Began working for Joe Gibbs' NASCAR team in 1998, where he served as Chief Dynamometer Engine Specialist.
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He also signed with Joe Gibbs Racing as a development driver under a partnership with former NFL player Reggie White.
Zipadelli is most notable for being the crew chief of the #20 The Home Depot car for Joe Gibbs Racing drivers Tony Stewart and Joey Logano from 1999 to 2011.
Denny Hamlin, who replaced Leffler in the No. 11 FedEx Toyota in late 2005 had his car repainted to resemble Leffler's variation.
Shortly after being replaced at Haas, Leffler signed a deal to re-join Joe Gibbs Racing for 2005, taking over a newly created Cup team sponsored by FedEx.
In 2005, Addington was named the crew chief for Joe Gibbs Racing's #18 car driven by Bobby Labonte, replacing Mike "Fatback" McSwain.