How could I lose 10 mpg in a week of BETTER driving?!

[H-Man]

I don't know, I'm just guessing at this point.
38 on regular is a bit low IMO, so that is why I considered it.

[wickydude]

Not all tank guns switch off at the exact same point, in my experience.
I stop when I see the fuel reaching the edge of the opening. That way I'm sure it's always filled to the same level.

[redpoint5]

I noticed relatively poor economy when I switched from 92 PON to 87 PON in my TSX. The after the second fill on 87, everything was back to normal.

The other theories as to your drop in mpg sound plausible too. Check the pump next time and see if premium contains ethanol, and what percent. Since ethanol only contains about 2/3 the energy of petroleum, that can make a big difference in fuel economy.

In Oregon, all fuel contains 10% ethanol, regardless of octane rating.

[Cobb]

If the fiat is like the smarty and required preminum fuel then you should use it since its a new car that is something that could even void your warranty.

I go to sams club and get 93 octane for my insight. Its not required, but it gives a little boost in mpg and maked for much better perforformance in agressive driving.

[RedDevil]

My previous car, a Nissan Almera, was hard to fill to capacity on the level and would take over 2 gallons less when the side with the filler cap was slightly lower.
Used to park it on the wrong side when sloping towards the pump. Maybe this applies too to the 500?
Also cold weather, esp. cold starts, really ruin FE. If your commute is shortish that alone would explain the fallback.
Bet you will get 50+ mpg from that baby by summer

[RobertISaar]

if Fiat themselves recommend premium fuel, use it.

anything with a functioning knock sensor can run on 87 octane fuel, but if the calibration and engine is setup for premium, you WILL see a drop in efficiency.

[ksa8907]

isn't that a turbo 1.4? use the premium. our pacifica recommends 89 and if we use 87 it drops 1-2 mpg.

i had to dig, but the non-turbo 1.4 has a cr of 10.8:1, turbo version is 9.8:1

anything over 10:1 needs at least mid grade, but if they recommend premium you should use it.

[Saks]

Originally Posted by Atheria

If a knock sensor is being triggered, how can I tell? I've never even heard of a knock sensor.

Yes the temperature in Albuquerque went down the past week. It was only 17 degrees this a.m. but warmed up into the 50s.

A 5 mpg difference between premium and regular is sizable. It's worth noting.

My Scion xD's avg. mpg. sensor was about 2 off also. My new car is confusing and I am still trying to figure out all the gauges and buttons.

A knock sensor is a sensor that can sense if your engine does have bad combustion, and that can lead to harming your engine components. So if a knock sensor is triggered then it regulates the spark on the spark plugs trying to fix that bad combustion. Usualy it can lead to bad combustion because of lower octane fuels (the bigger the octane the less chance the engine will have a bad combustion). So that's the first thing.
The second thing you should know that on newer cars while driving, if you put the car in neutral and let it flow, it spends fuel because it needs to keep the engine running...but if you leave the car in a gear and just release the accelerator pedal and let it float the fuel consumption will be 0! Because the car uses the inertion to keep running (to understand to keep the car running a engine has to use fuel, to rotate the crankshaft then the gearbox and then the wheels...but as I said if you leave the car in gear and just release the accelerator pedal then this whole process is going backwards...then the wheel are rotating the gearbox and the crankshaft and the result is, no fuel is needed to keep the engine running). But as I said this system is only on newer cars, and your Sophia is one of them

[Atheria]

So during this morning's 22-23 mile cold commute, after finally finding the current time avg mpg sensor, Sophia was getting up to 48! I ended up mostly in the 45-46 mpg range. My car has a mind of her own...but don't all women? ;-) What IS different between my Fiat Sophia and 08 Scion xD named Bob, is that with Bob the mpg sensor would jump to 99.9 if I coasted in gear, but if I do that with Sophia the RPMs stay up and the mpg doesn't improve. My mpg only goes up coasting in neutral. She is not a turbo, FYI.

I'm curious about my next fill up.

Thanks all!
Atheria

[H-Man]

Odd. My Prizm has DFCO that works.
I tested for it by keeping the car in gear with my foot off the pedal and turning the ignition to accessory, I felt no difference in acceleration.
Your car may be cold or simply needing to be broken in.