Air con causes big drop in mpg on mine...combined with big drop in power because it's a mechanical system. I reckon about 24-25mpg on a gentle run with the air con blasting. But you get the same drop opening the windows and sunroof so it's all the same in the end!!
Get the system checked out by an expert ? helped the system on my 1991 work much better and more efficiently. The bloke I used to get it done was awesome and after some lying git told me the whole system needed to be drained, flushed and rebuilt (?350) this bloke did a recharge for about ?65.
Even though mine worked OK once it had been recharged with new refrigerant it was incredible how much better it worked.
