i was looking over "new" to me 2009 trek 6500 disc MTB it has 26 x 2.25 front and 26 x 2.2 on the rear. what is the reasoning behind the two different sizes ??? my 1999 marin quake has the same size. what did they find out over those ten years.

Probably not as deep of a reason as you might suspect. Possibly one tire busted and they used a different tire they already had laying around.

That being said, a lot/most riders have different tires for their front and rear, often having deep center tread patterns on the rear for climbing, and deep edge tread on the front for cornering. Maybe the previous owners favorite tread pattern only came in a different width tire..

Lastly, difference between 2.25 and 2.2 is... ...small

My mtn bikes all came with slightly wider tires on the front. The fork has more clearance so will hold a wider tire.

This goes all the way back to 1970's BMX days, maybe before ---- was customary to run a 1.75" tire in the back and a 2.125 in the front ---- the back tire needs great acceleration and the speedier smaller tire is ideal since the front is most crucial for handling --- with a bigger contact patch

-- I'm planning on going 1.9/2.1 with my new build as well for similar reasons , --- but then again , the answer could be as simple as everyone else said -- its a 6 year old bike, maybe the PO just used a tire he had laying around

big front tire for confident steering, smaller rear for acceleration. I have a 29x2.4 front and 29x2.25 rear.

I ride with a 2.1 on the rear and a 2.3 on the front on the dually. The hardtail is 2.1 on both only because it's tubeless and I never got around to changing the front tire.

thanks guys.. was not concerned with was just curious if there was a reason for doing it.