While I'm at it...
for you piston guys, in the Australian regs there used to be a requirement for twin aircraft to ensure that they have enough fuel to fly (on two engines) to the single engine CP and then fly one engine out to an airport.
I don't know off the top of my head if this requirement survived the "just don't run out of fuel - OK?" changes where the CAA decided to get fuel reserves etc included in Company Ops manuals (of which maybe 10% have been ammended after 5 odd years, I bet...) however as most pilots learnt CPs at CPL stage, and that was, pehaps, two - three years before they really started flying twins, I bet that hardly anybody does this.
Of course the whole situation is not helped by the manufacturer's manuals, very few of which (especially Piper's) give any numbers for single engine cruise, but that's another whinge...
So, time for a "Rule of Thumb"
When a piston twin loses an engine, it has to slow form its normal cruise speed (with lots of parasite drag) to close to the long range speed. You will find that the aircraft is flying more efficiently on one motor at the single engine cruise speed, than on two at normal cruise! (but not as efficiently as on two at long range cruise, due to the added drag of the engine out thing.)
So the aircraft will fly furthur (through the air) one one engine than on two! Doesn't work for turbines, as they are forced to operate at a (inefficient) lower altitude.
So, the rule of thumb? If the headwind is less than 25% of the single engine cruise speed, planning normal ops fuel will also cover the engine out case.
So now you can say to the CASA guys when they jump out from behind the fuel bowser - yeah, I planned for that, and proceded to baffle them with the above!