Modern Buddy

It's time to replace my rear tire again but the front is still good.
I have Pirelli SL 26's on now but I haven't been too happy with the grip or longevity so I would like to go to Michelin S1's.

Any problems leaving the front Pirelli on until it needs replacing? The tread is pretty similar.

More than likely you'll be fine, I've done it before. That being said if the tires have wildly different grip characteristics your bike might ride funny.