The Stovebolt Site Forums The Shop Area General Truck Talk Speedometer isn't right, after all

could be a change in drive gear size is called for, especially if you've made a rear-gear change, but you could also just use the Garmin to "calibrate" it and live with it ..... might have to do with your wheel and tire size compared to what was original equipment ..... I'd also recommend you pull the speedo cable and clean the inner and outer well with a none-oily solvent like BrakeKleen, then use a dry graphite on it, the WD40 [which didn't clean the housing out] will gum it up over time mixed with dust, as will any oil

Bill