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This is not necessarily a "What mileage to expect" or "how to get better mileage" question but more of a what would cause a drop over 6 months.

I changed my plugs over the summer and could get around 20-21 at 60 mph and around 18 at 71-72 mph. I am pretty good at changing synthetic oil every 5000 miles and keeping the air pressure at around 36 cold. Also, I only use 89 Octane.

I just took an 90 minute all-highway trip and reset my mileage. I noticed it went from 12.6 (From 8 mile 13 stoplight daily commutes) and started climbing from there once I did the reset. It only made to 15.6 on the highway. Here are some thoughts I have as to the fairly substantial drop:

1) Destination ATs are at 35K. Perhaps rubber is getting softer?
2) 18 degrees outside? I thought cold would perform better.
3) Maybe headwind? I was heading east, however.
4) Denso iridium plugs? Changed at 30K now I have 40K. Don't think so.
5) Chrysler's calculations that "learn" driving habits aren't very good?

I was a little frustrated with the results as I have seen much, much better. Any thoughts would be appreciated.

BTW. I found threads on improving mileage but couldn't dial in somewhat sudden (a few months) drop in mileage. As per usual - Thanks in advance for any help.
 

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There are numerous things that cause MPG's to drop, tires- the type, air pressure, unbalanced and just being bigger. The next one could be the air filter type, if it's dirty or it might be a cheap one. The gas that you use, I always try and stick with the same gas station and i never let my gas tank get below a 1/4 tank. Fuel injectors could be dirty, but not clogged there's a big difference. Cold weather??? I don't know.

I get about 17 on the highway and about 15 in the city, but then again I am carry extra weight because the winter time, my emergency box which weighs about 80 lbs, my hi-lift jack, air compressor and other stuff. I don't worry about gas mileage unless it gets below 14.

Just my .02
 
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