So I've done some searching on this but I don't have a clear answer... I recently added 305/55/20s to my 09 suburban and I find that it likes to drop to 5th when going up even small hills. This is hard on MPGs. I have 3.42 gears. Has anyone on here upgraded to 33s and then changed over to 3.73s or 4.10s? Did the steeper gears improve mileage any?

Let's play with some numbers. Your tire size mathematically works out to 33.21 inches tall. Stock tire (265-70-17) is 31.61 inches. Stock tires, 70 mph, 5th gear is about 2,175 rpms and in 6th 1,700 rpms. 33s, 70 mph, 5th gear is about 2,075 rpms and in 6th, 1625 rpms. Now put a 3.73 ring and pinion set with those 33s and she'll turn about 2,250 rpms in 5th and 1,775 rpms in 6th at 70 mph. With a 4.10 gear and the 33s she'll turn about 2,475 and 1,950 rpms respectively.

Yep that looks about right. Has anyone swapped gears after a tire change? was there any improvement in milage? (I know it's higher rpm, but it's also less downshifting to 5th and less load on the engine)

I did, it definitely helps but don't expect stock gas mileage. My city driving gas mileage improved more than highway, but my highway mileage can tank pretty quick if I want to cruise at 80. It drinks A LOT less in the 2000-2200 rpm range than 2400+ Mines 6.0 with 6L90E, stock gears were 3.73, new gears are 4.56, running 35s... YMMV (literally)