Tire fitment question

I just bought a new/used 2013 WR250R. I want to mount a D606 on the rear and the Pirelli MT21 front. I have read through several threads and now I am totally confused. One says to use the 120/90-18 on the rear and the other says 120/80-18 on the rear. Same goes for the front, different sizes.
Can you guys help me solidify the sizes of these tires for my bike, I hope to order them up Saturday and get them mounted. Thanks.

I'd go with the 120/90-18 on the rear.
21 is the front wheel diameter.

Hey mate there will be no confusion on the size for the rear if you are going with the D606, as it's only available in the taller 90 profile.
( Also you have listed the rear as being a 120/~/21, but i think that we all know you really mean "18"...as pointed out above by YZEtc ).

Also as for the front, the size increase to the 90/90 is a good compliment to the taller "90" profile on the rear - so all good mate.

Cheers.

Hey Big-O,

I have the 120/90-18's on the rear (D606) and the 90/90R-21 on the front (MT21). It's a great setup. Be aware, however, that the bike feels a bit squirrely (on road) for the first 50-100 miles.

Direct swap. No fitment issues. Much improved on the trail.

Great guys, just ordered them up.