I have one side of my flip flop I have not taken advantage of. I'm currently 46x18 and want to try 42x15 as an option to mess around on. I'm obviously swapping out the front chain ring from 46 to 42 to do this. But is this a significant enough difference to have to get/ make a smaller chain? Or will my chain tensioner pick up the difference?

1/8" movement in drop or track end, whatever you're using, per tooth difference. so 7 teeth, you need 7/8" room. i think i would run a smaller chain, but i prefer my rear wheel really close to seat tube.

free/free?

I mean... you could easily try putting the gearing on and then installing the rear wheel with the chain tensioner and see what sort of slack it takes up? From there you can decide if you'll need to take links out. Unless you're trying to avoid purchasing the new ring and cog altogether?

Or, you could keep your big ring and use a 16T on the rear. I just calc'd it with "standard specs"

Wheel size: 27in nominal
Crank length: 170mm
Front: 46T
Rear: 16T

Gives you: 77.6 Gear Inches.

Change that to:
Front: 42T
Rear: 15T

Gives you: 75.6 Gear Inches. Almost the same... ur tensioner would almost certainly pick up that slack.

Edit: Do you mean chain tensioner as in Single Speed derailler style? Or did you mean chain tensioner like... a surly tugnut for fixed? It wouldn't make a difference in anything but skid patches. Maybe not even that... I don't happen to have that chart right in front of me.

here is a chart.
https://software.bareknucklebrigade.c...it.applet.html

Thanks for the info everyone! Much appreciated.
Yes fixed/fixed. The tensioner is a MKS for fixed, pretty much a tugnut.

you mean 46/18 from above? Those ratio calculators confuse the hell out of me.