Custom tune is where you can fine tune your truck . hand helds will do just so much but a custom tune will do more.Its expensive but you can tune several vehicals where as a hand held will do only one. Heres a web site to give you more info.http://www.hptuners.com/
HPTuners.com >> Performance At Your Fingertips
HPTuners.com >> Performance At Your Fingertips