Few have driven a Tesla to the point at which the vehicle really starts to show its age. But Tesloop, a shuttle service in Southern California composed of Teslas, was ticking the odometers of its cars well past 300,000 miles with no signs of slowing.

The company’s fleet of seven vehicles—a mix of Model Xs, Model 3s and a Model S—are now among the highest-mileage Teslas in the world. They zip almost daily between Los Angeles, San Diego, and destinations in between. Each of Tesloop’s cars are regularly racking up about 17,000 miles per month (roughly eight times the average for corporate fleet mileage). Many need to fully recharge at least twice each day.

These long days have pushed Tesla’s engineering to the limit, making Tesloop an extreme testbed for the durability of Elon Musk’s cars. Tesloop provided Quartz with five years of maintenance logs, where its vehicles racked up over more than 2.5 million miles, to understand how the electric vehicles (EV) are living up to the promise of cheaper vehicles with unprecedented durability compared to their conventional combustion-engine counterparts.