Any thoughts on the following numbers:
I bought my Fuso FG in NC and drove it home to CO with just the service body and tanks. I was running the AC for virtually the whole trip. Got 13.74 mpg on average over the 1466 miles traveled.
I drove it from Denver to Alamosa with the camper fully loaded this past weekend. Got 11.07 mpg going there. Had the AC on the whole time. On the trip home it was obvious from the fuel gauge that I was getting better fuel economy. It got 14.34mpg. I was not using the AC.
It appears that the AC is consuming a fair amount of fuel. Is this typical? I'd expect perhaps a 1mpg difference, but more than 3mpg? I don't think there is a substantial difference in amount of climbing involved going to or from Alamosa to Denver.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13