2014 CX5 2.5 fuel econ horrible

[MikeM.]

I'm not upset about the actual number. I am just trying to make sense of the gross mismatch between measurement methods, and the fact that when I overfill the tank, it matches what the computer is saying. When I take the advice from this forum to not overfill, I get inflated MPG numbers. Which is correct?

(1) First of all, it makes no sense that your trip computer would report different MPG results depending upon how full you fill your tank. The MPG displayed by the trip computer is based upon the distance traveled as measured by number of wheel rotations and the amount of fuel consumed as metered by the fuel injection system since the last reset of the Trip Computer. The fuel consumed is NOT based upon the fuel level meter in the fuel tank. Therefore the MPG reported by the trip computer would not vary depending upon how full the tank is filled.

(2) It's not clear why you consider filling the tank full by adding two gallons after the pump clicks off "over-filling". To my way of thinking, any pump that shuts off 2 gallons before the tank is full is out of calibration (has a shut-off nozzle that is overly sensitive). The nozzles at the stations I fill at never leave 2 gallons of extra room at the point they shut-off. I just fill the tank until auto-shut-off kicks in. Experience tells me I could usually fit in an additional quart or two at the most but why bother?


(3) Your belief that the MPG numbers returned by the Trip Computer are "inflated" when you don't top off the tank is odd. Why not assume the numbers returned when you don't top off the tank are deflated? But, as explained above, the MPG reported by the Trip Computer is unaffected by level of fill-up.

With these facts in mind, your "observations" are difficult to take seriously.

 

[MikeM.]

I don't know if my cx5 2.5 awd is defective. ..or if I'm lucky..so far first 3 tanks 32 mpg. 27mpg and another 32. Mine is mostly highway. I'm happy
Edit..this is actual mileage calculated at the pump..not the on board estimated

That's far from defective, just a sign that you are a good driver who is reasonably aware of the driving environment and doesn't do stupidly inefficient things like accelerating on freeway off-ramps (I'm surprised how often I see this) or leave the car idling while running into the store or talking to their friend or neighbor. My MPG numbers started out about the same as yours but MPG has continued to gradually improve over my two years of ownership as the moving parts wear in. To say I'm pleased with the economy would be an understatement.

 

[brandini]

Try filling at a slower speed once you're nearing the end of the fill.

 

[donkeykong]

(1) First of all, it makes no sense that your trip computer would report different MPG results depending upon how full you fill your tank. The MPG displayed by the trip computer is based upon the distance traveled as measured by number of wheel rotations and the amount of fuel consumed as metered by the fuel injection system since the last reset of the Trip Computer. The fuel consumed is NOT based upon the fuel level meter in the fuel tank. Therefore the MPG reported by the trip computer would not vary depending upon how full the tank is filled.

(2) It's not clear why you consider filling the tank full by adding two gallons after the pump clicks off "over-filling". To my way of thinking, any pump that shuts off 2 gallons before the tank is full is out of calibration (has a shut-off nozzle that is overly sensitive). The nozzles at the stations I fill at never leave 2 gallons of extra room at the point they shut-off. I just fill the tank until auto-shut-off kicks in. Experience tells me I could usually fit in an additional quart or two at the most but why bother?


(3) Your belief that the MPG numbers returned by the Trip Computer are "inflated" when you don't top off the tank is odd. Why not assume the numbers returned when you don't top off the tank are deflated? But, as explained above, the MPG reported by the Trip Computer is unaffected by level of fill-up.

With these facts in mind, your "observations" are difficult to take seriously.

Thank you for your reply, but perhaps I was not clear. I never said the trip computer was all over the place on its readings. The different MPG results I observe are regarding the difference in (1) trip computer vs (2) hand calculation. To illustrate, I will lead you through an example.

Let's say the trip computer is saying "0 miles to empty" when I have driven 375 miles since the last fillup, for simplicity's sake. I fill up until the pump auto-shuts off, at 12 gallons. This would indicate ~31mpg. However, the trip computer is indicating ~25mpg. Now, I know that a full tank is 14-15 gallons (depending on FWD/AWD), so I click the pump on again and add in two more gallons which it is perfectly happy to do...no complaints from the gas tank (i.e. spillage) until it clicks off again. Now for those same 375 miles the 14.5 gallons drops my average to ~26mpg, which is wayyy more consistent with the trip computer readout.

I consider the extra 2 gallons "over-filling" because of the advice on this thread to not add extra fuel after the pump clicks itself off, that the tank is full when it clicks itself off, anything else is no longer in the tank but in the "neck".

