New tires change how your truck handles everything from gravel roads to weekend mud. They are expensive, but waiting for a holiday sale at a local shop can shave a few hundred off that $1200 price tag.
You want to negotiate the mounting and balancing fees before you commit. Once you have the final out-the-door price, you can use a financing window to break that cost into manageable payments.
Remember that the 18-month window is a deadline, not a gift. If you still have a balance when the time is up, you will start paying interest on whatever is left over.