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Discussion Starter · #1 ·
I have been reading about fuel consumption on this forum and it strikes me as odd that many members seem to think the car's fuel consumption calculation is somehow "off", and they are relying instead on the gas station pump.

WHY?

I would much rather trust a new Lexus's computer system to tell me how much fuel I'm consuming rather than a gas pump that has likely been beat-up over years of use in all kinds of weather. Are you sure you want to trust an oil company's meter reading!? They're not out to maximize profits at all, are they?

In addition, how do you know the pump is stopping at the exact same point every time you fill up? The auto-stop feature varies by location all the time, and how far you insert the nozzle, etc.

And how do you account for variation between pumps at different locations?

As an engineer, I cannot imagine how a rusty gas pump would be more accurate than the car's computers, which precisely measure miles driven and fuel injected into each cylinder.
 

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I have been reading about fuel consumption on this forum and it strikes me as odd that many members seem to think the car's fuel consumption calculation is somehow "off", and they are relying instead on the gas station pump.

WHY?

I would much rather trust a new Lexus's computer system to tell me how much fuel I'm consuming rather than a gas pump that has likely been beat-up over years of use in all kinds of weather. Are you sure you want to trust an oil company's meter reading!? They're not out to maximize profits at all, are they?

In addition, how do you know the pump is stopping at the exact same point every time you fill up? The auto-stop feature varies by location all the time, and how far you insert the nozzle, etc.

And how do you account for variation between pumps at different locations?

As an engineer, I cannot imagine how a rusty gas pump would be more accurate than the car's computers, which precisely measure miles driven and fuel injected into each cylinder.
I was concerned about the gas consumption (have posted here about it). The car's computer was showing 6.1L/100Km (38.56mpg). At my first fill up the car took 32.353L of gas and the odometer showed 525Km. A quick calculation shows the consumption to have been 6.16L/100Km, which is fairly close.

I'm still not clear what the gauge shows (whether it's the average since the time the car was first driven or some other time) - I'll have to read the manual on this. In my case, however, where the car was brand new, I don't think it mattered.

Now, why is my consumption so high, it's a different matter...
 

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I've been using the same gas station about every time, and it constantly shows the milage off by about 2.5-3mpg. Instead of 40mpg, I'll get 37.5 or so.
I'm with SurgeCT on this one. Your gas stations pumps might be miscalibrated to charge you more, e.g. give you only .9375 gallons when it says 1.000 gallons. How accurate are your pumps? When were they last inspected and calibrated? Do you ever get gas at another station and if so is it off by the SAME 2.5-3 mpg ?

You also realize that tire circumference and frictional slippage affect the odometer readings ? Tire circumference can vary by wear and/or changing tire air pressure.

Accuracy is only significant is all variable factors are absolutely controlled. Dividing a numerator of one significant figure and dividing it be a denominator of one significant figure does not mean the result is more significant than one significant figure even if you carry the calculation out to many more decimal places. Filling a gas tank and reading an odometer are not the most accurate and controllable metrics. However, significantly more accurate than the one forum member who was lamenting about his mileage based on the 3/4 empty mark on the gas gauge

YMMV, Happy motoring in my CT cruiser,

MidCow3
 

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I don't know... that article said around 6% of them were off. It's not like 95% of them overcharge you. Even then, they are only slightly overcharging everyone by a few pennies.

I don't think there is a more reliable method than reading the pump and odometer. I can try another gas station down the street but I can't imagine it's going to be much different. Every single car I've had the trip computer overestimates the mpg.

How accurate is everyone else's trip computer vs fuel pump readings? I can't imagine I'm the only one that's off by 2-3mpg.
 

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Mine's off by a few %, which is consistent with a speedo probably overreading by a few % ... Not calibrated the CT speedo yet so knowledge is lacking there ...

By calibrating the speedo, I mean having an independent satnav in the car, heading off to a motorway and putting a steady 70mph on the car speedo. I then see what the gps on the satnav is saying. That needs a steady speed being held to though, as satnav speeds come from position averaged over a short period (if I was writing the software it would be 5-10seconds...) Recent (ish for the Puma!) results are :
Puma - 75 on speedo, 68 on gps
Focus - 75 on speedo, 72 on gps
Volvo C30 - 75 on speedo, 75 on gps (actually did it as 70/70 I think ...)

So for the Puma overreading by 10%, think of the following logic :
Bad speed signal - speedo & odometer will read high
Signal good, speedo cal bad - trip mpg high, odometer good (they may be doing different things with the data)

Depends how the speed data is coming really and how accurate all the measuring things are. Newer digital speedos should be a lot more accurate than the old analogue "stick on a spring" speedos.

Petrol wise - I fill at different stations and usually get a 3-5% (ish) overread on the trip mpg, which hopefully isolates the gas station thing. One thing I always do is fill up slowly, thinking that if I just fully grab in the pump lever, I'll get froth as well as petrol.