I suppose I shouldn't overfill at all, and just be happy that I am putting in less gas and hand calculating a great MPG that blows out the trip computer calculation. But the engineer in me can't accept this for the two reasons above, (1) disagreement between my calculation and trip computer and (2) the amount of "extra" space in the tank. It's not an insignificant amount, it's 2+ gallons. On every other car I've owned, if I keep trying to top the tank off, it will keep clicking off almost immediately, rather than letting me put another multiple gallons in. To me, this would indicate a "real" full tank. But hey, nobody said you have to take me seriously.

Next time you fill up, try seeing how much extra the tank will take after it clicks itself off. If you can get another 2 gallons, then maybe it's the design of the CX-5. If not, either it's an issue with the auto-shut off of the gas station I like to frequent, or a manufacturing defect with my specific car.

 

[MikeM.]

Let's say the trip computer is saying "0 miles to empty" when I have driven 375 miles since the last fillup, for simplicity's sake. I fill up until the pump auto-shuts off, at 12 gallons. This would indicate ~31mpg. However, the trip computer is indicating ~25mpg. Now, I know that a full tank is 14-15 gallons (depending on FWD/AWD), so I click the pump on again and add in two more gallons which it is perfectly happy to do...no complaints from the gas tank (i.e. spillage) until it clicks off again. Now for those same 375 miles the 14.5 gallons drops my average to ~26mpg, which is wayyy more consistent with the trip computer readout.

It appears you are saying your hand calculated MPG is inconsistent depending upon whether you fill the tank to the brim or leave 2 gallons of air space?

This would only be unusual if the two different fill levels DID NOT return two different MPG calculations. If you fill the tank to the same level each time the inconsistency you noted will not exist.

My experience has been that the trip computer is very consistent with my hand calculated figures, almost always within 1 MPG (sometimes the hand calculated is higher, sometimes the computer is higher).

I suppose I shouldn't overfill at all, and just be happy that I am putting in less gas and hand calculating a great MPG that blows out the trip computer calculation.

Don't be ridiculous. Your actual MPG for practical purposes is the distance you have traveled divided by the fuel purchased. Averaging this over multiple tanks well minimize or "average out" any inconsistency in your fuelling technique but it's still a good idea to be consistent with your fuelling technique. Fooling yourself that you are getting better mileage doesn't do anything but fool yourself. If you want better MPG you should focus on things like driving smarter when in traffic, avoiding high speed driving, keeping your tires properly inflated, removing roof racks when not in use, etc.

Next time you fill up, try seeing how much extra the tank will take after it clicks itself off. If you can get another 2 gallons, then maybe it's the design of the CX-5. If not, either it's an issue with the auto-shut off of the gas station I like to frequent, or a manufacturing defect with my specific car.

I have already done this multiple times. Like most cars I can usually squeeze in another quart or two. I decided I didn't need my tank that full so now I just stop fuelling when the pump clicks off.

 

[piotrek91]

Thank you for your reply, but perhaps I was not clear. I never said the trip computer was all over the place on its readings. The different MPG results I observe are regarding the difference in (1) trip computer vs (2) hand calculation. To illustrate, I will lead you through an example.

Let's say the trip computer is saying "0 miles to empty" when I have driven 375 miles since the last fillup, for simplicity's sake. I fill up until the pump auto-shuts off, at 12 gallons. This would indicate ~31mpg. However, the trip computer is indicating ~25mpg. Now, I know that a full tank is 14-15 gallons (depending on FWD/AWD), so I click the pump on again and add in two more gallons which it is perfectly happy to do...no complaints from the gas tank (i.e. spillage) until it clicks off again. Now for those same 375 miles the 14.5 gallons drops my average to ~26mpg, which is wayyy more consistent with the trip computer readout.

I consider the extra 2 gallons "over-filling" because of the advice on this thread to not add extra fuel after the pump clicks itself off, that the tank is full when it clicks itself off, anything else is no longer in the tank but in the "neck".

I suppose I shouldn't overfill at all, and just be happy that I am putting in less gas and hand calculating a great MPG that blows out the trip computer calculation. But the engineer in me can't accept this for the two reasons above, (1) disagreement between my calculation and trip computer and (2) the amount of "extra" space in the tank. It's not an insignificant amount, it's 2+ gallons. On every other car I've owned, if I keep trying to top the tank off, it will keep clicking off almost immediately, rather than letting me put another multiple gallons in. To me, this would indicate a "real" full tank. But hey, nobody said you have to take me seriously.

Next time you fill up, try seeing how much extra the tank will take after it clicks itself off. If you can get another 2 gallons, then maybe it's the design of the CX-5. If not, either it's an issue with the auto-shut off of the gas station I like to frequent, or a manufacturing defect with my specific car.

