I've had a 14 Ecodiesel longhorn for about 3 years. It's always had P275/55R20 tires on it. Now its time to buy new tires and I noticed the correct tire listed for the truck is P275/60R20. I'm getting 28-30 mpg on what I have now and am used to the speedometer being off a little. I don't do any towing and drive 75-80% highway. Wanting to keep my mpg up, any ideas on which size to go with and any recommendations?