If your speedometer reads low or high so does your odometer. It is easy to calibrate and determine just how far out it is and nowadays most of us also have a GPS which will be very accurate. Throw in a Scan Gauge (you DO have Scan Gauge, right?), :S for good measure and you will have more accuracy then you will know what to do with. :B
We consistently, over just about any terrain, get 10 or better on the highway doing 60 and average 9.5+ for the overall trips in our 24' C with the 6.8L V-10. :C