You have actually raised a good point here, tilly - I noticed this morning that I need more fuel, I had a quarter of a tank the other day - I'd put it down to the petrol evaporating in the tank, in this heat.....
If you get (say) 40mpg in a modern car then you're paying about 15p per mile for petrol.
The absolute most that aircon will add is about 5% - unless you are stuck in stationary traffic for ages. Once you are over about 20mph the additional cost is actually negligible.
But let's assume the worst case of 5% - which means you are paying (at most) 0.75p per mile to be cool.
So a 20 mile trip will cost you a maximum of an extra 15p to avoid sweating like a [whatever] - that seems like a bargain to me.
Well, jno. I put some petrol in last Thursday and I had to put some more in yesterday.
The only thing was, I didn't have my purse with me and I had to 'borrow' £20.00 from one of my visitors. I wasn't expecting not to have any petrol. Maybe it does evaporate.