So I was kind of wondering what I am doing, I changed my tires from the 245 75 16 x tires to 33 11.5 16 nitto muds and was wondering if I am going faster then what the speedo says and the mpg says 13.5 is it more or less how exactly do you go about this thanks.

 

There are calculators floating around online, search around and you'll find them.

Or

Take it in to the dealer and pay them to reprogram for the taller tires (I believe 32's are the tallest they can do? But it'll get you closer)

Or

Buy yourself a programmer. AEV ProCal is the cheapest at about $130, but has a ton of functions for the money. There are several more expensive programmers available, that also have peformance mods included. Check them all out before considering the dealer...

 

Because your tires are bigger, you are going faster than the speedo says, it's still programmed for your smaller tires. You're fuel mileage is better too as you are travelling farther with each revolution of your tires over the stock ones.

The dealer can only reprogram you up to a 32" tire.

 

The 245/75R16 tires are a 30.5" tire. So if you now have 33" tires, they are 8.2% larger.

Therefore your speedo will be low by 8.2%. If your speedometer says 50, you are actually going 54.1 mph.

 

If you have any doughts you could get the AEV programmer. It works great.