New tires, up-sizing and load range?

P series tires are way different from LT series tires. P series tires are all universally rated at 30 or 35 psi (unless it's a designated XL load range, then it's rated at 50 psi). LT tires have load tables associated to them. You typically need more pressure in an LT tire to achieve the same load handling rating as a P series tire. However, LT tires do provide a stiffer sidewall versus the comfort oriented P series tires which tend to have a "Mushier" ride. On my F-150, I actually went from 275/55R20 to 275/65R20. I was 5mph off at 60 mph. The ride was much stiffer as the 275/65R20 tire was an "LT" tire. If the 275/60R20 tire is a "P" series tire, then I wouldn't expect any rougher of a ride, actually I'd expect a softer ride since you'd have a taller side wall and probably suffer just a tiny bit of inaccuracy at the speedo. However, what I found is that in "LT" tire sizes, there are lots of all season tires that have the mountain snowflake symbol on them (meaning no need to chain up when traction tires are required), which is what pushed me to get an LT tire instead.