I didn't make any particular effort to make the drive ratio exactly even. I was looking for something more than 20 counts per revolution. Also, the available gear had to have a 1/8 inch bore - that's the shaft size on the counter.
There were only a few options that fit well into the available space, so I picked one that worked and gave it a shot.
As for durability, I expect the thing to last well over a hundred miles, as there is virtually no load at all on the chain.
But time will tell. It will certainly wear out - the only question is when. With chain at $6.00 per foot it's not a big deal to replace when it starts getting some slack in it.