I had a rear flat tyre recently and put the spare tire which is 14 inch but the tires under the car are 15 inch. So i put a 10a fuse in the Fwd circuit and disabled the 4wd.

 

So my question is that is it OK to drive in the FWD on regular basis. Or is it doing some damage to the driveline and/or the tranny?

There has been a lot of discussion on the importance of maintaining a consistant tire size on AWD Subarus (See this post...) I think that the FWD fuse is for emergencies only, and leaving the fuse in for a prolonged period of time will be bad for your transmission, but I could be wrong. I'd play it safe and get the tire replaced, and turn the AWD back on (remove the fuse)

I got a new tire which is the same size as the others. I was just wondering if i could save some fuel by having the car in FWD

Well, this too is controversial, if it was me, I'd leave well enough alone...(See this post)

Well it seems to me after reading the post that there isn´t any gain in fuel by having the car FWD. Probably not good for the drivetrain.

 

The FWD circuit is only i think for when towing the car or have a mismatch tire.