In the LX with AC on, you manually set the temp. My assumption is
that this just mixes in a variable amount of air from the heater
core with the conditioned air from the evaporator, and it doesn't
affect the compressor operation at all (i.e. - make it cycle more
often, or whatever). Is that right? So if AC is on, it's always
Also, I drove from Dallas to Tulsa yesterday after filling up. For
the entire 240 mile trip, the car reported 45.9 MPG. I did have a
tail wind, and was using Economy mode, whatever that is, but still,
it's uphill. :-) And it said the remaining range was 270 miles.
So I'm kinda wondering about the MPG calculations.
By the way, checking the odometer against the mileage markers on
the turnpike, I find that the Honda slightly under-reports the true
mileage. It's the first car I've ever owned that did that.
No. The compressor cycles on and off to maintain the evaporator within a
certain temperature range. In the case of your Honda it may do this by using
a low side pressure switch (evaporator temperature and pressure are related)
or with an actual thermostat or both. In any case, the compressor runs only
long enough to drop the evaporator temperature to a certain level and then
is disengaged. Eventually the evaporator temperature rises to the upper set
point and the compressor is cycled back on. The lenght of time the
compressor is off is related to the volume and temperature of the air
directed through the evaporator. When you adjust the temperature setting
higher, less air is directed through the evaporator, so it takes longer for
it to be heated up to the upper set point, and therefore the compressor
stays disengaged for a longer period. Of course if the inlet air is very
warm, and the temperature selector is set very low and the fan is on high
speed, the compressor may be forced to run continuously in an attempt to
maintain the evaporator temperature within the intended range. Some vehicles
use a variable displacement compressor (but not your Honda). Instead of
cyclying on and off when the AC is engaged, these compressor attempt to
maintain a certain evaporator pessure (and thus temperature) by varying the
flow rate of refrigerant. This is done by altering the dispacement of the
compressor. These types of compressors are always "on" when the AC is
engaged, but the amount of power they consume varies drastically depending
on the tempearature and volume of air flowing through the evaporator. If the
load on the evaporator is low (minimal air flow through the evaporator), the
compressor adjusts the flow of refrigerant to a very low level and consumes
less power. In theory this style of compressor is more efficient, but in
practice I think the advantage is minimal and the compressor is much more
I don't trust the on board mileage computers. I have a Fiesta. I make the
same 23 mile trip to a job 3 or 4 days a week. Sometimes the mileage
computers tells me I am getting 43+ mpg (this morning for instance) and some
days it reports 36 or so. No difference in the route and almost no
difference in the traffic. I keep a gas log book for the car and althoguh I
do see tank to tank variation, these are of a much smaller magnitutde and
easily explained by variations in how completely I fill the tank (only a 1/4
gallon error when you only buy 6 or 7 gallons at the time is significant -
the difference between 34.6 and 37.5 mpg). Last month we drove the Fiesta to
Florida (600 miles). After every fill-up the computer would tell me
something like 425 miles to empty. However, by the time I had gone 50 miles,
it woul be telling me 350 miles to emply. On the otherhand, it does better
at the bottom end - I once didn't buy gas till it told me I had ten miles to
empty. When I filled up, it took slightly more gas than the advertised
capacity, so at least when it gets close to empty, I have a reasonable
indication of how much further I can go.
I have had cars with odometers on both sides of "correct" based on
Interstate mile markers. I like to check mine againt a GPS over a long
distance. Worst car I've checked lately was the SO's RAV4. It over reports
miles traveled by nearly 5%. The Fiesta is about 1/2% over reporting. My old
Ford Fusion under reported by almost 2%. Of course changing tires can have a
significant effect. Even for tires of the same "size" it is possible to have
significant variations in the rolling radius and thus the miles reported by
the odometer. For instance, the OE tires on my Fiesta are reported to
require 862 revolutions to go one mile. I checked at Tire Rack and tires of
the same size (185/60R15) have reported revoltions per mile from 862 to 881.
Today my odometer indicates around 100.5 miles for every 100 actually
traveled (based on comparisons to my GPS and consistent with mile marker
comparisons). If I change to tires with a revolutions per mile rarting of
881, then I would expect that the odometer error would grow to the point
that the odometer would indicate 102.7 miles for every 100 miles actually
C. E. White says...
Thanks very much for the reply, Ed.
> No. The compressor cycles on and off to maintain the
> evaporator within a certain temperature range.
