Given the age of your home, it is likely that the wall between the garage and the house is insulated just as well as the other exterior walls of the house. Therefore, we wouldn't be concerned about garage ventilation causing the house to be colder.
Ventilation should help to reduce the temperature in the garage somewhat, but just one vent may not be enough to notice a difference. Ideally, you should have at least one vent to allow air into the garage and one vent to allow air out of the garage. A rooftop vent should work well to keep air moving out, but there should also be a provision for air to enter. There may already be open ventilated soffits on either side of the garage; these would work well. Unfortunately, ventilation will only help to remove hot air from the garage. There is a better approach which will probably be more effective.
A significant amount of heat that you are feeling is due to the hot roof surface radiating heat down onto you and the contents of the garage and heating everything up. Ventilation will do absolutely nothing to stop radiant heat. Instead, a radiant barrier should be installed. The link below explains more, but this is effectively a sheet of aluminum foil that radiated heat back to the roof and away from the interior of the garage. This can be attached to the roof trusses just below the roof sheathing.
With the radiant barrier in place, you and the contents of the garage (including the floor and walls) will be spared most of the radiant heat transfer and will feel cooler. And since all of the surfaces in the garage will be cooler, the air (which was being warmed by the hot surfaces) will be cooler as well, with no need for ventilation.