Front Tyre/Brake Vertical Clearance

Hi - Just looking at my bike and I've noticed there is very little vertical clearance from the top of the tyre to the underside of the front brake.  Now, I've had these tyres on for a while now without a problem.  However, I've measured the clearance, and there is just 1.5mm.  Is this normal?  

The tyres are Specialized Roubaix Armadillo Elite, billed as "700x23/25", but I've got the digital calipers out and they measure 27.5mm width on my rims for some reason, and of course this wider width means they are probably taller too.

For comparison I put a 700x23 tyre on (actually they measure 24.7mm on the rim) and these give me 3.5mm of clearance at the front which looks fine. 

Anyway, I'm planning on replacing the Armadillo Elites with Schwalbe Ultremo DD, again 700x25.  Does anyone know whether the Schwalbe tyres measure pretty much true to size?  My concern is that the new tyres could be even larger than the ones I've got on, and then they wouldn't fit without dragging on the underside of the brake.

Any thoughts, advice?

ok - that does sound close clearance.  My concern would be your tyre picking something up that jams the front wheel ? 

I'd not risk a bigger tyre - I guess you'd only find out for sure when you put the new tyres on. 

What bike is it you've got ? 

Thanks for your reply.  I have a 2008 Specialized Allez Elite.  It came with 700x23 tyres (which measure 24.7mm width).  They give 3.5mm of vertical clearance. 

I wasn't sure whether it was simply that the tyre I've got on is just very generously proportioned for a "25", being measured at 27.5mm width, or whether I shouldn't really be using a "25" on it at all. 

I've never had a problem yet with anything jamming the wheel, fortunately.  Those "25mm" tyres on it have been on for about 2000 miles (including a load of Paris-Roubaix cobbles).  My concern is whether the Schwalbe tyre is likely to be even bigger - despite it being billed as a "25", because anything bigger is not going to fit.