(I think I really confused the pumps in the US when I was over there trying to fill up slowly!)

I wonder - Might be worth over a very long 100mile+ run, zero the odometer at the start, see what it is at the end after following the satnav exactly, compare odometer readout to distance according to the route planner. Reason - calibrate the odometer.

PS What I'm really trying to say is that there's lots of reasons for the numbers to be different :D But without good data from independent sources, you'll have trouble figuring out why ...
 

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Discussion Starter · #8 ·
Notice how the CT (at least the nav version) has a setting to indicate there are new tires installed.
Not sure if it affects to odometer and therefore fuel consumption reading, or just the navigation route guidance.
 

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... Filling a gas tank and reading an odometer are not the most accurate and controllable metrics. However, significantly more accurate than the one forum member who was lamenting about his mileage based on the 3/4 empty mark on the gas gauge
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MidCow3
Ha, ha, that was me reading the 3/4 empty mark :). I did re-do the calculation after my first fill up and it was pretty much dead on with what the car's computer was showing. Mind you, I'm still lamenting about the mileage (38.5mpg) :(
 

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In the US, the department of weighs and measures goes around and makes sure that the pumps are dispensing accurately and applies a sticker stating its accuracy. Have dealt with them in the past in a different industry, and they are sticklers for accuracy otherwise face a steep fine.
 

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Most of the gas pumps now are digital and long gone are the mechanical pumps from most stations. Thus I do think what you pump in is pretty accurate. Today, near 1/8th of a tank left I put in 8 gallons to fill. Miles driven 353 putting the mpg at 44. This is down from my summer miles of 47 mpg. Colder weather or different gas formulations for winter weather, I'm not sure which is to blame for the lower mpg but I am not complaining. In nine months of owning the car i am very pleased with all aspects of it. CT200H Premium, Starfire Pearl, Navigation
 

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For my first three fills I checked odometer-gallons vs the car computer. On the first fill the computer was actually just shy of 1 mpg low of measured. The next fill it was a little over 1mpg high of measured. On the 3rd fill the computer was again slightly higher than measured, by less than 1 mpg this time. That was close enough for me. I don't bother measuring any more. I figure the computer is probably slightly higher than actual, but not by enough for me to care about.
 

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Most of the gas pumps now are digital and long gone are the mechanical pumps from most stations. Thus I do think what you pump in is pretty accurate. .....
The display is digital; the pump is not digital. The pumps still pump fluids mechanically and have to be periodically certified and calibrated
 

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In the US, the department of weighs and measures goes around and makes sure that the pumps are dispensing accurately and applies a sticker stating its accuracy. Have dealt with them in the past in a different industry, and they are sticklers for accuracy otherwise face a steep fine.
Thanks, I was about to write the same thing. I feel the pump method is more accurate when using the same gas station and pump, as well as a consistent pumping method (i.e. how many times you let the nozzle click off when topping off the tank). I keep track of these #'s vs. what the computer is telling me and my #'s have been anywhere from 1.1 MPG to 3.6 MPG off (computer is ALWAYS more conservative in my case), which is 2.3% to 7.4%.

Here is why I don't think the car computer is as accurate. The car computer is instantaneously calculating what your MPG is during that entire tank of gas, measuring (not sure how) how much fuel is consumed during that distance traveled and then doing the math for you at each instance and then averageing it over time...and when you're done with the tank of gas, the average MPG is what's on the display. Call me crazy, but there seems to be a decent amount of room for error here, no?

Now if the car calculated it by measuring only at the very end of the tank (by using a flow meter on the fuel line), by measuring how much fuel was consumed and the total # of miles traveled, then I would think that would be consistent with the pump method.

I'm curious as to others thoughts on this logic? While I agree there is also some small margin of error using the pump method, I can't see how that method would deviate by as much as it has for me, when using the same gas station and pump.
 

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Let's see man versus computer. My money is always on the computer, faster and more accurate.
Also, external factors and unwanted variables can be taken account for and controlled. In any environment, accuracy always depends on accuracy of measurement and controlling the environment to exclude superfluous inputs.

Computers are ubiquitous and we depend on them for everything. We should have never doubted Hal in “2001 Space Odyssey”.

A typical modern car has 30-100 computers controlling the various functions. I personally would hate to go back to a car without any computers or advanced electronics. Why should we tust instantanous mpg and average mpg any less than we trust any of the other myriad of functions computers perform. This is the computer and Internet age!
 

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I've heard other service departments of certain car manufacturers can alter the trip computer data by percentages. If I know it's about 5% too high, they can adjust it. Maybe I'll go to my local dealer and see if they can do this.
 

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Tire Calibration Affects Mileage Calculations

How to change you trip computer if it is off:

 
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