Posting this in the E60 forum because everyone in our E61 forum busy replacing their failed rear suspension self-leveling air compressors... :|

I have a sidewall puncture (very small, slow leak) that cannot be repaired. Further revenge of the damn road to my summer cottage. :angry:

My E61 has four, M5 19" 166 M (non-staggered) front wheels for summer, with Continental Sport Contact 3 tires, size 245-35-19.

My local Continental shop cannot get this 325 EUR tire for several weeks, if not longer. However, they do have 2 used (6mm tread) Continental Sport Contact 3, 255-40-19 tires for sale at 40 EUR each. Wtf?

Anyway, can I mount these two tires to my rear wheels? Or will this screw something up with the speedometer, rubbing, etc.

The 255/40x19 tyres are 1.6" larger in diameter so your gearing and speedometer accuracy will be affected.
A cheaper correct size tyre would be the Hankook Ventus V12 evo,recommended by many members.You should be able to get them for around half the price of the Continentals that you have quoted from mytyres.co.uk.
By the way,their price for Continental is around 260 euros.

Does anyone know whether the speedo reading on the rear or front?

You definetly cannot do this on an x-drive car, your differential will go to hell and break within a week of having different diamter tires.