Poor gas mileage on new car

Give it up Matt. Honda is not going to do anything after this much time. You have not posted what kind of mileage you get on a highway trip at a reasonable speed or your mileage range. All you post is the 20 MPG number.
Also waiting until the low fuel light comes on is not a good practice and we all hope you are stopping when the pump clicks and not topping off.

Check for a dragging brake:
Drive at medium to high speed for a few minutes, then let the car coast down to low speed, all without using the brakes (do this in an empty road).
Stop the car, get out and carefully feel the wheels.
If one or more are hotter than the others that brake is dragging.
Also could be wheel alignment, get it checked.
Another possibility is a defective thermostat.

@matt357, I believe they replaced the computer. The car was under warranty (this all occurred in her first year of ownership).

Regarding EPA fuel mileage, all the EPA does is provide the manufacturers with guidelines on how to test their vehicles; the manufacturers do this themselves. There have several reported instances of manufacturers over-reporting their EPA mileage. As I recall, Honda was one of the guilty manufacturers.

Have you checked the air filter and spark plugs? This is a simple task yet important.

Do you know what test the dealer ran to find out it was the computer that was the problem?

Iā€™m guessing the dealer hooked up the scanner, looked at the software version, and realized it was wrong

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Yes, dbā€™s spot on , it wouldnā€™t take much effort for a dealership to verify the software configuration is correct. But a correct software configuration wouldnā€™t rule out that the ECM itself isnā€™t faulty. The only way to test that is to temporarily replace it with another one. The problem with going down that route is that doing that experiment might damage something else, leaving the OP with a problem when before no problem existed. With the amount of electronic gadgets in cars these days, in the absence of the computerā€™s own diagnostic functions, thereā€™s really no economically viable way to verify everything is working 100%.

I would think a bad ECM could be identified by the dealer without swapping it out.

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Do dealerships have text fixtures for comprehensive testing of 5 year old ECMā€™s?

As far as I know, no dealer has the capability of really testing an ECM for a problem. Itā€™s still unclear to me as to whether your mileage is really that bad or not. You seem to be saying 20 MPG both city and hwy. Your car is 6 years old and has a measly 15k miles on it so that means it;s seldom driven.

You infer that most of your open road driving is on dirt roads and you seldom go into this large city that for reasons unknown has no traffic jams. You also state that you donā€™t have the luxury of filling the tank and doing a road test. Come onā€¦

Save some money, Schedule a weekend. Fill the car. Hop on the interstate, go at least 50 miles out, and 50 miles back. Refill tank. Now do the math.

As to Lemon laws that ainā€™t gonna happen.
The ECM should be covered for at least 8 years/80k miles BUT Honda is not going to replace it under warranty on a random guess without a verifiable problem.

Lemon laws? For 3 miles under predicted mpg? Try having some real problems. Some ppl are way too litigious in this country. This is why Iā€™m embarrassed sometimes by other liberals

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To me it isnā€™t a ā€œliberal vs. conservativeā€ problem. To me itā€™sā€™ a problem with our tort laws and that they allow, and by so doing encourage, lawyers to bring damage suits for all form of nutty things in attempts to win yet another ā€œtort law lotteryā€.

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More like self-reliance or dependency. Do we attempt to resolve our own problems with our own devices, or do we want to depend on laws and regulations to resolve our problems? Eventually even those who depend on the laws and regulations to fight their battles will find them suffocating at some point.

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While there is some variation in the exact wording of the Lemon Law from state to state, most of them that I have read contain the following words, or a rough equivalent of these words:
The alleged defect must be one that materially affects the safety or drivability of the vehicle.

Does reduced gas mileage materially affect the safety of the vehicle?
Nope!
Does reduced gas mileage materially affect the drivability of the vehicle?
Nope!

Case closedā€¦

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Wow, that is worse than my Matrix that only once got 22.5 MPG and of course they claimed at 30mpg on the highway for this 2.4L engine. The dealer suddenly did four small recalls and one of them was updating the car computer. When they updated the computer it suddenly can get 3.5 mpg more or 26mpg. To get the highest possible MPG you have to go 60 mph without stops and of course meaning in the highest gear. Air filter must be like new too. Thatā€™s about it these days, No more tuning the timing or the cap/rotor. Stick with the usual spark plugs and replace them at 78% of the highest mileage claimed. A sharp edge is what counts for a spark. This way you save your catalytic over cheap spark plugs too

you may want to review again that sticker that was in the window given the estimated miles per gallon. It will say ā€œUP TO xx city/cc highwayā€. It never guaranteed this performance.

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Maybe Iā€™m missing something but itā€™s two months away from 2017 and the car is a 2011? This is not a new car. Lemon laws and fuel ratings are for new cars, not used cars, so you takes what you gets.

Maybe OP has a 5 year warranty of some type, and wants to make sure all problems are solved while still under warranty. Makes sense. Some inde shops will do a pre-warranty-expiration inspection for this purpose. So that if thereā€™s a problem just under the surface thatā€™s covered by the warranty, the dealer will be on the hook for the repair, not the owner. My sense however is that there isnā€™t a problem. Itā€™s just the way this make/model/year behaves is all.

Filling up a car , noteing the odometer reading and driving until the gas light comes on and trying to figure out how many miles you got on that ankful is NOT the way to measure gas mileage.

You fill up the tank, note the mile reading drive until you fill the tank again, note the miles again, get how many miles you drove and divide by the gallons.

You are driving 3000 miles a year. Your car spends a significant portion of its miles warming up. 20 mpg on a car with an EPA rating of 23 city is a reasonable mileage under those conditions.

The way you are using your car, rust and depreciation are costing you much,much more than gasoline, 3000 miles divided by 20 mpg = 150 gallons times $2.33 = $349.50 a year.

Getting 23 mpg would save you less than $1 a week.