I assume you mean the common zonal type geraniums, Pelargoniums? You don't say where you are located or how cold your winter may get, but these plants can take light frosts with relative ease. Hard frosts or extended periods of freezing temperatures are another matter and require some sort of winter protection.

You can force geraniums into dormancy by cutting them back to 4-5" of stem (no leaves) and removing them from the soil. Gently shake as much soil from the roots as possible and place in brown paper grocery bags. Leaving the bags open for air circulation, these can be stored over winter in a cool, dark area that remains at 45-50F - a garage is a good bet. You may need to spritz them occasionally to prevent complete dessication or have your house-minder do so while you are gone. Once every 6 weeks or so is sufficient.

Here we are again disagreeing about the temperature of a garage in winter in a zone that does have winter temperatures that approach the frezing mark.
Open the garage door to let the car out and what temperature is outside comes inside the garage.
The only thing about a garage is it might not be as windy.

Any garage that is open to winter's cold is not a place to store a geranium....or any plant for that matter.
The geranium can renew itself when the sun returns in February/March.
But, it must not be touched by frost.

Into a cool area of the basement...the crawl space might...might be above the temperature that would harm it.
A coldcellar or fruit cellar or a cool area of the basement that is not heated is ideal.
There it spends the winter...alone, untouched by water, by light, or heat.
It can be left on a shelf, in its pot or not, or in a paper bag that is left open to allow moisture to escape.
It can be hung upside down (that's the easy way to hang) and left as is.

In February, bring it out, cut it back to about 4 - 6 inches, given fresh potting soil in a clean pot, given material that will prevent soil from clogging up the drainage holes, watered and given a sunny window.
Soon new leaves form. Its at this time the plant is given some fertilizer--1/4 rate -- until the plant is well leafed.
That takes about 3 - 4 weeks.
Maybe before it goes outside again, it might show a flower bud--in any way, it will flower again as good as ever.

Every day or so turn the plant 1/4 turn to allow full sun measure to the whole plant.

Allow the plant to dry down somewhat...don't let it dry out, between waterings and always water to drainage.
Dump the excess.

But, it must never be touched by frost.

The other way is to let the plant live on as it was outside by placing into a pot and given a sunny window until the flowering stops.
AT this time, it can go into the coolness of the basement until February.
Some just continue to treat it like any houseplant. Water when it needs it, give it as much sun as weather permits, and in February, cut it back, re-pot with fresh potting soil and continue to treat it as a coming houseplant.

Any garage attached to a residence, unheated, will NOT be equal to outside temperatures. Anyone who states such has no understanding of radiant heat and conductivity. Most newer garages are also insulated. And it is extremely easy to measure - just place a thermometer against any freestanding surface (not either the interior or exterior walls) and compare to one placed outdoors. A detatched unheated and uninsulated garage is another matter.

That 1000's of gardeners in scores of locations across the country DO winter over tender plants in garages is also a testament to that facility's suitability for this task. It does depend on your location and the plant in question. however. Heck, in my location I even overwinter some tender plants in my carport, which is open on all 4 sides. Because it is out of the wind and out of the elements and contains various objects that provide some shelter, it is a microclimate that remains 5-10 degrees above outside temperatures.

Well, experimentation may be the only way anybody is going to be assured their plants are not going to suffer from being in the garage during a northern winter without giving them protection.

I don't know about where you are GardenGal, but not too many garages I'm aware of are insulated....and why would you insulate a garage for in the first place, the only resident is the family car.

If you wish to insulate the wall that serves the exterior wall of the house, then that is the homeowner's choice but insulating a garage must be for a purpose.
Your assurance that plants can survive well in a lean-to effect of a carport is also dubious if not given protection from the elements.

Try to leave a bottle of water in the garage. If you wish, try leaving the case of Coca Cola out there as well; see if any of them pop their lids.
You can try some Budweiser at the same time.
Out of the wind....yes, to be sure, a garage is generally a place that one could stand and be out of the wind.
Unless, of course, the wind's direction is aimed straight at you.

Now as for me...us, we finally get around to moving things from the garage fridge to indoors to prevent damaging the compressor unit trying to compete with temperatures below what it normally runs at.

Now before you start to suggest what you have done in the past, the temperatures we are discussing are those that fall below the freezing mark for extended times.
Saying the temperature outside the garage is 30º...and a plant is quite safe inside the garage is, I'm sure you are aware, is not quite the argument we are discussing.
With ambient temperatures of 10º - 15º...or below, are the temperatures that must be looked at for how we protect what's inside the garage.

Water does not have to be at the freezing mark to freeze...it can freeze at temperatures above that mark if certain factors are also present.

Hi there - just saw this posting as I was looking for info on caring for my geraniums during the last few days of freezing crazy temperatures in New Jersey! Needless to say I'm scrambling to cover everything I've planted. So far so good though.
However, in reference to the garage part of this conversation, I have learned my lesson. Nothing survived the winter in my attached garage where the temps outside regularly hover around zero - 10 degrees F. And this winter was not nearly as bad as ones we've had in the past. I had a couple canna lillies,some elephant ears and a beautiful dahlia die in the cold of my garage. So there is 1 arbitrary result we can consider amongst others as we research the "sanctuary" of the garage. :-)