I don't think that driving at upto 20% slower than the posted speed limit is wrong. More than that and you can actually become a hazzard to others. Most speedos in cars have a tolerance of upto 10% built into them and by law it has to be a positive error. So driving at 70 by the speedo, you might only be doing 63. Granted this is at the extreme end of things, but I have had first hand experience of this.
I bought a car last year (identical MX6 to my own, except it was a 2lt V6 with 159bhp rather than a 2.5lt V6 with 198bhp), on the long drive home from Stoke on Trent via Melton Mowbray then down to North Bucks... I noticed the speed seemed to be higher than my other car, which was quicker and more powerful. Upon checking with a friends GPS locator, we found a 13% error in the speedo. I later discovered a previous owner had fitted smaller tyres than stock, which gave a 5% error. But that still left an 8% error in indicated speed. The error can also increase or decrease depending on the speed you are doing... It's not consistent.
The MX6 I still own has been extensively modified, I've fited new gauges and wheels/tyres, going from a 205/60/15 to a 235/40/17. Inspite of the problems this could cause with speed readings, when testing it one weekend with gps systems, we found it was under reading by just 1mph. I had in fact made it more accurate with the mods.
On a funny note, I got clocked by a mobile speed camera the day after getting the car. I was testing to see if the speed limiter was still in place (jap import) and it was... kicked in at 112mph (speedo said 125). Road was clear and in mid summer, weather was perfect. As I rounded a long sweeping bend, I saw the camera van on the bridge over the road and looked at my speedo. It read around 99mph.
When the NIP arrived, it said 87mph. However as the DVLA had screwed up when the car was imported and they registered it as a Mazda 626 rather than an MX6.. They couldn't pursue it. I had notified the DVLA of the error and even when I sold the car, they still hadn't rectified it. Should have kept it, no speeding tickets ever again.