Originally Posted by Bean Burrito

I was just looking at the Sirrus and had my LBS throw on some 38s just to show me that they would work. That front fork has a good bit of clearance, and the 38s fit with plenty of room to spare for fenders. Just from looking at how the 38s fit, 40s would be fine. The point at which I would start to think maybe things wouldn't fit would be like a 45.

See if your LBS will try fitting a tire the size you'd like just to test the fit if you want to really up your confirmation of the fit.