Tesloop, a Tesla-only intercity shuttle service for Southern California commuters, has reached another milestone with its Model S 90D. In a recent announcement, the company revealed that their Model S, dubbed eHawk, has passed the 400,000-mile mark, making it as one of the highest mileage Teslas in the world today.

eHawk entered service on July 2015, driving from city to city in Southern California and Nevada. By February 2016, the Model S 90D had logged its first 100,000 miles, and by August that year, the full-sized family sedan passed the 200,000-mile mark. In a recent blog post, Tesloop stated that roughly 90% of eHawk’s trips were driven using Autopilot, with Pilots (as the company refers to its drivers) only taking over active driving duties when needed. Tesloop’s Model S 90D currently travels an average of 17,000 miles per month. On the company’s recent post, Haydn Sonnad, Tesloop’s founder, expressed his optimism for the coming years.

“Vehicle connectivity is about to transform the car ownership and user experience. We are close to the point where increasingly sophisticated autonomous driving features and deep connectivity are coupled with electric drivetrains that last hundreds of thousands of miles, a whole new approach to mobility can be offered, that will transform the economics of car ownership and usage, while offering a greatly superior customer experience,” he said.

Over the past 3 years and through 400,000 miles on the road, eHawk has accumulated roughly $19,000 worth of maintenance costs, equating to about $0.05 per miles. This cost is broken down to $6,700 for general vehicle repairs and $12,200 for regularly scheduled maintenance. According to the company’s estimates, a Lincoln Town Car or a Mercedes-Benz GLS class would have accumulated maintenance costs of $88,500 ($0.22/mile) and $98,900 ($0.25/mile), respectively, had the vehicles been driven for 400,000 miles.

The Model S 90D’s high voltage (HV) battery unit was replaced twice under warranty since July 2015. The first battery HV battery replacement was at 194,000 miles, while the second was at 324,000 miles. Average battery degradation over the vehicle’s first 194,000 miles was around 6% with multiple Supercharger stops every day. Between 194,000 – 324,000 miles, the HV battery degradation was estimated at around 22%. According to Tesloop, this was likely due to the company’s practice of constantly charging eHawk to 95-100%, instead of Tesla’s recommended 90-95%. On its blog post, Tesloop shared Tesla’s reminder to the company after its first HV battery replacement.

“Found internal imbalance in HV battery due to consistent supercharging to 100% from a low state of charge (SOC) without any rest periods in between. HV battery has been approved to be replaced. Also recommend that customer does not Supercharge on a regular basis and does not charge to 100% on a regular basis. We also recommend that the customer use scheduled charging to start charge 3 hours after end of drive at low SOC.”

Apart from its HV battery, Tesloop’s Model S 90D also had its front drive unit replaced under warranty at 36,000 miles. No issues with the vehicle’s drive units have emerged since. The Tesla-exclusive shuttle service also opted to upgrade the rear seating of eHawk to the executive seat option for maximum passenger comfort. According to the company, the seats have held up well over the thousands of passengers the electric car has transported over the years.

Considering the endurance showcased by its Model S 90D, Tesloop estimates that eHawk should be able to last another 600,000 miles over the next five years. If the vehicle achieves this, it would be the first Tesla Model S to reach the 1 million-mile mark.

Tesloop currently operates a fleet of Model S and Model X vehicles. One of its Model X, a 90D named Rex, also achieved its own milestone last month, after it hit 300,000 miles on the road since being deployed. When the all-electric SUV reached the 300,000-mile mark, its battery degradation was estimated at roughly 10%. Since achieving its milestone, however, Tesla has changed the vehicle’s rear drive unit.