it shouldn't be wrong from just disconnecting and reconnecting it, though there is the remote possibility that some gunk was in the works of the speedo and you worked it loose (if you ever turn the
tranny end of a speedo cable you'll notice it has to turn at an even rate to keep the needle raised) if you had gunk in there it could've put resistance against the cable, but even that's not likely to have made more than 1mph difference or so.
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1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4
tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".)