[SentraSE-R]

You're not telling me enough, so it looks to me like you're comparing apples to oranges. I can't believe you get 28-29 mpg at 72 mph with AC on, and only 30 mpg at 60 mph with AC on. You say AC beat windows down @ 72 mph. How far down were your windows? How high were the AC temperature, duty cycle, and fan set? Was the cabin air set for fresh air intake or recirculating?

My short answer is yes, AC robs mpgs v. windows cracked down 2"-3". The reason is your 2. The slow speeds I run on the commute ( 60 MPH max) makes opening the windows a much less aero hit.

The difference in mpg between my cars at 72 and 60 mph (windows up) is more like this:

Car...........mpg @ 70 mph...mpg @ 60 mph
Sentra.......26...................33-34
'06 Elantra..31...................34
Scion.........32...................38-41
'12 Elantra..32...................44-48

You have the capability to test the compressor by driving the same route with & w/o AC. Crack the windows a bit for both A&B (AC on v. AC off) tests if it's a scorching day. Do a 3-5 mile route, traveling the same direction, the same day. Do three tests (ABA). That eliminates different gasoline, different weather conditions, different engine health, etc. Note all the AC parameters. I recommend setting to minimum or medium temperature, maximum fan, and recirculated air for AC parameters. Do it (ABA) @ 72 mph and again @ 60 mph.

Then do ABA testing of windows down 1/3 v. AC @ 72 & 60 mph, and windows down all the way v. AC, @ 72 & 60 mph. That's a lot of driving, but at the end of it, you'll have your answer.