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Is your LS an automatic or manual? The automatic LS has abysmal MPG ratings, 22city/35hwy and 27combined. Even though your drive is what your calling all hwy, the distance driven 10miles means your engine/cat are not up to full operating temps. Short trips constantly would get a MPG number somewhere between your city & combined rating.

If your outside temps are above 80degrees try running some mid grade 89octane or even buy some gas with 10% ethanol(better engine knock prevention). Where did you find 87octane with no ethanol? Only stations that sell that here are not top tier.

 

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I would agree to change except that I've been running 87 for three months and this plummeted out of nowhere. THAT is my primary concern.

What has your weather been doing? Is your car parked in the sun? If your weather is anything like the weather here the temps have been 80+ degrees daily and if parked in the sun the car is easily 120+ degrees.

There is also the chance you got some bad gas.

 

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That was actually with a different gas station. I had to fill up before the trip.

I love this car, I do, but I am really irritated as to why the mpg dropped so far so quickly. Ugh.

You never did mention where you happened to find no ethanol 87 octane. See one thing about filling at no name stations they buy their gas from the cheapest place they can so you will never get consistent quality. What you might be experiencing is gas from one of their crappier suppliers. At least with top tier or a name brand that should not be an issue.

I would also not be using no ethanol 87 octane in anything other than a lawn mower and this is coming from a guy who runs 91 octane no ethanol 99% of the time. See once the temperature is above 85-90 I switch to 89,90 or 93 octane with 10% ethanol for the better high temp knock prevention.

What engine temps does the 1.8L run? Is it as hot as a 1.4T? Suspect that low octane with no ethanol runs like crap when its hot.

 

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I am now waiting to see if the tune will improve that even!

A tune will not improve your MPG, with your short trip with an always cold engine moving farther away from the park & ride would be the only improvement you could make.