I have a 2006 Cobalt SS with the 5 speed manual transmission. The car is all stock. It seems lately, that at higher speeds (over 50) that my speedometer reads low. For instance I travel with cruise control at 68 mph, but my buddy in a brand new car is matching me for speed and claims 71-72 mph. I'm sure there is a chart of RPM vs MPH somewhere out there, but I can't find one. Anyone know rpm for say, 50, 55, 60, 65, 70 mph? At least I could use the Tach to make sure I'm not speeding.