Not so safe, but it all depends where you live in the city and all that. Downtown / midtown is the best, anything else is hit or miss. Areas on Moross St. by Grosse point are just as nice as anywhere else. Also, even though Detroit is arguably the most dangerous city in the country, suburbs are very safe.
Air conditioning tonnage has nothing to do with weight. A ton, as used in the HVAC field, is a term that describes how much heat the AC unit can remove from a home in one hour. The measurement for heat is the British thermal unit (BTU). One ton of air conditioning can remove 12,000 BTUs of air per hour. A four ton unit can move 48, 000 BTUs and so on. Therefore, the more tonnage an AC unit is rated at, the more air it can cool.
One of the first things I do is to apply my rule of thumb. Understand here that the standard practice among HVAC contractors sizing air conditioners for new homes is to use a rule of thumb. It's often in the neighborhood of 1 ton of air conditioning capacity for each 600 square feet of conditioned floor area, usually abbreviated 600 sf/ton. So, when I get a Manual J, I find the capacity and the conditioned floor area and calculate that number. I find out what the square feet per ton is for the house. If this is supposed to be an ENERGY STAR home, it shouldn't be 600 sf/ton. It should be more like 1000 sf/ton or higher. That's my rule of thumb.