Adjusting the Speedometer

[tla11384]

Originally the truck came with 225/75/15. I have 245/65/16 on them on now. The speedometer is off about 5 mph. How do I adjust this and am I getting worse MPG??? Thanks

 

[Clemsonfan15]

Since the tires are bigger I think you are getting worse gas milage. How much, I don't know.