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Discussion Starter · #1 ·
Yesterday the wife and I went on a trip. 250 miles each way. Set the cruise control @85. Temp outside was 68 -70 for the ride. MPG average was 35.

Return trip again set the cruise @ 85, temp was 40 -45. MPG's dropped to 28

Ride was on the same roads going the opposite direction, there was no measurable wind either way.

Does my car hate the cold?
 

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Did you check the mileage the old-fashioned way, dividing the actual miles traveled by the gallons used, or are you referring to the car computer's per-trip mileage readout?

My sense is, the computer estimates are ballpark figures, in a really big stadium.

If you're doing the real-word calculation, it's a genuine mystery why the mileage would vary that widely. It would seem the drop in temperature might pull down the mileage a little, but not by that much.
 

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Discussion Starter · #3 ·
Of course I did. Also used 1/2 tank one way 3/4 tank the other.

This isn't about my calculations it's about the car mpg'd dropping off
 

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Of course I did. Also used 1/2 tank one way 3/4 tank the other.

This isn't about my calculations it's about the car mpg'd dropping off
Sounds like you bought gas at your destination, I'd perhaps suspect that as well; also tire pressure probably dropped a bit due to outside temp change (not suggesting this would cause that much of a drop by itself).
 

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Discussion Starter · #5 ·
Gas was bought @ Shell. Put in 93 like I always do. If Tp dropped it would be miniscule. It was checked before I left.
 

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Gas was bought @ Shell. Put in 93 like I always do. If Tp dropped it would be miniscule. It was checked before I left.
If you filled up at your regular station when you got home and the mileage comes back up, I'd still partially suspect the gas, however that is quite a drop even if you were running 87 octane. Unless something did change mechanically I'd suspect it was a 'coming together' of several factors that caused it.
 

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This is probably way off... but all I can base my ideas on would be my own experience...

Air temp will make a difference, but it should be more in the 1-2mpg range... nothing as severe as what you are describing.

Here in Texas we have a "funny" little law - only counties with a certain number of registered vehicles are required to use "oxygenated fuels" (read: high alcohol content) - I can tell you from personal experience that cars running on it get significantly worse gas mileage (dont get me started on the EPA unless you have a whole day).
Which brings me to my question: Does your neck of the woods have a similar requirement? If so, is it possible that the gas from leg 1 was untainted and the gas from leg 2 contained alcohol?
I know you said you got the fuel from Shell... but is is possible that you simply got some funky fuel? Its common around here for gas to vary from station to station even within companies. If your mileage goes back up now that you are home, maybe thats all it was...

Finally... Tweak might be on to something... If the ratio of "uphill" to "downhill" wasnt the same both ways then that could cause the difference...

I think its a mystery wrapped up in a riddle tied up by a conundrum... but its also possible it was a fluke....
 

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With a car that weights as much / has as little power as ours, I'd imagine even slight elevation changes could account for this...
Having driven several hours and dealing with multiple hills along the way I noticed the major impact it had for me so I imagine this is likely the largest cause with the other mentioned factors being partial influences to all come together and result in a significant loss. I also notice with cruise on and dealing with even small hills the 500 will romp on the throttle to attempt to maintain speed which burns a significant amount of gas.
 

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Discussion Starter · #11 ·
I live in South Florida and was driving on Alligator Alley. One of the straightest flattest,most boring roads in the world.

I've also noticed a similar drop in my daily commute, but it's much harder to gauge on a shorter trip. I get less mileage in the morning when it's cool, then afternoon when it's warmer
 

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Discussion Starter · #13 ·
That's what I'm thinking. Into the shop we will go. There has to be a sensor somewhere not working right
 

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That's what I'm thinking. Into the shop we will go. There has to be a sensor somewhere not working right
Do you have a OBD2 scanner? I know you can get the pretty cheap at Pep Boys, etc and it would probably be a worthwhile tool to have and if it shows no codes, you can avoid the time-suck of going in to the shop.
 

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That's what I'm thinking. Into the shop we will go. There has to be a sensor somewhere not working right
as a shop owner let me save you some $$...

save your money for now.

If there was a sensor going stupid you would know - it would set a CEL. Even if it was a "two trip code" it would set a pending code.

One of the little pocket OBD scanners will help to let you know if something is developing and they can be had on ebay for $20 - a lot less than a "diagnostics" at the dealer (which, I promise you, will come back with no useful information).
If you dont wanna wait on mail-order, Autozone, Oreillys, Advance, etc will either run the codes for you or rent you a scanner for free (pay a deposit which you get back in full when you return the tool)

I will tell you what I tell all my customers - once is a fluke, twice is a problem.
 

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Discussion Starter · #16 ·
First it doesn't cast anything to bring the car in and have it checked. Second A false reading may not always throw a code.

I'll let the Studio deal with it
 

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That's pretty bizarre. In the cold the car's engine is making more power maybe youre just sucking down more gas. Also...cruise control is not the best for gas mileage.
 

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Discussion Starter · #18 ·
That's pretty bizarre. In the cold the car's engine is making more power maybe youre just sucking down more gas. Also...cruise control is not the best for gas mileage.
Cruise is the best choice here. The roads I was on have no hills, or over passes, and are straight as an arrow
 
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