Ok, so I might actually save a bit by using the warmest
comfortable setting, but at the risk of wearing out the
compressor clutch sooner.
> I don't trust the on board mileage computers.
Well, this is my first one, so I have nothing to compare it
to. Do you have any idea over what period of time, or
miles, or trip meter, the calculation is done?
And is it fair to assume that it measures actual fuel
useage through the injectors, and doesn't try to measure
changes in fuel remaining in the tank?
On the return trip I got 39.5 MPG. That's a bit downhill
overall, but I had no tail wind. If the 45.9 mpg
measurement was accurate, the only explanation would be the
I believe (but don't know for sure) that it uses injector "on time" and the
odometer input signal to estimate fuel consumption. I feel certain it also
uses the signal to the fuel gauge to estimate miles to empty.
Tuesday morning my Fiesta claimed 41+ mpg on the drive to work. This morning
it claimed 38. Same car, same driver, same road, close to the same speed. No
wind either morning. I think the estimate just varies that much (if the true
mile is 39.5, then the range from 38 to 41 is only +/- 4%, which is pretty
good for what I feel certain is an indirect estimation of fuel economy).
Lot of things can effect you estimated and actual fuel economy:
* air temperature (cooler air is denser, increasing aerodynamic resistance
at high speed) - minor factor
* altitude - thinner air, lower aerodynamic resistance - minor factor
* wet roads (significant moiture on the road can increase rollling
resistance) - minor factor
* snow covered roads - pushing around snow or slush takes energy -
* actual energy content of the fuel being used - gasoline is a blend of
compounds, these days including ethanol, the energy contnet can vary by
measurable amounts depending on the exact blend - signficant factor
* vehicle load (how much stuff do you have in the car?) - significant factor
* tire pressure - can be significant if you let it drop much below the
pressure recommended for the vehicle
* traffic conditions - anything that causes you to use the brakes is upping
fuel consumption - significant factor
* how fast you drive - once you pass 55 or so, aerodynamics are significant
and higher speeds will reduce fuel economy - particualrly significant if you
are driving something big like an SUV - significant factor
* road surface - not all road surfaces produce the same rolling resistance -
minor factor, unless you are on loose sand or gravel
* wind - head winds increase fuel consumption, tail winds reduce it - can be
* terrain - lots of up and down will affect fuel economy - significant
* driver - some people just burn more fuel than others becasue of their
driving style - significant factor
Having said all this, my fuel economy is remarkably consistent if I
calculate average fuel economy over 4 or 5 fill-ups. Single tank average can
vary a lot, partially because of actual changes in fuel economy, but also
becasue of the inability to fill up the tank to the exact same level each
time. Averaging over several tanks greatly reduces the effect of variations
in filling up the tank.
One thing I have wondered about for a long time is the effect of driving in
a "convoy" like situation. For sure "drafting" another vehicle can reduce
the aerodynamic drag on a vehicle and increase fuel mileage. I wonder what
is the effect on fuel economy of being in the middle of one of the high
speed trains of vehicles that develope on busy interstates. I drive down
I-95 from NC to SC several times a year and often find myself in the middle
of lines of cars driving 75+ mph. At other times I have been on I-95 when
there is very little traffic (say early Xmas morning, trying to get back
home before lunch). My gas mileage records indicate that I get better
mileage when in the heavy but fast moving traffic. I've always wondered if
the heavy fast moving traffic reduced the aerodynamic resistance on the car
and therefore increased the fuel economy. Note - I tend to drive faster in
the packs that develope on I-95 than I do when I am on my own....
Cooler air means a cooler starting temperature for the engine
increasing time for engine to reach effcient operating temperature.
On trips under 20 minutes, can be a major factor.
If it is really cold, it will be a really big factor even for
long trips. At -20C engine may never really get to efficient
I really don't think it safe to be close enough that "drafting"
will make more than a minor difference. I.e. if you are close
enough for the drafting effect you are way too close. However,
merely staying a safe distance behind a big truck is still likely
to improve mileage
a) you likely are driving more slowly (reduced drag).
b) you are likely driving in a smoother manner. Acceleration
(or trying too hard to maintain speed while climbing)
makes the engine use a richer mixture (more power
but big drop in efficiency).
vehicles with oxygen sensors and electronic fuel injection don't go
"rich" until close to w.o.t. [wide open throttle]. the reason you use
more gas on a hill is usually because of the greater energy output with
the wider throttle opening, not because the mixture has become richer.
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