Any tyre fits just about any rim or the right size, the markings regarding tubes only tell you what it can be used for. So:
130/80 - 17 Tubeless
It's 130 mm wide (so will it clear the swing arm and fit the wheel?)
It's depth from rim to tread is 80% of the with. (So will it hit the mudguard)
It's for a 17" rim
It can be used without a tube if the wheel is set up for it.
A 120/90 -17 (nothing else) is 10mm narrower, a little "deeper" and won't seal to the bead so needs a tube regardless.
There are also older imperial sizes eg. 4.00 R 17 is 4 inches wide on a 17 inch rim with a fixed ratio on the depth.
The other numbers are date codes, load and speed ratings.
I know your XT has spoked wheels. The front is a 21" 1.85" wide, the rear 17" 3.00" wide. These seem narrow and will have a tube. There are metric equivalents, but I think you need to get them from Yamaha, not by cross refering the numbers you have.
On the tubeless thing, take a tourer and the rim could be the same size, but will be cast wheels (or special type) hence there is no way for the air to get out, hence with a tight bead (where the tyre grips the rim) it will be air tight.
You could take the tyre off the tourer and fit it to the XT (same size), but here's the crux of it, it will leak unless there is a tube and even though the tyre will still grip the rim. This means there are three practical combinations and puncture fixes:
Spoked wheel, Tube, Tube type tyre: use levers, get the tube out, fix.
Cast wheel, tubeless tyre: Fix with an external bung, remove with a bead breaker later and fix with an internal bung.
Spoked wheel, Tube and Tubeless tyre: Remove with a bead breaker and fix the tube.
(Cast wheel, tube, tube type tyre: Only ever used when the casting is porous, acts just like a spoked wheel.)
The bike manufacturers often go for option three because they think you'll never want to get your hands dirty and will ride 2000 dry miles a year to and from your local coffee shop. They fit tubeless tyres with a tube simply because they are available.
The options that were discussed in the other thread involve hardware that will sort the spoked rim so it'll run a true tubeless set up and allow an easier bung fix for a puncture. These solutions are currently rather experimental, Heath Robinson or expensive, hence the discussion.
Hope that helps
Last edited by Threewheelbonnie; 9 Oct 2008 at 13:30.
Reason: Made no sense!