Depends on what the speed limit is. I usually drive about 10% over the limit but it really depends on the weather, time of day, how much traffic is on the road, etc. I've cruised along at 100+ mph very early in the morning when the weather was perfect and there wasn't any traffic. I've also crawled along stop and go stuck in traffic when commuting.
+10% is the margin of error that is the approximate tolerance that is built into most speedometers so they will always read a higher speed. This is to compensate for tire wear, wrong tire size, incorrect tire pressure, etc., or to never read a lower speed than the vehicle is actually going. If this was not the case (of reading a slightly lower speed) then there would be lots of class action lawsuits for speeding tickets. I use the +10% too because of this. Lots of adventure and touring bike riders have GPS and they are always complaining their speedometers read a lower speed than their GPS, and that's because of the of the speedometers built in margin of error. If you have a GPS with a speedometer check it against your speedometer. GPS speedometers are also not always perfect but it may give you an idea. Or set the cruise on 60 mph and stopwatch check it for 60 seconds between mile markers. Or 51.4/52.9 sec @ 70 mph this way if you think you were going over 75 mph when you passed that cop you may not have been and that's why he never went after you.
Checking your speedometer's accuracy is a fun thing to do on that boring long haul drive
