As was stated before... unless you changed your tires the ODO is still accurate. Thus how exactly are they screwing anyone over, at all. They're not getting any extra 'services' out of you, they're not skewing the gas millage your getting as it's based off the ODO, not speed... Where's the hurt?
Such a bs thing to worry about. If you trust stock gauges to start with, unless it's like a Lotus.... you're in for a world of hurt. You think a few mph off on the speedo is bad, how about that "15 PSI" gauge on the supercharged Colbalt LOL LOL LOL LOL. My 3000GT's oil presure gauge was at 'normal' on the one tick... stock gauges are inaccurate. If you hate them, get an autometer. Simple.