2012 Civic LX - temp control knob, MPG calculation

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 on.
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.
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On 8/7/11 5:55 PM, Peabody wrote:

Did you install different (larger) tires or wheels?
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You sure it's not displaying kilometers?
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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 complicated.

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 traveled.
Ed
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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 tailwind.
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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 - significant factor * 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 significant * terrain - lots of up and down will affect fuel economy - significant factor * 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....
Ed
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On 8/11/2011 8:16 AM, C. E. White wrote:

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 temperature.

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).
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On 08/31/2011 11:14 AM, Alan Bowler wrote:

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.
--
nomina rutrum rutrum

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