Pros/Cons of 110/90 Rear Tire on 450?

Not enough difference between the two that any amateur is going to feel (it's all in your head)...run what you have, and don't over think it. 

You cant go by just what the tire manufacturers call them. 

A 110/90-19 from Bridgestone is substantially wider than a Dunlop, or a Michelin. Almost to the point that a 120 Dunlop is a 110 Bridgestone.

Bridgestone, Pirelli, Dunlop, Michelin. In order of width of 110s. Have not measured latest Pirelli MX32 or Michelin 6 so based on previous models.

And yes its a big difference in cornering, just as big difference between a mousse and a tube in the front tire. 110 much easier to turn with if you compare same tire brand and model. Same in front, 90 much more difficult to turn than the 80.

If you specifically can feel it, is hard to tell.

I like the 110/90 because it gives more cushion on chop and seems to lean over a little sharper.

The 120/80 hooks up better on hard packed flat turns, especially useful for AX style tracks and jumps right out of corners.

It's not a huge difference but it is noticeable. 

As mentioned, tire brands vary in their sizing.

I’m a nobody, but I can feel the difference between a 110 and a 120 on my 500. I usually run a 120 in back and a 90/90 in front, it absolutely needs more effort to initiate and maintain a turn. It’s worth it for the extra acceleration, braking and turning grip, at least to me. 

I agree. I would be willing to bet that most would fail the Pepsi challenge in this scenario.

I didn’t realize that the new 450’s came with 120’s because I hadn’t had a 450 since 2006. When I ordered a replacement tire for my 2023 CRF450R, I got a 110/90. I’m a Vet B rider and I couldn’t not tell you the difference between the two. I would absolutely fail the Pepsi challenge. Some talk about 110’s being able to lean in on corners easier than a 120, but I can’t tell the difference. I’m not out there dragging bars though. 

I tried a 110. Didn’t notice any difference in corners but traction wasn’t as good on hard packed. I think the only time you would want to run a 110 on a 450 is if the track is crazy rutted. Even then, I don’t think it would provide a noticeable improvement over the 120 in the corners or ruts. 

If you're intune with the geometry of your motorcycle the difference is immediate and staggering. 110 on a 450 has a significantly faster turn in feel, if you're used to a 120/80 or a 120/90 it'll feel like the is falling or dropping in on initial lean, once you're over on the edge of the tire after that initial lean in, you won't feel too much difference. It'll also break traction quicker compared to a 120/80 and 120/90. The 110 will also move through ruts more comfortably and won't try to crawl out of it less compared to the 120s. If you're in a rutted area I'd go for the 110.

The 120/80 is a good all around tire for a 450 and if you're not inclined to going and purchasing a 120/90 online, and want to just hop into a store, snag a tire and go. I'd just go that route. I personally enjoy a 120/90 the most, the extra height of the tire can make a more comfortable ride while going over bumps, like desert whoops. I also found that a 120/90 seems to want to drag itself out of ruts more than a 120/80. All this comes from using Dunlops, I've played around with some other tires like Mitas, and Pirelli's but I don't really remember much about them. Other than the front Mitas TerraForce Soft being absolutely amazing on the brakes.

I always ran 110 on my 450’s no matter what size came stock. I prefer the way the bike handles with the 110 and didn’t notice any loss of traction. 

It gets a lot of down votes…but the reality is, for novice and amateur riders, inconsistencies in body position and technique, from day to day, moto to moto and lap to lap will cause the bike to “feel” different…far more so than the tire or even suspension settings, or bar position or lever position, etc etc.

In order to really test a specific “something” (in this case tire width) on a bike, the rider has to eliminate as many variables as possible between the tests…amateurs and novice riders just aren’t that consistent in technique and body position to know if the changes in “feel” are the thing being tested or them…that’s why I think, as a group, we dirt bike riders (myself absolutely included) over think pretty much everything about riding dirt bikes…

There are no downsides to a 110. The downsides to a 120 are: kill cornering and sucks in ruts. 

Not true at all. If you can't tell the difference my assumption is you primarily ride hard pack. 110s are MILES better when it comes to riding in ruts. The 120s catch constantly. 

This! A narrower tire will turn in faster, and track better in ruts. The wider tire grabs the edges of the ruts more, and tries to climb out of them

I prefer the 120. Ya, I'm just a 30 novice, but I can 100% tell the difference between a 110 and 120. I get more wheel spin with a 110 vs the 120. 110 will turn better, but I've ridden with 120's for so long I've been able to adapt to the change. As far as fronts, I ran 90/100-21's for a while in my early days on 450's, which were great for flat corners, but they sucked in ruts! Finally decided and have stuck with 80/100-21 front and 120/80-19's in the rear. I haven't been riding much so I haven't had a chance to try any of the new 120/90-19's, but it is on my list of things to try.

Are you using the same brand tire every time? A 110 Bridgestone measure significantly wider than a 110 Michelin and so on. 

Yes. Dunlop 2013-Now, but Bridgestone 08-12 

I run the 120 on the rear of my KX500 and I think it makes a difference.  I run the same 80 front tire as all my other bikes.

I mostly ride the 500 at the clay based tracks around here like Washougal, Burnt Ridge and the now closed Ward Creek, so it's extra help on the blue-groove sections later in the day.

I think the 120 has application on a 450F as well.

I ride a 350F and a YZ250 the most and they get the 110 rear tires with 80 fronts.
I've never ran anything but the 100 on the back of my YZ125 and have no plans to change it.

Learn to turn better with the 120, bigger tire has more traction!

I think the 120 rear pushes the front tire in the corners. 

If you only care about straight line traction, sure, a 120... But in my experience, I'd rather turn better and flow through ruts without fighting the bike. Im a 30B guy and can absolutely tell the difference. Im not picky about many things, but a 110 is one of the things I am picky about. Brand new bike with a 120 gets swapped now before i even start the bike. The slight difference in bump compliance is negligible so ill lean 110 every time in that regard. I can only comment on Dunlops tho. Still get plenty of traction on my 450 with a 110. I usually start decent even at red bud with just a 110 34.