The correct answer is Creakyknees's answer.
Or, if you want to know before you make changes, measure the circumference of your current tire (pumped up, no weight, on bike - mark where valve is on a floor with a line type pattern like a linoleum or tile floor, roll wheel along line until valve points down again, measure in mm if possible).
Now ChainringTeeth/CogTeeth*circumference = roll out. Chainring/Cog gives you the multiplication ratio the actual gear gives you (45x12 to use YMCA's example). A 45x12 is 3.75, i.e. the cog turns 3.75 times for every turn of the chainring (or crankarm).
Remember that usable ratio relies on tire circumference. One very good rider in CT, many years ago, got DQed in the state TT. He won by a couple minutes but he'd fitted an oversize tire on his rear wheel. He (wrongly) assumed that any wheel/tire would work with a 53x15 for the limit, but when he went to rollout his rear wheel consistently failed by probably half a centimeter (or some minor thing like that). He was duly DQed.
What was frustrating about his experience was that he didn't need that - he was so good that he started carrying a spare tire in the TT because he could have a flat, stop, change it (tubular), keep going, and still finish with a reasonable expectation of winning the TT by a minute or so. He flatted one year and didn't finish, and based on his prior multiple minute winning margin he should have been able to change the flat and still win.
Remember that the gear limit really has to do with your tire's circumference. The gear is simply an easy way to refer to a given gear in a language others understand.
"Dude, what's your biggest gear?"
"7.465 m"
"What?"
"Well, it's 7.452 if I use that other wheel."
"Huh? What are you smoking?"
vs
"Dude, what's your biggest gear?"
"53x15"
"Oh. I'm running a 49x14"
"Oh."
cdr