Depends on what the original chain is. You need to post more information if you want helpful feedback. 38x18 would be a slightly steep gear for an 'all purpose' single speed bike (depending upon your level of fitness). It would be good for double track and getting around the block. Try mating a 34t chainring to your 18t cog for an intro to ss mountain biking. As far as the chain goes... 8spd chains can frequently be used with 3/32 singlespeed set ups. Try to avoid using chainrings and cogs designed for multi-speed drivetrains.
a 38/16 would be hard to push on anything but a flat trail and a 38/14 would be even harder.
if you are riding trails, a 2 to 1 ratio is what a lot of people preach. if you are on a 26 inch bike, 38/19 would get you to the 2 to 1. myself, i like an easier gear to pedal, so when i had a 26 inch ss, i ran a 32/17. on a 29, you basically add 2 teeth to the rear sprocket to get the same gearing, 32/19. on my 29, i usually run a 32/20 or 32/21 on all but the flatest of trails. i prefer to be able to pick my speed up quickly out of corners and up hills rather than grinding it out.
Try the search function located directly below the advertisement towards the top of your screen. I would say that out of all the topics that get covered in the SS forum, this one has popped up at least once a week for the past 5 or 6 months. Just search "SS gear ratio" and I bet you will come up with a lot of information.
Good luck figuring out your ratio.
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