Most cases it will be off by 10% or so. At 10mph 1mph off 20mph 2mph off, and it just increases like that.

 

Cool thanks. I think Ill invest in a GPS anyway. It might actually help MPG on certain days I travel to places that I dont no where the hell i am lol.

 

You can get a basic GPS off of Ebay or Craigslist for $50 or so. Also remember since it throws off the Speedometer the Odometer will be off too. So your MPG calculations will not be accurate. A good tip to check how off your Odometer is, check it against mile markers on the highway/freeway.