I’ve got 5k miles on my bike now and feel I have arrived at my set up sweet spot for my riding in SoCal.
Tires- ran stock Dunlop, ran Kenda 110/100 Tackmaster, ran Mitas 110/80 ED 09 (skinny tire), and now have a set of Continental TKC 80 90/90x21 and 120/90x18. I’ve heard the talk about running smaller rear tire with KLX anemic power but see no difference pulling hills with bigger- heavier reat tire. Front tire runs quiet without howl.
Gearing- I just replaced chain with 108L gold EK oring, 14/44 JT sprockets. I have run 14/42, 13/42, and 14/44 which is a good compromise for street and trail. I ride roads to off-road sections when riding.
14/42=65mph indicated
13/42=60mph
14/44=65mph. With this set up I can see 80mph indicated @9,200rpm. GPS speed is 75mph.

Freakdaddy

 

Just because you can't feel it on some hills doesn't mean it doesn't exist. The 120/90 is only ,maybe a fraction of a pound difference to a 120/80-18. But the 4.10 -18 will be two to three pounds lighter than the 120. That and the major difference besides rotating mass taking up less power, is the narrower rear tire will allow a bit of spin when needed. Plus they're usually lower cost than the 120. The 120 is around 4.50" where the 4.10 is around 4.00" (the 4.10 is a designation of a lower profile than the regular 4.10). In my use in some deeper sand the 4.10 would spin up without bogging, the wear has been good too.

I learned a lot of the thing about tire size when racing a 125 two stroke. Lots of guys ran 4.50 and 5.10 knobbies, thinking they were the hot ticket, but the fast, knowledgeable riders were running 3.75 or 4.00. I figured if I wanted to compete it would be smart to do what they did and find out why. I did all three - find out what, find out why, and compete,. Take a look at the tires on the back of MX bikes, you'd think they'd run wider bigger diameter if it was better.

I don't really care what size tires you run or what brand, but the facts relating to running a smaller tire do exist.