common reasons for decrease in mpg


Old 02-19-10, 03:18 PM
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Join Date: Jul 2001
Location: Indiana (formerly Massachusetts)
Posts: 323
common reasons for decrease in mpg

i have a 1995 dodge ram 1500 V8 5.9 gas engine. i used to get 500 miles to a tank. lately, i get 400.

what can explain this? i had a tune up relatively recently. i just changed my air cleaner thinking that was it.

is it just winter driving?
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Old 02-19-10, 04:29 PM
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Join Date: Feb 2009
Location: Calumet Township, Indiana
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When did you notice this 20% drop in fuel efficiancy?
Do you have consistant information to estimate how long a time period this drop took place?

'95 Ram 1500 5.9L gasoline w/ auto trans is estimated to get 11 mpg city and 15 hwy
If you have the 30 gallon gas tank and get 400 miles per tank you're doing great.

is it just winter driving?
Cold air is denser and contains more oxygen = better volumetric efficiency. And that's a verifiable opinion.
Old 02-19-10, 04:47 PM
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Location: Hamilton County, Ohio
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Also, the more ethanol, the poorer the mileage.
Old 02-20-10, 11:24 AM
Join Date: Oct 2008
Posts: 1,312
In general, do all the recommended maintenance outlined in the factory service manuals.

These will say at 30,000 miles, do this and that.

45,000 miles do this, that, and something else.


And along in there with the regular stuff will be to lubricate various things. Repack wheel bearings, etc. Service transmission, etc. Check air pressure in tires.

Then a good one is to replace the oxygen sensor every 60,000 miles. When you do this, remove a battery terminal for 10 minutes, then reconnect. Let the vehicle idle for 5 minutes, then drive over 45 mph as soon as possible.

This helps the engine computer to "relearn" the response of the new oxygen sensor. The engine computer will slowly adjust things as an oxygen sensor gets older. Then it would take a long time for it to readjust to the new sensor. By removing power, you clear the memory of the engine computer and place it at factory defaults. And this is of course where it should be with a new oxygen sensor.

I got about 6 mpg more on my 4 cylinder car after changing the oxygen sensor. Note that it had over 100,000 miles on it and the sensor had never been changed.

Then I did the same on a used truck I bought. I didn't notice any change. I think they had replaced the oxygen sensor recently. (Previous owner took pretty good care of that vehicle.)

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