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

[Atheria]

I bought a 2013 manual transmission adorable Fiat 500 on March 17th and am in LOVE with this tiny car! Her name is Sophia...as in Loren...after all, she's Italian. ;-)

Anyway, the car is rated at 31/40 but the salesman swore on his mother's grave that some of his customers were getting 45 mpg. I was totally excited when after my first 117 miles, I got 48.5 mpg! And that wasn't even with perfect driving...some heavy traffic. Since then, I've been extra diligent about throwing the car into neutral and coasting as much as possible, etc. I was expecting to get 50 mpg on this fill-up, but nooooo....I averaged only 38.2 mpg! How can that be?! I've done the same percentage of highway vs. city driving I think, or at least approximately. I've not been gunning the car or anything. I can't figure it out and am really upset. I would have kept my 35 mpg 2008 Scion xD and saved $10,000. Although I love this Fiat for other reasons....money is money.

Now Fiat recommends 91 octane gas but says you are safe as long as you used at least 87 octane. Being cheap as I am, I got mid grade 88 gas. Is it at all possible that I could lose 10 miles per gallon by not using premium gas? That sounds weird to me. I just can't figure out what else could have affected my mileage that dramatically.

Thank you,
Atheria

[Frank Lee]

Tank-to-tank fill error; environmental changes; route changes.

[Atheria]

My routes are basically the same. It's a little colder but not enough to lose 10 mpg. Tank to tank error? I made sure both times to stop filling the moment the handle popped instead of trying to shove in extra gas.

[Frank Lee]

Get a few more tanks through there before getting too excited.

Check the tire pressure.

Does everything sound OK?

[Atheria]

I am panicking with the first 326 miles. :-)

[gone-ot]

GASOLINE -- the 91 octane was 100% pure gasoline while the 87-88 octane probably contains 10-15% ethanol, leaving only 85-90% pure gasoline, and that means lower ENERGY (BTU per volume) content, which manifests as reduced milage.

...also, one tank-full does not a "trend" make!

[Atheria]

I didn't realize that 91 octane doesn't contain ethanol! I know that regular unleaded here has 10% ethanol so I'm assuming 88 octane has it too. Who buys that E85 I'd like to know? In answer to Frank's question, the engine sounds fine.

Thank you. I will use up this full tank and try 91 next time to test it.

Atheria

[H-Man]

Your engine may have a knock sensor that is being triggered by the lower octane.

[Cobb]

As an insight owner I see a 5mpg different between tier 1 and tier 3 fuel. At that it takes a hit in colder weather too. Much of the US has had colder weather, has you? The computer too maybe off, the Insight is 2mpg+ than actual.

[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.