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

On my sister's minivan it loses about 2-3mpg if the A/C is on all the time.

Not sure what the impact is on our 300E....I did a 250 mile freeway trip at about 74mph with the car full of stuff and the A/C on high the whole time (it was 97 degs out)....and still got nearly 22mpg on midgrade gas.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

With my '96 Ford Contour, I see about a 2 mpg loss with the AC on. I see a gain of 2 mpg with criuse control. I would say modern AC compressors use about 2-3 HP. I was told the old York style compressors consume 10-15 hp to run. It has been a while since I've run my '75 Bricklin, but I do remember a big power loss running AC. I can note some amount of power loss with AC on, on newer cars, but not that amount.
Also, fuel can have an impact. I have notice a +2 mpg with some gas. Also, if it has 10% alcohol, I usually lose 2 mpg.
Tom

I would expect less than 1 mpg diff on such a large vehicle. I would look to calculating differences, fuel quality or altitude rather than use of ac for the explanation.

Glad to hear you are using that beast!

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Maybe it has a bigger compressor to cool off larger cabin than a normal car would?

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1985 CA 300D Turbo , 213K mi