This is how I see your situation:
Fill up #1
1. You overfill your tank and have approx over 16.5 gallons in your CX-5
2. You use 12.5 gallons to drive 375 miles and your indicated range is now 0 ~2 gallons in your CX-5

Fill up #2
3. You fill up your tank to the point where the pump cuts your off 14.5 gallons in your CX-5
4. You overfill your tank, have gas in the filler tube, and some of it might have been sucked back into the gas pump 16.5 gallons in your CX-5

Please read this:
http://www.epa.gov/donttopoff/
especially the part this part:
Topping off the gas tank can result in your paying for gasoline that is fed back into the station's tanks because your gas tank is full


Here is the formula that the CX-5 uses to calculate range remaining: (notice that indicated range is 0 when approx. 2 gallons of fuel remain)

sorry about the crappy screen cap..

 

[CC58]

First time I did a long drive of 100 miles (without pulling a trailer). Went from Denver to Boulder to Nederland to Black Hawk and back to Denver. The 19 and 119 are 45mph twisty mountain roads gaining 3000 ft in elevation. Got 31.5mpg in my 2.5L Touring. Amazing.

 

[MikeM.]

This is how I see your situation:
Fill up #1
1. You overfill your tank and have approx over 16.5 gallons in your CX-5
2. You use 12.5 gallons to drive 375 miles and your indicated range is now 0 ~2 gallons in your CX-5

The math does not add up in this section.

If you fill to ~16.5 gallons and burn 12.5 gallons you will be left with ~4.0 gallons, not ~2 gallons.

What gives?

In any case, I think it unlikely the tank can hold even 16 gallons. Even if the fuel filler neck is filled to the top.

 

[camellochapin]

Try filling at a slower speed once you're nearing the end of the fill.

Ehmmm nope... that is the worst thing you can do for your own economy. Flow sensors have low accuracy at low flows; the flow-accuracy chart for a flow sensor shows its peak performance for measuring flow is with high flows (regardless of the measurement technology being used). That being said, just stop filling when the pump automatically senses you are over filling; slowly feeling is just going to give you a very bad reading, and you will be paying more $$ for that last liter/gallon of fuel.

 

[brandini]

Ooooh i want to see that, have a source?

 

[camellochapin]

Ooooh i want to see that, have a source?

There you go:

1. Orifice Flow Meter

I could try to find official documentation from flow meter manufacturers such as Rosemount, MicroMotion, etc... but basically they are all similar. Hope it helps...

 

[V8toilet]

Just went 376.5 miles and filled up at the same pump I filled it with previously with 10.65 gallons. The trip computer was indicating 33.5 mpg (was up to 34 mpg until I hit traffic) and the average speed was 47 mpg. This was mostly interstate driving 45 miles one way with some city driving at the end of each trip and some runs to some restaurants. Calculated at the pump is a very good 35.35 MPG. (wow) This is with a 2014 FWD Touring 2.5.

Also noted that on the pump it specifically says "do not top off" and the manufacturers also warm against it so I think I'll avoid taking the chance of possibly screwing something up.

 

[CXVille]

Ehmmm nope... that is the worst thing you can do for your own economy. Flow sensors have low accuracy at low flows; the flow-accuracy chart for a flow sensor shows its peak performance for measuring flow is with high flows (regardless of the measurement technology being used). That being said, just stop filling when the pump automatically senses you are over filling; slowly feeling is just going to give you a very bad reading, and you will be paying more $$ for that last liter/gallon of fuel.

Based on a quick search, it appears that most gas pumps us a gear or positive displacement meter. The graph referenced is an orifice meter, which is significantly different in terms of construction and performance characteristics. I expect that they will start, if they have not already, using mag meters for measurement because they are totally solid state and introduce no line loss or pressure drop to the system. Their accuracy at low flows can also be pretty good.

However, the gear pump meter will have the similar issues with less accuracy at low flows. All flow meters have a range of acceptable accuracy, and almost all types taper off at the low end, under registering flow. In the case of the gear meter, leakage around the gears, especially as it wears, will reduce its accuracy at low flows.

 

[brandini]

SO fill up fast and as you're nearing full slow down, it's not exactly difficult, especially if you fill up at about the same indicated level each time.

 

[jjw6455]

Don't tell my county's weights and measures department that gas flow is inaccurate at low flow, they'll immediately shut down the station until it is corrected. Last year they shut down several stations that registered a penny when the pump was started. Pumps in ohio are certified accurate at any delivery point or speed.

unless you fill the tank all the way when you are measuring mpg manual computations mean nothing. I can fill the tank, then drive till near empty, but if I only fill up the tank partially, I don't have a true count of gallons used.

the only way to know if the tank is filled is if you see it in the neck. But, this is very bad for the fuel vapor recovery system in the car, and maybe even void the warranty because it can send raw gas to the recovery system. The cars computer calculates the mpg better more accurately than you can.

