04 1500 SCSB, 5.7, black, average 18-19 A/C on and 65 MPH @1900 RPM, down a little due to 91 tune,

the timing increase with the 91 tune over the 87 is 2 degrees at WOT, at cruise timing ranges 40-47 for both tunes, I'm not seeing the MPG increase with 91 tune due to timing advance...

As for the A/C mileage decrease 1-2 is typical, if the RPM at cruise is to low in the power band (large diameter tires or 3.55 gears) additional throttle may be required to maintain speed.

Also to consider is how often the compressor cycles.

Any extra insulation can reduce the heat gain.

Has been 95-105 here lately and I set the fan speed to 1/4- 1/2.

Make sure the system is fully charged air flow thru the condenser is good.

I spliced a ball valve into the heater hose so hot coolant would not get close to the heater core and increase the heat load on the A/C evaporator.

 

hmm couldnt tell ya...i've only used my a/c like 3 times in the 4 yrs i've owned it lol.....

 

Most people don't know this but every time you use the defrost setting, you're in fact using your AC system too.
I'm guessing that living in Michigan might make you use that defroster more than 3 times LOL

 

may be the timming is increased, but My Hemi feels more powerful with the air on, mileage is not effected that much....