With a garage vent it can get very hot inside the garage during the summer months. Installing a couple of vents to encourage air flow will lower the temperature in the garage as well as help ventilate the room from toxic fumes when the doors are closed.
The More, the Better
When purchasing your garage vents, the more the better. Each vent will allow new air in while evacuating the hotter, toxic air. If you are only purchasing two vents, then get the largest vents you can.
Install Garage Vents on Two Exterior Walls
When installing the garage vents, make sure they go onto at least two of the walls. The best configuration would be directly across from each other, but this is not always possible. If you can not put a vent directly across from the first, install one on a diagonal.
Seal with Caulking
As you are installing the garage vent, you should run a bead of caulking around the vent before placing in the hole. This will help seal the edges of the garage vent from water and leakage.
Install a fan in the vent if you have a large garage with high ceilings, or if you can not put multiple vents in on an opposite wall.