Yes, the miles - odometer and speedometer - will both be off.
Take an existing tire from your truck. Put a chalk mark on the side of the tire where it meets the pavement. Also mark the pavement in the same spot. Now roll the tire one complete revolution. Put another mark on the pavement in this new spot. Now measure the distance between the two spots on the pavement. When you get your new bigger tires, do this same marking procedure. Whet you will find is, for the new tire, the distance between the two spots on the pavement will be greater. So with each revolution your truck will be covering a greater distance with the new tires. So lets say for example, your new tires cover a 5 percent greater distance. This would mean that for every 100 miles that the odometer shows you traveled, your truck actually traveled 105 miles.
Edit: Take this into consideration when you calculate fuel mileage as well. If you know the percentage of error, you will know to include this into your MPG calculations.