October 30, 2011

well i have had two different gps in the truck and both showed that my truck mph was off by three mph. i have my custom tune on the truck for the tires im running so im not sure why its off but now im wondering if this would cause my gas mileage to be off a bit. i have to do the math to find out what im running gas wise but is there a way to find out what it is if the mph is really off? just wondering.