I recently stopped using Mint, but from what I remember goals are tied to accounts and you cannot directly categorize transactions. Even if you have a separate checking/savings account for "Car Repairs", when you spend from the account Mint will subtract the amount from your progress, which I don't think is what you want to do.

A better way to do this is to have a budget category for "Car Repairs" and check the "carry remaining balance forward". You can then contribute a fix amount (say, $20) each month. When you don't have an expense, the remaining money carries forward to the next month. With this method you will have to keep track of the monthly contribution yourself.

I recently stopped using Mint, but from what I remember goals are tied to accounts and you cannot directly categorize transactions. You will have to have a separate checking/savings account and then pay from that account for Mint to recognize the transaction. Note that when you spend from the account Mint will subtract the amount from your goal progress, which may not be what you want.

An arguably better way to do this is to have a budget category for "Car Repairs" and check the "carry remaining balance forward". You can then contribute a fix amount each month. When you don't have an expense, the remaining money carries forward to the next month. Then, any car repair transactions you can categorize as normal.