2005 Honda Civic LX gas mileage

When I picked up my Honda Civic LX that I just bought on 9/7/2005, it
had a full tank of gas. I drove it over 400 miles, did a fill up,
divided the gallons into the miles and it came out to 39 mpg. I thought
this was great, since I had the AC on at various times, did more city
driving than I normally do, etc. But, with my next fill up, less AC and
less city driving it came out to only 37 mpg. Then 36 mpg with the next
fill up. It keeps going down. The last fill up it was 32 mpg. I don't
do fast start ups or slowing down and I drive the car at 60-65 mph on
the highway with no AC. I now have 3,200 miles on it. I called the
Honda dealer and asked them if they could check it out to see what the
problem is. To my surprise, they said that the better gas mileage I was
getting before was a fluke! What I am getting now is normal. They said
there was nothing for them to check unless I start getting around 19
MPG or the service engine light comes on. They said the engine is run
by computers so there is nothing for them to do. Is this right?

Thanks,

Roland

 

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Keep on dividing the distance traveled by the volume of fuel you put in
.. . That's your mileage. Don't overfill the tank past shut-off. That's
covered in the Owner's Manual.

'Curly'

 

I never overfill. I take the pump out on the first click.

 

I always give it a one more chance. Sometimes they click off by mistake.
But if that was the problem, you would get one high reading, follwed by a
low reading. Over the course of many fillups, a gallon difference wouldn't
be noticed.

I suspect you were driving the car very gently at first, and you're just
driving it harder now.

 

No, I actually have slowed down on the highway. Recently I have been
driving 55-60 mph on the highway. I guess the bottom line question I
have is - is there any way to hook up the engine to the diagnostic or
whatever it's called machine and have it tell you if the engine is
tuned properly? To be specifc, what can cause the engine to not work
efficiently? Clogged fuel injector, dirty fuel filter, air filter, etc?
I know the car is brand new so, none of the engine parts should be
dirty but, I don't know. It just doesn't make sense. I had a 2002 Dodge
Caravan with a 4-cylinder engine and I averaged 24 mpg. Now I have this
little compact car with an even smaller 4-cylinder engine and I only
get 8 mpg more?

 

Your mileage should decline as the weather cools. In a nutshell, the engine
requires more fuel to achieve the same speeds etc. at lower temperatures.

There is a fair amount of variation in these calculations, so it's important
to watch the averages over several fillups for similar driving conditions
and ambient temperatures. I have had seemingly freakish 46 to 47 mpg
calculations yielded for my 91 Civic LX (manual transmission, no air
conditioning) from the data twice in the past 1.5 years, once in each of the
past two summers, whereas the average mpg is closer to 40 mpg.

In July and August, for seven fillups, the mpg varied from 40 mpg to 45 mpg
(45, 44, 41, 40, 45, 40, 41). I almost always put in over 8 gallons but
rarely lately over 10 gallons. It does seem that I get better mileage if I
don't drive as near to an empty tank as possible. (The fuel pump doesn't
have to work as hard, reducing energy consumed to run it? That may be out in
left field.) Also, I have read that driving until the tank is near empty
permits the pump to pick up sediment at the bottom of the tank, which is
generally not good for the engine etc.

My mileage recently slipped a tad below 40, whence I remembered the weather
had cooled and I hadn't checked my tire inflation pressure. Sure enough, all
four were down 4 psi. This won't affect mpg a lot, from my experience, but
my impression is it will be noticeable.

So I'd say with your new car, keep collecting data; be aware that cold
weather will affect mileage; and check your tire pressure at least once a
month, particularly when seasons change.

 

yup. my 98 CX hatch averages around 30. if youre getting 32, thats
pretty good for a brand new car. might get better... who knows?

 

id say with the new car... drive it. enjoy it. be glad you werent
retarded enough to buy a hummer H2.

 

I don't think waiting for the pump to click off on it's own is an accurate
fill-up method, unless you're using the exact same pump and your able to
keep the flow constant. My method is to fill-up until the gas is about to
overflow from the nozzle, which makes for easy spillage if you're not paying
attention.

Also, the filling stations will be switching to winterized gas, which will
reduce the fuel economy by about 5%.

Are you filling up with premium or regular gas? and which filling station
are you using? Personally, I favor Esso for Regular gas and Shell for
Premium gas.

