I've noticed that as I increase speed my speedometer becomes more inaccurate.

I've used an app to compare. At a about 35 to 40 mph, it begins to be off by a few mph. Over that it is but 5 mph.

 

That's because the error is a percentage of your speed. So if it's off by, say, 10%, it would indicate 27MPH (or 33MPH) at an actual 30MPH, a difference of 3MPH. But at an actual speed of 60MPH, it would read 54MPH or 66MPH, for a difference of 6MPH.

 

Using oversized tires will cause such an error.

Some OBD readers have the capability of correcting this. I have a Vident iLink 400 and it apparently can do this but I have not attempted it yet.

 

Thank you.