If the value of the tires is more than $750 (in aggregate), then it is a felony (Malicious Mischief 1 or 2). Otherwise, it is a gross misdemeanor (Malicious Mischief 3).
I agree with what Mr. Brandli wrote. By way of further answer to your question:
1) What are the chances of your getting charged with a crime?
Under the facts that you've described, chances are very high that a charge will follow. In addition to the breakdown of charges Mr. Brandli gave, be advised that because the complaining witness (also called "the victim") and you shared a dating relationship at one point, your charge will have the added tag of "domestic violence." This includes certain other penalties upon conviction, such as loss of a right to own or possess firearms.
2) What are the penalties for a first-time offender?
If the value is less than $50, the crime would be a misdemeanor, punishable by 0 to 90 days in jail and/or a $1000 fine.
If the value is $50 to $750, then the crime would be a gross misdemeanor, punishable by 0 to 365 days in jail and/or a $5000 fine.
if the value is more, then you are facing a felony (if 2nd degree - class C felony, up to 5 years in jail with a standard sentencing range (assuming you have no criminal history) of 0-60 days; if 1st degree - class B felony, up to 10 years with a standard sentencing range of 0-90 days.
3) How much proof is necessary to convict?
That is always the prime question in every case. The most correct, but also most frustrating answer, is "exactly enough evidence to convince every member of the jury that you committed the crime, beyond a reasonable doubt."
Your case presents several interesting issues, and if you are charged, you should contact your attorney immediately. Also, if you are contacted by the police on this issue, you should politely decline to answer any questions or talk about what happened and contact your attorney immediately.