Differential Gear Change

Greetings,... If this has been addressed already feel free to post a link, but I couldn't find anything on this topic,...
Was told by an installer that I should think about changing my differential gearing to get my fuel economy back. I went from the 16" 3rd gen tire size to a 285 70 R18. Yes there's a speedometer difference. But I'm not knowledgable about the tech side of these vehicles.
What do you think? He thinks I'll burn my tranny out if I don't change the gearing as well.
Thanks for listening.

 

Gears will help with fuel mileage and the speedometer, the transmission is another story. Look for the 5.29 mega thread, setup for third gen trucks with everything you could ever want to know.