I finally got around to replacing the leaking A/C compressor on my 92 D250. It's got an automatic and 3.54 gears. For several years it has returned 17 mpg in the city.
I recently started driving it again for my daily commute. The last week's tank of fuel returned 16.6 mpg.
I replaced the A/C compressor so I could fire up the life support system again. This is the first time in several years that the A/C has worked.
This tank of fuel has really taken a beating. Instead of 90 miles per quarter tank the truck has returned 70-75 miles per quarter tank. That's looking more like 13 mpg.
I've never seen mileage that low with the trunk unloaded. In fact, that's worse than my typical towing mileage.

Maybe I'm being paranoid about this one tank of fuel but there's no way I can afford to drive the thing if that's what I can expect for summer mileage. The fuel was purchased at the same station I've used for years.

Are you guys seeing drops like that?

When the compressor kicks in at idle I can hear it tug the engine down a wee bit. The difference is barely enough to notice. It shouldn't be putting much of a load on the engine.

 

the fuel we have theses day's is Junk it's colored Water that's the biggest thing also how you control the skinny pedal is alsoa factor . the compressor will make a difference in sound but shouldn't hurt fuel econ.

 

Mileage the same for me whether I have the AC on or off (17 city 20s highway). AC kicks out the cold, so it's working fine and cycling as normal.

 

I read once where a AC compressor takes 25hp to run. I don't know if that's right or not. Your AC will engage with heat defrost on as well

 

The HP demand depends on the compressor type. The GM R4 take 11-15 hp. The older Yorks took more.

 

Perhaps I should have stated that I'm not a newbie when it comes to either my truck or working on cars.
I've built hot rods and several a/c systems too. I simply found it disturbing that the mileage dropped off that much when the A/C was working. I don't remember that happening in the past but it's been 8 years since the truck was my daily driver.

An A/C compressor should not consume 25 hp. Even the old York and RV2 systems don't suck that much power. They top out around 15 hp at higher line pressures. You'd be smoking belts and killing tensioners at 25 hp drive.

A typical Sanden style compressor should not take more than 8-10 hp. I expected the Cummins to shrug that off with no appreciable difference in performance.

 

Yeah those older non-rotary types sure sucked the HPs, the rotaries not nearly as much. At highway speeds, any draw by the compressor is offset by having the windows closed, that was proven by the Mythbusters...Mark

 

If the system is over charged enough so liquid droplets (super saturated) are getting into the compressor, power draw will increase significantly. It's also hard on longevity.

The suction line to the compressor should not get frosty.

Poor heat transfer in the condenser will also increase power dramatically. It shows up as excessive high side pressure.

If you're charged with R134, try C12 (propane-isobutane) instead. It's more efficient by far.

 

I converted the truck to 134 back in 2002.
The system pressures are in good shape. The low side line got down to 38 degrees but that's it.

Years ago I had trouble with out of control high side pressures. It would actually get high enough to slip the belt on the compressor when the truck wasn't moving. I installed an 8" fan on the condenser and the problem went away.
My little fan is till alive and kicking.

 

I would say your issues lie elsewhere. Mine actually improves MPG when it is extra hot, not sure exactly why, but when it really gets rolling towards 90 degrees, she picks up another 1/2-1MPG, and the AC is running all the time.

Run another tank through and see if it is the same.

 

I have found this as well. Hot summer months, with the coolant temps up around max temps, the truck gets better mileage. I think this is because with the warmer weather the engine can get up to optimum temp, and thus reach a better combustion temperature, and therefore better fuel economy?

 

California fuel is real bad. It is getting to the point that it unstable.

 

Problem is it's not just Calif it's all over the Place .

 

That doesn't surprise me.

 

My 93 will drop a mpg or so with the AC. i am in and out all day, shoeing horses, so I dont normally run it til the last horse is done nd I am headed home for he night.

Wife's 04 seems to drop some with the summer and AC. Got it in Dec, so this is first summer.

I dont see how running the AC with the windows up or windows down/ no AC would make a bit of difference aerodynamically. Have Mythbuters seen our trucks? They may also think a bug shield affects MPGs, too, lol.