 

[CXVille]

Don't tell my county's weights and measures department that gas flow is inaccurate at low flow, they'll immediately shut down the station until it is corrected. Last year they shut down several stations that registered a penny when the pump was started. Pumps in ohio are certified accurate at any delivery point or speed.

unless you fill the tank all the way when you are measuring mpg manual computations mean nothing. I can fill the tank, then drive till near empty, but if I only fill up the tank partially, I don't have a true count of gallons used.

the only way to know if the tank is filled is if you see it in the neck. But, this is very bad for the fuel vapor recovery system in the car, and maybe even void the warranty because it can send raw gas to the recovery system. The cars computer calculates the mpg better more accurately than you can.

The normal method of checking pump accuracy involves a 5 gallon can so fill rate is not considered. they might do it at different rates but I doubt they do very much of that. It would take a long time to check each pump at several different rates. camellochapin is correct when he says that accuracy decreases as flow rates decrease. How much the accuracy degrades depends on a number of items, but with most all flow meter types there will be a low flow point there the accuracy decreases significantly.

Cars calculate mpg based on the time the injectors are open. They are open for milliseconds and it assumes a fuel flow rate based on averages for the injectors, based on the specified pump pressure. The injectors are not flow matched or flow tested individually and the car adjusted. It is my understanding that the injectors vary up to 5% (+/- ~ 2.5%) so that, combined with the accuracy of the fuel pump pressure, determine your accuracy. My experience is that cars claim 2-5% better mileage than shown based on manual calculations. The gas stations are regulated, as you noted, the output of the car computer is not. I trust my manual calculations rather than the car, although I find the car to be pretty good.

 

[MikeM.]

The normal method of checking pump accuracy involves a 5 gallon can so fill rate is not considered. they might do it at different rates but I doubt they do very much of that. It would take a long time to check each pump at several different rates.

Actually, the State I live in checks each pump at high and low flow rates. They discovered that, on average, you get more fuel for your money when filling at a slow rate. So, if you have the time, you can save money by filling at a slow rate.

camellochapin is correct when he says that accuracy decreases as flow rates decrease. How much the accuracy degrades depends on a number of items, but with most all flow meter types there will be a low flow point there the accuracy decreases significantly.

I'm surprised there is so much talk of accuracy decreasing at low flow rates but no talk of whether it decreases in your favor or the station's favor. It makes sense that it accuracy decreases in your favor and that is what Washington's Department of Weights and Measures found.

 

[jjw6455]

A couple years ago there was a chip that was installed in gas pumps that cheated the customer by charging more per gallon than the posted price. The ingenious part of the software was that when the number of gallons pumped equaled the number of gallons that is used by the auditor the amount was accurate. It wasn't until the auditor started using something other than 5 gallons that the fraud was discovered.


From an Ohio auditor " Each pump can only deviate 3 ounces in either direction to be marked compliant." That's roughly .5% and there is no criteria for the flow rate, the .5% is at any rate.

But, assuming the accuracy of the injectors is +-2.5%, to match that when you refuel your tank you must be within .375 gallons (assuming a 15 gallon fill). I don't know about you, but when the nozzle on the pump shuts off, I wouldn't bet it is within .375 gallons of my last fill up, especially if I am using a different pump fill up to fill up. Not to mention the tank baffles (to minimize the sloshing noise) which trap air preventing a complete fill, etc. Even parking on a slight incline can effect the amount of gas in the fill. Also, assuming the pump is correct you are neglecting the temperature expansion rate of gasoline (about 1%/10C) while the injectors measure actual volume used regardless of temperature. It is not unusual in Ohio to have a ground to ambient temperature difference of over 10C.

 

[MazSter]

The cars computer calculates the mpg better more accurately than you can.

I don't think so..my opinion. You can fake a car meter out several ways in your driving habits that will give you different readings..
Another reason I believe they are off. I only have 4 tanks so far in the cx5..meter tells me I'm getting 28 average. Manuel calculated tells me 30 to 32. I can only back that with gallons used in the cx5 vs. The jeep I used to have. My 5 day work commute uses half the fuel my jeep did and I consistently got 15 mpg with it..
This car at 30 mpg is going to save me 200 bucks a month.
Also I buy new every one to 2 year's and have yet to find an accurate on board mpg calculator. .even on a lexus I had...
Other factors..on board calculator does not account for tire diameters with worn tires or even tire manufacturer..If you pick a tire size and compare specs..almost all are off 1 to 2 % in diameter from say good year or Firestone
.that tricks the factory calculator that uses a tire diameter created by what a tire size should be


Sent from my SPH-L720 using Tapatalk

 

[jjw6455]

Don't forget that the tires also affect the miles driven which you use in your manual calculation so both manual and computed are off by the same factor.

In general I think that neither method is super accurate, but I believe that the computed method is more consistent as it removed human error and a variety of other outside influences. As a result it can be used to compare gas brands, driving methods, weather affects.... and while it won't give a super accurate value, it will give a consistent value.