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Civic Hybrid '08 - Went from 36 MPG to 30 MPG? Please Help!

The tires are listed as ‘LRR’ tires, so I’m not sure they’re the problem.

FWIW . . .

I recently had Costco install some Michelin Premier tires on my car

Apparently they are “low rolling resistance” tires . . . I couldn’t care less, that’s not the reason I chose those tires

I calculate my fuel economy the correct way. Fill up 'til it clicks, then stop. Reset the trip meter. When it’s time to fill up again, divide distance traveled by amount of fuel required to fill up 'til it clicks, then stop.

I never continue filling after it clicks

Anyways, those “low rolling resistance” Michelins have made absolutely no change to my fuel economy, versus the Goodyears they replaced. Not even a tenth of a mpg difference.

It’s also possible OP’s main battery pack is failing, in spite of no warning lights. I know when the batteries get weak, the system is designed to rely ever more on the ICE, which naturally lowers the overall fuel economy. Perhaps the threshold has not yet been reached, for a warning message or fault code to appear

Unless I hear otherwise, I’m going to assume OP’s car has the “updated” software . . . the version which is programmed to reduce the number of “idle stops” and also relies more heavily on the ICE, in an effort to prolong the life of that rather pathetic main battery.

In a cruel twist, it’s possible the vehicle is actually behaving correctly, as per its programming

There’s nothing particularly miserly or fuel efficient about those Michelin defenders. Considering they’re also standard replacement fare for tons of other non-hv and gas guzzler vehicles, that leads me to believe they’re not the “correct” tires for OP’s purposes and fuel economy goals

Could also be there’s more than one problem

Nobody here places any weight to this interesting clue.
Something could have happened when the tires were changed.
I really doubt a change in formulation of the same model tire could lower mileage by nearly 20%.
Could botching a tire change upset wheel alignment (as I speculated in a previous post), or cause a dragging brake?
Did they do any other work when the tires were changed?

FWIW,

For several cycles, I’ve used Goodyear Assurance all season tires on my '98 Civic. I like them for their performance in the rain. They usually started to dry rot before they wore out, so the last time I bought tires, I went with another brand, Firestone’s cheapest tires available. I think they were about $39 each. Pretty much simultaneously, my fuel economy dropped. I admit the car also needed a new head gasket at the time, so there were other relevant factors, but even now that I have the head gasket fixed, the fuel economy isn’t quite what it was with the Goodyear Assurance tires. I’m thinking next time I need tires I might try Goodyear’s Assurance Fuel Max tires.

P.S.- I measure my fuel economy the same way you do, although the reset button on my trip odometer has stopped working.