Pars

 

I only "get" this because recently one of my offline buddies was telling me
how his boss had just bought a hummer. The boss paid over $40k for it. He
says it gets 10 miles per gallon of gas. fueleconomy.gov says the 2006's get
16 mpg around town.

Owning "things." Dear lord.

 

The load on the fuel pump is virtually constant. It delivers fuel at
pressure
to the engine compartment where it is filtered and applied to the injector
pump while the volume not required is returned to the tank.

 

The difference in pressure across a pump, among other things like flow rate
and fluid density, is what determines the pump load.

A higher head at the pump inlet reduces the amount of work it must do.

Higher levels in the fuel tank translate of course to a higher head at pump
inlet.

Thus my proposal. Admittedly the difference may be negligible. The theory,
with practical consequences in many applications and being a routine part of
pump design, is correct.

 

My '05 Civic LX coupe with automatic transmission delivers between 23 and
25 mpg in-town driving. Freeway driving will produce low 30's; so I'd say
you're doing good 32 mpg.

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I did say "virtually constant" and you missed a significant factor
(for in-tank pumps in the Honda/Rover cars) which is that the fuel is
sucked into the pump without being replaced by air from the outside
thus reducing the absolute air pressure in the tank. You can verify this
by listening for the 'whoosh' of air into the tank when you undo the
tank's filler cap.

From time to time, the depression is great enough to activate the
ingress of air through the filtered vent used as part of the emission
controls. This causes a strange sort of 'groan' from the valve when
the car is stationary and engine is off.

Since the head at the inlet varies by only a few inches, I doubt that
the difference in electrical load is measurable.

 

Now it's "significant" huh.

Contradicting your earlier statement.

I haven't run numbers but I do /not/ maintain it's significant. I don't
know. I raised it only as a possibility above. The possibility is in fact
consistent with theory.

Now I'm outta this exchange because I know a pig when I read one.

You know what I'm talking about.

Others who have read my posts here: Graham is the one. Open emails from him
with caution.

 

I have never ever emailed a person at your e-address. Please show
the headers of where I did or apologise for making false allegations!

I do not give other correspondants Trojans or Viruses or any other
nasties.

 

I did say "virtually constant" and you missed a significant factor

One should note here that, as long as the tank is not vented to atmosphere,
then the pressure due to the air above the fuel level in fact will be
reducing (contrary to the implication above) as long as the fuel level goes
down. Thus the pressure at the pump inlet will be even lower than if the
tank were vented to atmosphere.

With no venting, even more pump work is necessary than with venting.

It's mere theory, and in this application out in left field, because the
pump is such a low power device, raising say 1 gallon / hour a few psi, well
less than 1 hp of motor power, I'd guesstimate with some rough calculations.

 

It isn't contrary - that is exactly what I said - "reducing the absolute
pressure in the tank"!

Agreed again.

Your figures are a bit adrift though. Yes I know they are guesses but
since the pump needs to raise the delivery pressure at the filter so
that the engine's FI is supplied with enough fuel at the most extreme
of power demands and RPM, the volume at a guess is far higher than
1 gallon / hour. That fuel which isn't consumed by the FI injectors is
returned to the tank through a pressure relief valve. So at 90 mph
the engine will be burning around 3 gal / hr or more and that figure
doesn't include the amount returned to the tank.

Here are some figures to play with. The pump output pressure runs
at 2.3 to 3.0 bar (that's about 33 to 44 psi). At 16volts it can deliver
at nearly 60 psi. Figures taken from Haynes for my Honda engined
Rover 216 16 valve 1.6 litre.

How do those numbers run out at HP needed from the pump?

 

You seemed to be implying this reduced the pump work. On the contrary, lower
pressure on the fuel in the tank increases the pump work necessary to move
it to the injectors, etc.

That junk you posted did nothing more than bolster my original point, but in
a manner that muddies the issue.

Whatever "at a guess" means.

Of course the flow varies. Why would you suggest otherwise, except to be a
jackass?

I don't work with absurd numbers such as 90 mph, but instead numbers for the
typical driver in the U.S.

You're not greeted with seriousness or patience by me, because of that
vulgar email you sent.

 

But I didn't say that.

The same as your guess.

Ah, but the flow remains essentially constant. This is fact that you
have failed to grasp. Any fuel at say 2.5 bar which is not consumed by
the FI is returned to the tank.

Show me the headers! In fact show me the email.

And read more carefully what I have posted - you are trying to twist
what I say to suit yourself.