Infinite mufflers, batteries, and tires
After you have to put down Clements, leave Washington D.C, but stop immediately afterwards. If you stopped fast enough after leaving, a "Town" tab will appear. Tap "Auto Shop" and notice that the mufflers, batteries, and tires are free. Tap on each of them and continually tap "Buy". You should be able to purchase as many of each item as desired at no cost.