When I put up my garage addition, the city inspector said that I had to put a smoke detector in each bedroom, in addition to the two I already had in the hallways. This I gladly did. My question is, how come the city code does not require smoke detectors in the garage(s). Vehicles can and do self combust, and garages are often filled with flammable stuff. In the interest of safety, shouldn't garages have these detectors? I have them in mine. When I do some welding, I have to put the detector in the laundry room to keep it from going off. A small inconvenience.

Your cars can be replaced, the ones in the bedroom will keep you from dying in your sleep. There should be one in almost everyroom. To catch things, You also should consider a cabon monixde alarm also.

And I would make sure they are wired ( which should be most code) and also monitored for response when you are not there, and esp for the cabon monixide alarm.

I agree that cars don't matter when lives are at stake. The point I am making is that a garage in my opinion is a very likely place for a fire to start, and being alerted before it makes its way into the house would be a good thing. I am saying that the codes shoud be stricter to ALSO require smoke detectors in garages.

You won't get any argument from me, that they need to be stricter, and in some places they are, we have fairly strict here ( learned that since I have been working for a fire alarm company part time this last month)
Here not only do there have to be alarms in each bedroom, upstairs ( top of stairs, kitchen but they have to be wired systems, with batter backup, they can not just the walmart type. They don't have to be monitored but they do have to be wired into electric.

Good point. I have 2, one in the upstairs hallway and one in the garage. No codes enforced here.

Depending where you live, may also need one for each floor.
Carbon monoxide is excellent idea.
If you need to install additional detector, difficult to get wires between detectors and need to be interconnected(when one goes sounds,they all sound)
Google "Wireless Interconnected Smoke", may be solution.

The reason a smoke detector is not required in an attached garage, is the typical building code requires an attached garage to be separated from the living quarters with a fire rated construction method, usually fire rated drywall.

Having a SD in the garage may cause nuisance alarms due to fumes, dust, humidity, etc.

All states subscribe to the National Fire Protection Association codes, but any may amend the codes, so always best to check with your local building or fire official.

The required detectors can be viewed in Section 11.5.1 of the NFPA 72 Fire Alarm Code :

NFPA 72: National Fire Alarm Code®

There is no problem installing a detector in a garage, I have done so many times, with the customer made to understand the risks of nuisance alarms.

Having a CO detector also is a good idea, not just in a garage but also in areas with combustion type heating systems.

How about using a heat detector? I have a central alarm system (ADT) and upon the advice of the installer put a heat detector (not a smoke detector) in the attached garage, near the door to the house. We also placed a heat detector in the kitchen to cut down on false alarms. We have smoke detectors throughout the rest of the house.