OMAHA, Neb. (AP) — Billionaire Warren Buffett decided it was time to upgrade his 2006 Cadillac DTS after General Motors CEO Mary Barra pointed out how many improvements had been made since then.

Buffett wrote a letter to Barra praising his car-buying experience at Huber Cadillac in Omaha. GM posted the letter and a story from the saleswoman who handed the deal online at its blog, http://fastlane.gm.com , this week.