In a Friday blog post, Elon Musk wrote that Tesla will remove mileage limits on its warranty policy for all Tesla Model S drive units. The warranty, which will still span eight years, won't have a cap on the number of owners for each vehicle.

People who purchased Teslas before today were told that the warranty period for the drive unit expired after eight years or once the car logged over 125,000 miles.

The revised warranty applies to new vehicles and Model S cars that are already on the road.

The change in rules may have something to do with recent reports that the drive unit of the Model S has been defective on a relatively large portion of its cars. Well-respected car reviewer Edmunds recently concluded its long-term test of a Tesla Model S, and it had to have the drive unit replaced three times in 30,000 miles. An unofficial poll on the Tesla Motors Club forum showed that of 98 Tesla users, 31 percent had to have their drive units replaced. (Granted, the poll came from a thread geared toward people looking for “Drive Unit Replacement/Fixes,” so the outcome of the poll may be higher than what Tesla users generally experience, but it's still a surprisingly high number.)

When a user is out of warranty the cost to replace the drive unit can be as high as $15,000. Seeking Alpha suggests that this high cost could mean Telsa's warranty program is underfunded.

“In hindsight, this should have been our policy from the beginning of the Model S program,” Musk wrote in his blog post. “If we truly believe that electric motors are fundamentally more reliable than gasoline engines, with far fewer moving parts and no oily residue or combustion byproducts to gum up the works, then our warranty policy should reflect that.”

The move is somewhat similar to Tesla's voluntary retroactive engineering of the vehicle's battery shield in March of this year, when the company created extra titanium and aluminium armor for the battery and offered the upgrade for free to all Model S owners. Tesla had promised at the time that the extra armor would not raise the car's sticker price.

This time around, the looser warranty terms may very well have an impact on Tesla's bottom line. As Musk warned in his press release, “I must acknowledge that this will have a moderately negative effect on Tesla earnings in the short term, as our warranty reserves will necessarily have to increase above current levels. This is amplified by the fact that we are doing so retroactively, not just for new customers. However, by doing the right thing for Tesla vehicle owners at this early stage of our company, I am confident that it will work out well in the long term.”