Windows vs. A/C

[MyQ45]

I have tested my 02 Q45 and a previous 99 XJ Vandan Plaus Jag. Sraight level stretch of Texas hwy. Windows up & A/C on - Windows down. bth cars have internal mpg information systems. At same point I cleared the mpg and ran the test at 65 for 3 miles. The jag dropped 4mpg with windows down. The Q dropped 3.6 mpg with windows down. Both cars have strong V8s. jag 4L at 290hp Q 4.5L at 340hp. So the /c really doesnt 'pull' on the engine.