Hmm I doubt very much that A/C costs 2mpg, unless its a really really hot day. It will only work when required, otherwise an electromag clutch disengages it (I think
Especially in a 3litre, the extra loading on the engine is minimal compared to say a 1.3 Micra with air con on. Perhaps you just had a lucky steady flow journey.
I've managed a 550 mile round trip and got 35mpg (2.5) with air con on but only becaue it was motorway almost door to door and average speed was 70, max 90 never less than 70 and a wee wee/espresso break.
Speed up/slow down is a killer with the V6 as you just can't resist that heavy right foot when the traffic clears.
Anybody else notice that cruise control seems to have better acceleration on the recall button, than using your right foot on motorway? Or is it just me?