How about this:

Dear ______,
I had my vehicle serviced at your shop on _____. Soon after reclaiming my vehicle, I noticed the spare tire was missing. After several attempts to contact the service manager and not getting my phone calls returned, I finally was able to catch the SM and was told the shop did not have my tire.

Given that the tire was there when I took the vehicle in for service and was gone when I received the vehicle after service, I can only conclude the tire was removed at the service center and not replaced.

I am requesting that the tire be returned with no more delay. I will be sending a copy of all pertinent documents to the Director of Customer Service, the Attorney General and the Better Business Bureau.

Yours, _______

Don't say anything about the month in between or anything else that could give them a way to deny responsibility. You want to make them think that giving you a new tire will be cheaper and easier than defending your charges.

Stop playing nice and get to the point. They don't care about power outages, typos on the bill or anything else. Just tell them what you want and tell them what you are going to do if you don't get it. I would also make plans to take the dealership to small claims court. At the least you could get money to replace the tire.