Even Worse Fuel Consumption

I knew that Air Con worsened fuel consumption but I didn't know by how much.
So I did a test. At 78 mph (true) over 250 miles on autoroute without Air Con I got 32.7 mpg.
With Air Con I only managed 28.8 mpg - a 12% hit.
Does that sound about right?

Doesn't sound right. I get 40mpg Bristol to Copenhagen with Air Con.