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


My mpg have changed overnight from 35-36 to 30. I currently have 180,000 miles and I’ve mostly taken this car to the dealership for repairs and regular maintenance. I’m hoping to get this car to survive to 250,000 miles.

Here is a step by step of my situation.

  1. July 1st, I change all 4 tires to the same brand and model I purchased a few years back. Mileage magically drops to 30 miles per gallon.
  2. August 1st, I complain to Costco and they replace all 4 tires. Mpg issue persists.
  3. I need regular maintenance from Honda so I go in and mention my car has been sputtering infrequently up hills, on a scale of 1 - 10 I rate the sputtering a 2 or 3. I can almost here like liquid slurping around in the engine.
  4. Dealer says I shouldn’t wait till 200K miles to put in the new spark plugs, so they perform a Valve Adjustment and replace the spark plugs, as well as do a fuel induction (add additive, clean system).
  5. They do the job and I’m still getting 29mpg…
  6. next step?

At this point my dad is telling me it might be the coils. We checked the computer and there are no error codes.

Should i spend money on the coils? Is it just the tires from Costco are crap? I want my 5 - 6 mpg back. I’ve been calculating how many miles I’m losing a tank and its a lot. I drive about 300 miles a week.

I hope you can help!

The two things to have checked when a sudden drop in fuel mileage is realized are, a faulty coolant temp sensor for the computer and a thermostat that’s stuck open.

If the coolant temp sensor is telling the computer that the coolant doesn’t reach operating temperature when it actually does, the engine will use more fuel.

If the thermostat is stuck open where the coolant can’t reach operating temperature, the engine will use more fuel.


@Tester-This vehicle being a hybrid could it be as simple as the main battery getting weak and having the gas engine operate more. I always wonder when someone states a consistent mileage figure how they are figuring the MPG. I know on our main vehicle a trip to the Doctors office two times on one tank gave us a MPG 6 miles above the previous tank that had only in town short trips.

Hi Tester,

Wouldn’t the analyzer or computer have triggered the check engine light if it was either of those?

Not necessarily.

The coolant temp sensor is one of the primary inputs into the computer. So a faulty sensor might not cause the Check Engine light to come on.

Just like a crankshaft position sensor is a primary input into the computer. And when those fail, they don’t always cause the Check Engine light to come on.


Thanks Tester, it looks like something I should be able to do on my own. I haven’t had to work on my car since high school but it seems simple enough. Hopefully its not expensive!

Hybrids require special low rolling resistance tires. Did you buy the same tires that came with the car? If not, that could be your answer.

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I bought exactly the same tires as I had before, so I didn’t expect the drop off from the tires. The only thing I can think of relating to the tires is they changed their manufacturing process somehow and its not the same model anymore despite it being sold as that.

Tire pressure the same???

Yes I checked the tire pressure. It has the correct PSI.

The fact that the mileage dropped so dramatically right after the new tires makes me suspect those. I have a friend who purchased a Ford Focus and it never got anywhere near the EPA mileage. It had some very good tires on it but he had no need for tires of the rating that came with the car for commuting to work. He eventually wore them out and bought some fuel saver tires. His mileage immediately jumped to the EPA rating or a tad bit higher.

Like you said, they may have changed something about the tire even though it is sold as the same tire. It may not be the same as your old tires.

The sputtering is something to be concerned about. Did this start It is probably fuel or spark. You might check and see what the fuel pressure is when the engine is under load. If you have never changed the fuel filter, you might do this for good measure. It might also be an in tank unit and no-replaceable. If that is the case, replace the sending unit and pump as well as they are also old. It might be spark related as well.

One sensor that I have seen not throw a code when it is getting out of range is the oxygen sensor. It seems these must be about completely dead or have a broken wire to throw a code and trigger a check engine light. If this has never been changed, I would do that first thing as it is surely hurting your mileage by this time.


It’s possible they upset the wheel alignment when they changed the tires.
I can’t picture how they would do this if all they did was remove and replace the wheels, but it might be worth it to have the alignment checked.

I suppose it’s possible the tires you bought were redesigned, so perhaps the same make and model of tires has a higher rolling resistance, along with better traction?

If your thermostat is the original, it might be stuck open.

I suppose it’s possible the hybrid battery pack is starting to malfunction, but wouldn’t that lead to some kind of error message on the dashboard?

Maybe your tire gauge is worn out and malfunctioning, leading to you having lower tire pressure?

What IS the make and model of the tires you bought? And the size?

As @whitey says above, an original thermostat is almost certain to start showing signs of failure by 180K miles. That’d be my first suspect if it is the original thermostat, or one that has been in place over 75K miles. . And it is usually pretty easy to check, just ask your shop to remove it and take a look. Any obvious signs of distortion, replace. Before installing it again, or installing a new one, test it in a pot of water on the stove that it opens at the right temp and the spec’d dimension.

This is my tire:

I’ll check out the thermostat this weekend. I’m going to have to youtube and see if I can do this myself, it seems easy enough. Hopefully its this.

I’m sure its going to end up being a Michelin manufacturing process error and in 1 or 2 years I’ll get a settlement from the impending lawsuit with a $5 off coupon off a purchase of $500 or more…

As long as the tires are the same size and pressure as the previous ones, I doubt they could cause that dramatic a drop in fuel economy. You need to address the problem with the engine not running properly, as noted by the others.

Those tires were never OEM tires on this car

As such, unless you have the specific OEM tires on the car, I’d say all bets are off, as far as fuel economy goes

These are the OEM tires for your car

We have several 2008 Civic Hybrids in our fleet, and I know for a fact they never came from the factory with Michelins

As you know, hybrid fuel economy can drastically decline, if you put on regular tires

AFAIK . . . Michelin defenders are fairly standard tires. Unless I’m completely mistaken, there’s nothing that would make them particularly suited to hybrids and super high fuel economy

I still think maybe your old set was specifically for hybrids, and your new set is not, in spite of the tire shop’s response to the contrary

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That’s the best theory I’ve heard so far. If everything else checks out, the OP should try different tires when the current tires wear out. The OP might find his fuel economy improves as the tires wear, but he should make sure the next set of tires are specifically made for hybrids. They will have less rolling resistance.