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I have a 2007 Civic hybrid. I had been getting up to 55 mpg. I had my oil changed and all new tires installed and aligned and now I get 40mpg. Can someone explain what has caused this dramatic drop in efficiency?

Honda claims the '07 Civic Hybrid gets 49 mpg(city) and 51 mpg(hwy).

Please explain exactly how you calculate the miles per gallon.

Also, what brand and model of tires did you have, and what are the new ones?

Where did you get the oil changed and what kind (viscosity) of oil did they use? Where did you buy the tires and what kind of tires are they?

Assuming you’re calculating mpg correctly (MOST people who’ve asked questions here DON’T) then the culprit could be the tires. The oil change isn’t going to make that much of a difference.

It’s possible that the wrong type, or quantity, of oil was addedm but that shouldn’t impact fuel economy much.

However, the OE tires are a very low rolling resistance (part of the slight of hand to get good mileage) and if your replacements were not the OE, that would have an effect, as would improper inflation pressure. However, it would not affect the mileage by as much as you’re seeing.

Also, depending on where you are, cold weather will reduce fuel economy, as will driving in rain or snow or high winds, so if the weather recently has been poor, that will have an effect.

Was the work done by a dealer? If so, do you know if there was an update to the ECU?

It could simply be that your old tires were so worn that they were actually smaller. It happened so gradually that you probably didn’t notice your fuel economy estimates creeping upward, or you attributed the increase to something else. Then you bought new tires and that gradual change suddenly reversed itself. The new tires are larger since they are not worn. So you are actually travelling a larger distance per tire rotation. The computer that tells you your fuel economy on the dashboard doesn’t know you have switched to tires with a slightly larger diameter than the old ones. Perhaps you were never really getting 55 MPG. Perhaps your car’s computer thought you were travelling farther than you actually traveled.

If I had to specualte, I would say it is probably a combination of tire size and rolling resistance. I would expect it to gradually get back to normal after the new tires are broken-in.

If tire wear is the cause, your actual fuel economy didn’t change that much. Only your calculation of your fuel economy changed because the car is travelling farther than it thinks it is traveling. If rollikng resistance was the only cause, your fuel economy did change. However, I think it is a combination of the two causes.

Are these MPG measurements coming from the dashboard read-out or are you doing the calculations manually?

Measuring miles per gallon can give you deceptive results. You are really better off measuring gallons per mile, or gallons per 10,000 miles. An increase from 30 MPG to 40 MPG will save 83 gallons per 10,000 miles, while an increase from 10 MPG to 12 MPG will save 167 gallons per 10,000 miles.

I say this from several years experience in the field of Metrology:

If the OP is measuring MPG the same way before and after the oil/tire change and he was getting consistent numbers then it doesn’t much matter how he measured MPG. It is possible for a measurement to be offset but repeatable.

I always assume hybrid owners are using the dashboard display rather than measuring and calculating fuel economy manually.

Answers to questions: The oil change and tire change and alignment were all done by the Honda dealer where I bought the car. I calculate mileage by filling my tank and setting the mileage trip marker to 0. When I fill the tank again I record the number of gallons to the tenth and then use the actual mileage and actual gas used to make the calculation. The calculation reported was based on the tank just before the oil change/tire change and the tank after the change (as well as all subsequent changes). Most of the summer I got 49/50 and only the last two were at the 55 level. I am now hard pressed to get 40/41 and of course it is now dropping even more with the advent of cold weather. I got only 27,000 on the factory tires and the dealer told me I would now get 60,000. He never mentioned that my mileage would drop like a rock. Is the concensus that the change in tires is the culprit?

It could have been a very poor alignment, but in addition to the other issues already mentioned, I would suggest that maybe just maybe that winter just hit your area and that could be the difference. BTW what exactly does getting up to 55 mpg mean? does it mean you got between 35 and 50 or between 48 and 50mpg?

I would say it’s the tires, and possibly in incorrect alignment. Hybrids come with special low-rolling-resistance tires. If you replaced them with something else it can have a significant negative effect on fuel mileage.

However, one tankful of gas is not enough to tell anything. A long term average is the best indication of vehicle mileage. A slight variation in driving routine, or weather conditions, from one tank to the next can make a HUGE difference.

Where do you live?

Most of the gas stations are switching over to the winter blend now, which tends to decrease fuel economy. We notice a 10% decrease in our cars during the winter months.

My mileage stayed very close to the 47-50 range for several months. My lower mileage has now persisted through five to six tanks of gas. I don’t think my alignment is off because the car moves in a straight direction when I loosen my grip on the steering wheel. I am getting no vibration. I get gas at several different stations so I suspect the change in fuel is not the culprit. The tires remain the best scientific explanation.