It's been a few days since this was posted, but I'd still like to recap a warning when switching to LT tires from OE "P" tires. Basically, LT tires will typically require a higher inflation pressure to keep the same load rating as the original P type tire, and an even higher pressure to be able to handle a higher load. This guide is long, but worth a read for anyone switching from OE "P" tires to LT metric (no relationship with Toyo, just really like this guide, which is pretty much universally applicable):
http://toyotires.com/tires-101/load-and-inflation-tables
In a typical example, if you have P245/75 R16 (usually 109 load rated) at a factory recommended 32 psi pressure, each tire can carry 2271lbs, and that's the max, higher pressure won't usually give them a higher load capacity. See pg. 16 on the linked pdf.
Now, if you switch to a LT235/85 R16 and keep the same pressure your tires can now carry 1700lbs each. You've just lost ~25% cargo capacity! This is mostly due to construction and heat management.
Now, the upside is, your new LT's will usually be at least "D" or more typically "E" rated, and now you can go to higher pressures. Per the table on page 20, you'll see you should go to at least 55 psi to be near your OE load rating (2331 lbs). Any less than that your load carrying capacity is less than the original "mall-rated" P type tires. Of course, if the tires have the usual "E" rating you can now go to 80lbs and enjoy >3000lbs per tire for a comfortable safety margin over your vehicle's Gross rating (which of course, shouldn't be exceeded).
I'm no tire expert and I know everyone has a different experience and opinion (especially on that last comment with regards to the truck's GVWR). There are also a million caveats (passenger & cargo load, terrain, etc) but I think that's the main reason you saw such a jump in MPG from increased pressure, your tires were probably underinflated at 3X psi. Also, every disclaimer ever uttered may apply here: Closed course, unprofessional driver, these are only my comments which should not be taken as a rule by anyone ever, and you should always consult your owner's manual and obey all traffic laws, keep hands and feet inside the vehicle when moving, etc, etc).
HTH.