Need new tires. Questions about upgrading..!

I did similar when I owned my 2010 F-150. Went from 275/55R20 "P" series tires to "LT" tires that were 275/65R20. I did notice them and it dropped me a few hundred RPM at highway speeds from where the truck was at previously. I had about a 5% error on my speedo given the new tires. I actually bought Goodyear Duratracs and I loved them. They rode rough for the first 1,000 miles until the tire wore in and it didn't howl as much after that. What's nice about the Duratracs is that they are mountain/snowflake branded, so they can be considered a traction device on light duty vehicles, so no need to chain up or get winter specific tires if that's a concern in any of your travels. Yep, LT tires on average put on from 10 to 15 lbs over the "P" rated tires. But what you're getting is a tougher carcass. Also note, to achieve the same load ratings as your "P" rated tires, you need to actually run MORE air in the LT tires to achieve the same load capacity. In my instance, I only needed to run 35psi to achieve the load ratings for my F-150 with the OEM tires. To achieve the same load handling in an LT tire, I consulted an inflation table and ended up going up to 45 psi so your ride will in general be rougher. I also added Bilstein 5100's at the time since I wanted a nice leveled look on my truck. Then I realized I was overloading my F-150 with the chores I do every spring and summer so I traded for a new truck and am now waiting to buy a trailer of some sort.

Hope this helps some!