works for you. you'll notice that it will be harder to accelerate. if you ride in an urban area where you need to stop and start often, a bigger gear might be slower, or it could cause more fatigue. that will be slower in the long run.
do you want to feel faster, reaching higher speeds for the thrill of it, or do you want to get from one place to another faster? there are too many variables to give you an answer and anyone who can prescribe a specific number for you is full of hot air, but those are the kinds of questions you need to ask yourself to figure it out.

Rolla

This one:

Altimis

^
^
This one probably need vehicle drafting though. You will dies, fast, alone.

the sci guy 

I ride 48/17, and can easily ride anywhere from 14-22mph depending on my cadence. And I’m pretty small, so someone with good legs and muscles could probably really fly.

Jmpierce

I ride 46/16, and rode a thirty mile all flat route today. My average speed was 13.7, high speed 18.5 and I'm 65, out of shape 265 lbs. This also seems to work good in city traffic and on hills.

motobeCarnage 

I rode 46/16 (or 17, cant remember) in a very hilly city for years before buying a road bike. I assumed it would make me faster in my favorite strava segments (no that's not the reason I bought the bike) but I was surprised that while I did make a couple PRs they were mere seconds over my previous times. I was forced to do more work which offset any benefit of a larger gear ratio in terms of speed. BUT I've gotten faster since then because of the added "resistance" I guess you could say.

Point is, unless you're already strong in a larger ratio then you're not suddenly going to get much faster just by increasing your front ring for example. I think you should, 40 seems tiny to me, but don't expect an immediate speed boost.