Larger tires on a road bike
Well, it depends a lot on the specific model of bike, and likely even on the exact frame size.  You can probably eyeball it to get a good idea.  Remember that larger tires are both wider and stick out more from the wheel.  On a road bike, odds are high that you can put a slightly larger (say, go from 25mm to 30mm) tire but not a hugely larger tire.  If you want to make a big jump you probably also want to get wheels with a wider rim. Note that for off-road tires, the knobs aren't usually counted in the mm size but definitely do affect whether or not the tires fit on your bike.

Look for:


Front and rear: Brake clearance &mdash; can a larger tire fit in the brakes? If this is the only issue, you can get different brakes
Rear: Stay width &mdash; will a wider tire rub on any of the stays (rear)?
Rear: Stay bridge clearance* &mdash; will a larger tire hit the bridge between seat stays or chain stays?
Rear: Seat tube clearance &mdash; many bikes have a geometry where too large of a wheel/tire will hit the seat tube above the cranks
Front: Fork clearance &mdash; how much room is there under the fork crown? (height and width)
Front: Down tube clearance &mdash; how much extra room before the front tire hits the downtube?