Having ridden a bmx bike for 6 years now, distance isn't really it's forte. So I asked my grandpa for one of his really old bikes so I could fix it up and use it for distance rides.
Now, having ridden a bmx for so long, I'm a big fan of just having a single gear and no brakes. Too many gears becomes a hassle for me, so on my grandpas bike, I've removed the front brakes (left the back), and all the gear systems and am looking to change the sprocket and attach to the back hub a freewheel.

What I'm asking is what's a good all purpose gear ratio? On my bmx I ride 30 - 12 and it seems alright but I'm open to other combinations. Also, I'd really appreciate any other advice on rebuilding this bad boy just cause I'm new to this sort of thing.

tl;dr What's a good gear ratio for casual distance riding. Would also like other bike info.