Defrost = Poor MPG??
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Senior User
Joined: Mar 2000
Posts: 358
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From: Tri-Cities, WA
Defrost = Poor MPG??
Took a round trip of about 150 miles yesterday, one that I make every 6 weeks or so. Anyhow, the only difference on this particular trip was that I left my HVAC set on "vent" instead of "defrost", thus my A/C compressor was not cycling... I was greeted with just over 9mpg, as opposed to my typcial 7. Now, I fully understand that the compressor causes some extra parasitic drag on the engine, but 2mpg worth??? Is there some way to check the compressor for excessive drag or anything like that? I have to admit I felt no difference in power or anything like that, but this truck has NEVER given me 9mpg before.
And no, it's not differnent fuel (same tank from prior 150 trip) or different driving conditions (same highway, same 68mph cruise setting)... This is, however, the FIRST trip I have taken where I was not using an HVAC setting to engage the compressor.
And no, it's not differnent fuel (same tank from prior 150 trip) or different driving conditions (same highway, same 68mph cruise setting)... This is, however, the FIRST trip I have taken where I was not using an HVAC setting to engage the compressor.
Laughing Gas
Joined: Oct 2001
Posts: 957
Likes: 3
From: Winchendon MA
Yup its a ford thing.
Seems some dumby engineer years ago made it so the A/C is on any time the selector is in the defroast position! So your sucking power by useing the A/C.
Now I've had this argument on how dumb and wastefull this is with many poeple and while they all make the point the windsheild defrost faster with it, no one has been able convince that its a must as cars / truck with out A/C get the job done just fine.
I pulled my wire plug off the compressor at the 1st snow.
44Dwarf
Seems some dumby engineer years ago made it so the A/C is on any time the selector is in the defroast position! So your sucking power by useing the A/C.
Now I've had this argument on how dumb and wastefull this is with many poeple and while they all make the point the windsheild defrost faster with it, no one has been able convince that its a must as cars / truck with out A/C get the job done just fine.
I pulled my wire plug off the compressor at the 1st snow.
44Dwarf
Freshman User
Joined: Jan 2008
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Originally Posted by tobyw
Took a round trip of about 150 miles yesterday, one that I make every 6 weeks or so. Anyhow, the only difference on this particular trip was that I left my HVAC set on "vent" instead of "defrost", thus my A/C compressor was not cycling... I was greeted with just over 9mpg, as opposed to my typcial 7. Now, I fully understand that the compressor causes some extra parasitic drag on the engine, but 2mpg worth??? Is there some way to check the compressor for excessive drag or anything like that? I have to admit I felt no difference in power or anything like that, but this truck has NEVER given me 9mpg before.
And no, it's not differnent fuel (same tank from prior 150 trip) or different driving conditions (same highway, same 68mph cruise setting)... This is, however, the FIRST trip I have taken where I was not using an HVAC setting to engage the compressor.
And no, it's not differnent fuel (same tank from prior 150 trip) or different driving conditions (same highway, same 68mph cruise setting)... This is, however, the FIRST trip I have taken where I was not using an HVAC setting to engage the compressor.
FTE Legend
Joined: Jun 2006
Posts: 31,930
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From: Ottawa, Ontario
The higher the engine rpm the more energy required to rotate that AC compressor. At the 1800-1900rpm my 3.55 geared 5.8 1/2 ton runs at while going 70mph in OD I see no difference at all with any climate control setting, but at 3000rpm when it drops out of OD the drag from the compressor is substantial, it would definitely hurt milage if I had to turn those rpms all the time.
Posting Guru
Joined: Aug 2005
Posts: 1,069
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28percent better mpg wo ac? 150 miles isn't very far for a test of mpg. 460 shouldn't decrease that much, especially with that gear ratio. Maybe headwind/tailwind difference.
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Senior User
Joined: Jul 2007
Posts: 452
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From: Upper Pennsula Michigan
Well for some reason i'm getting very bad mpg with my truck now...I was getting almost 11 and now i'm getting 9.3 i just don't now...I know its like 10 degrees colder....The last 2 tanks i got the same....
Senior User
Joined: Jan 2008
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From: Shelton, Washington
Originally Posted by capn kirk
i think all cars us a/c for defrost to take the moisture out of the air
Elder User
Joined: Nov 2005
Posts: 834
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I don't believe your A/C compressor alone is causing that much gas consumption. Is it possible that you have a vacumn leak in the climate control system that only happens when defrost is selected? I know if there is a big leak, the default setting for the flapppers is defrost. I can't think of where a leak would be, that only happend on defrost. An easy test would be use a hand pump to draw a vacumn with the engine off on the line going thru the heater box. Then switch to defrost and see if it will hold a vacumn.
Good luck Frank
Good luck Frank
Last edited by fmr9; Jan 31, 2008 at 07:27 PM.