The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 02-08-2005, 12:45 PM
Anaamika Anaamika is offline
Guest
 
Join Date: Nov 2001
Why does a car's gas mileage go down over the years?

Question: What are the physical changes in the car that make the gas mileage decrease? Or is this even a true phenomenon, I think it is but I could be subject to "hearing-and-believing" syndrome.
Reply With Quote
Advertisements  
  #2  
Old 02-08-2005, 01:48 PM
Enola Straight Enola Straight is offline
Guest
 
Join Date: Jan 2001
Actually, after a few months the mileage goes up due to the break-in period where theinternals properly seat, reducing internal friction.


However, over the years, this same process also wears away the close tolerances of the parts that move against each other: things like cylinder compression rings, which, when worn, no longer contain the energy of combustion and simply blows past the piston instead of pushing against it.
Reply With Quote
  #3  
Old 02-08-2005, 02:44 PM
bump bump is online now
Guest
 
Join Date: Jun 2000
And... things like spark plugs, O2 sensors and other regulatory systems get kind of tired and worn out, causing the mileage to go up some.

I replaced my O2 sensors, spark plugs and some other emissions related stuff over the past couple of years, and have actually seen about a 3-4 MPG(about 17%) increase since that time, in a pickup with 170,000 miles on it.

I'm starting to think that's the real issue these days, not actual cylinder wear & tear.
Reply With Quote
  #4  
Old 02-08-2005, 02:48 PM
Philster Philster is offline
Guest
 
Join Date: Aug 2000
Many cars can expect consistent reliable performance and mileage, but some things can crop up that can lead to poor performance and reduced efficiency.

Computer sensors. As they age, they may become less accurate, and the fuel mixture and timing of ignition might be off. While it might never make you jump up and think something is wrong, just being a little off with fuel/air or ignition can reduce mileage. Some parts, esp polllution controls, might have reduced life spans.

As mentioned, internal engine wear can reduce compression. Less compression =-less power = you hit the pedal a bit harder to accelerate burning slightly more fuel for the same power.

Accessories, such as A/C compressors. If they wear and offer more resistance, then more horsepoer (via gas) will be used.

Disrepair: The actual maint of a car is extensive when the miles creep up. Is the radiator as clean as it was when new? How about the smog system...the PCV valve....and everything else...the diff fluid...tranny fluid...air filter...spark plugs...wires....etc.
Reply With Quote
  #5  
Old 02-08-2005, 03:07 PM
Stranger On A Train Stranger On A Train is offline
Guest
 
Join Date: May 2003
Quote:
Originally Posted by Philster
Accessories, such as A/C compressors. If they wear and offer more resistance, then more horsepoer (via gas) will be used.
This can be particularly important. Coolant--R12 (freon) in older cars, R134a in 1994(?) and newer vehicles--will leak out of the nominally closed loop eventually. If not regularly checked and replunished, it will cause a significant decrease in A/C efficacy, which will far outshadow any losses of mechanical efficiency. The same is true for house A/C, refrigerator/reefer, and any other device that uses a gas coolant heat exchanger.

Other than that, regular maintainence--check and change the plugs, plug wires, points (if your car uses a rotary distributor), serpentine belt, O2 sensors, fuel filter(s), and especially the air filter. For most modern cars, as long as the oil change regime has been followed with regularity, engine wear (piston rings and cam and crank bearings) shouldn't be an issue. A Most auto engines will far outlive the car they power if maintained. BTW, the high milage oils are a rip-off, as are oil additives. A synthetic blend (4:1) and an oil change every 3 months/5,000mi is more than adequate maintainence for a standard (non-high performance) car under normal driving conditions.

But the number one killer of automobile efficiency? Under-inflated tires and misaligned wheels. Check your air pressure.

Stranger
Reply With Quote
  #6  
Old 02-08-2005, 03:16 PM
Anaamika Anaamika is offline
Guest
 
Join Date: Nov 2001
Quote:
Originally Posted by Stranger On A Train

But the number one killer of automobile efficiency? Under-inflated tires and misaligned wheels. Check your air pressure.

Stranger
I am proud that this is one young lady who checks her own air pressure. And when the air gets low, I take it down myself, put my quarters in, fight with the hose, and fill my tires up with air.
Reply With Quote
  #7  
Old 02-08-2005, 04:04 PM
Rick Rick is online now
Charter Member
 
Join Date: Aug 1999
Posts: 15,235
You could be the perfect woman!


Another reason for decreased mileage is air leaks in either the intake or the exhaust. Any air leakage will throw off the calibration of the O2 sensor and will cause the car to inject more fuel for a percieved lean condition. Many times an exhaust leak will be so small it cannot be heard.
Reply With Quote
  #8  
Old 02-08-2005, 05:49 PM
Mr. Slant Mr.  Slant is offline
Guest
 
Join Date: Nov 2001
Anaamika:
According to my readings of the long-term testing conducted by one major US automobile magazine during the mid-90s, the phenomenon you described does not exist.
Cars tested routinely got better gas mileage after 2 years and 60,000 miles than they had when new. They also posted lower 0-60 times, lower quarter mile drag race times, and higher speeds at the end of a quarter mile drag race.
Properly maintained cars probably experience a modified bell curve for gas mileage.
Meaning, when the car is new, you take a small MPG hit from break-in stress.
Later, when the car is completely broken in (not until 50,000 miles or more in the case of some GM V8s), your vehicle would achieve peak efficiency.
From the end of break-in until the beginning of mechanical wear-out, you probably experience your best mileage. Then, as your engine begins wearing out, your gas mileage would begin to decline again.
I will point out that "wearing out" seems to happen somewhere on the high side of 200,000 miles for most any properly maintained motor in a production passenger car available for retail purchase in the US. This statistic excludes failures due to "bad luck" like your motor oil getting filled with anti-freeze.
Reply With Quote
  #9  
Old 02-09-2005, 01:29 AM
Stranger On A Train Stranger On A Train is offline
Guest
 
Join Date: May 2003
Quote:
Originally Posted by Jonathan Woodall
I will point out that "wearing out" seems to happen somewhere on the high side of 200,000 miles for most any properly maintained motor in a production passenger car available for retail purchase in the US. This statistic excludes failures due to "bad luck" like your motor oil getting filled with anti-freeze.
Fwew! That's a new one! Has this actually happened to you?

I did have one oil change place fail to put the farkin' drain pan plug back in place after dumping out the old oil. The oil pressure light went on before I got out of the parking lot. "Oh," says the turk who was too stoned to actually screw the thing back in. "Sorry," says the manager of the place, and offers me a coupon for a free oil change. Like I'm going to take my car back there. Yeah.

I'm glad that was the POS Corsica, and not a real car. I change my own damn oil now. Ease of maintainence actually something I look for in a car, and I have to say that Subaru pretty much has everybody else beat in that regard. JMHO.

Stranger
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 06:49 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.