Displeased with mileage

reshp1 said:
Looks like the computer on mine is a bit optimistic, but still, 28.7 is pretty good for mixed driving, and rush hour at that. Next tank should give me some more solid data.
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After nearly 3 years of tracking my MPG very closely, I've come to the inescapable conclusion that the trip computer is more consistently accurate than the errors introduced with the manual calculation method.

The trip computer can meter fuel more accurately than a gas pump gushing out gallons/minute. The biggest variable with manual MPG calculations is that pumps shut off at different levels of fill. The Trip Computer meters the fuel as it is used so it isn't affected by this variable. The other potentially large variable the Trip Computer doesn't encounter is pump accuracy. In Washington State, where I live, the Department of Weights and Measures does unannounced random pump accuracy and fuel quality testing. It's legal for the pump to read under but even a slight over reading is a violation.

If your state doesn't have such an effective program for keeping station owners/operators honest, your MPG numbers may be lower simply because you are not getting quite as much fuel as you paid for. Also, your fuel could have more than the Federally allowed maximum of 10% alcohol. Washington State tests alcohol content and levies enforcement fines when it finds alcohol over the limit. If your state doesn't measure and enforce this very effectively, I absolutely guarantee there will be considerable fuel over 10% alcohol in the marketplace. That's what Washington State found when they started measuring alcohol content but enforcement actions have brought the cheaters (mostly) back into line. There is a double incentive to cheat on alcohol content;

1) Alcohol is cheaper than gasoline

2) It's cheaper to make gasoline of marginal octane and a little extra alcohol can ensure it measures out to the desired minimum octane.

The bottom line is that you can trust the CX-5 Trip Computer at least as much, if not more, compared to the manual calculation method using fuel pump figures.
 
Mike, this is a very good point. I try to always use the same pump when filling up but who is to say how accurate that it. The PA state department of weights and measurements inspects pumps yearly but I don't know if the sampling.

However, I was under the impression that these trip computers guesstimate the amount of fuel used based on the MAF readout on air flow. Are you aware of how specifically the Mazda system works?
 
inlinev said:
Mike, this is a very good point. I try to always use the same pump when filling up but who is to say how accurate that it. The PA state department of weights and measurements inspects pumps yearly but I don't know if the sampling.
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Many states have similar programs, I imagine PA is similar. Some states are anti-regulatory and neglect this important consumer protection. Those are the states where problems exist. Even filling up at the same pump will not insure consistent fill levels, pumps shut off at different times depending on a lot of factors including ambient temperature.

However, I was under the impression that these trip computers guesstimate the amount of fuel used based on the MAF readout on air flow. Are you aware of how specifically the Mazda system works?
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The MAF sensor is only one of many things that helps the ECU determine how much fuel should be injected. I believe the Trip Computer calculates fuel consumption directly by knowing the dwell (time open) and flow rate of each injector. If, over time, your injectors get deposits on them that reduce their flow rate your actual MPG will be better than reported. On the other hand, if impurities in the fuel erode the injector orifices such that their flow rate increases, your MPG will be worse than the TC reports. Also, if your high pressure fuel regulator, over time, starts regulating to a lower pressure, your actual MPG will be better than reported.
 
My CX-5 seems to have a speed somewhere between 70-80MPH where the fuel economy falls off a cliff.

I used the trip computer to calculate average speed and fuel economy on two recent 170mi drives. Each time I filled my tank at the same station, reset avg. MPG and avg. speed and drove 170 miles to my destination. Both times I drove as quickly as traffic would allow (between 65-85MPH). The avg. speed calculated is lower than the avg. cruising speed because I drove several miles on city streets at low speed before an after getting on the highway. Temperature was similar on both drives and there was no wind.

Trip 1:
avg speed: 72MPH, fuel economy 37.6MPG
Trip 2:
avg speed 76MPG, fuel economy 29.4MPG

The only way I can explain this big drop is that at some engine load, the skyactiv motor simply gets much less efficient.
Up to some point it gets great MPG, but if you push it just a little bit harder, the MPG drops quite significantly.
 
MikeM. said:
After nearly 3 years of tracking my MPG very closely, I've come to the inescapable conclusion that the trip computer is more consistently accurate than the errors introduced with the manual calculation method. The trip computer can meter fuel more accurately than a gas pump gushing out gallons/minute.
Click to expand...

This is very likely not true. There is no flowmeter in the car AFAIK, the computer uses injector timing to estimate fuel injected indirectly. However that is subject to all sorts of errors including battery voltage, fuel pressure, temperature, etc, etc, the errors add up and compound fast. One of the reasons your car has so many sensors nowadays is exactly because this way of calculating fueling is lousy and the engine management software needs feedback to "close the loop" and adjust fueling accordingly in order to run efficiently and meet emmissions limits.

On the other hand, flowmeters measuring flow rate directly are very accurate. An application like a fuel pump has more than enough incentives to use a very good one (breaking the law in one direction and giving away thousands of dollars in free gas in the other direction).

The biggest variable with manual MPG calculations is that pumps shut off at different levels of fill. The Trip Computer meters the fuel as it is used so it isn't affected by this variable.
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I think this isn't that big of a deal. The pump senses when to shut off when the nozzle tip becomes immersed. The tube that leads to your tank is not that wide, an inch or two in level in the tube between when two pumps shuts off isn't that big of difference compared to the total volume in the tank. We're talking about 5% differences here in mileage readings, that's 1/2 a gallon or more when you fill an empty tank. I just don't see two pumps shutting off at that different of points.

The other potentially large variable the Trip Computer doesn't encounter is pump accuracy. In Washington State, where I live, the Department of Weights and Measures does unannounced random pump accuracy and fuel quality testing. It's legal for the pump to read under but even a slight over reading is a violation.
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But an under-reading pump costs the owner a lot of money and a good flowmeter isn't that expensive. Wikipedia lists fuel pump accuracy as 0.3%, which is way smaller than the errors we see between computer and pump calculations.
 
reshp1 said:
Wikipedia lists fuel pump accuracy as 0.3%, which is way smaller than the errors we see between computer and pump calculations.
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True. But using the fuel pump reading to calculate MPG also introduces the error of varying fill levels as I previously noted.

I do not agree with your assumptions that this is a small variation or error. The good thing about this error is it will equalize over a number of consecutive calculations so if you are interested in lifetime average MPG (or even a 6-8 fill up average) then, I agree, the pump method is more accurate. But for a one tank average I maintain the Trip Computer is more consistently accurate.
 
MikeM. said:
True. But using the fuel pump reading to calculate MPG also introduces the error of varying fill levels as I previously noted.

I do not agree with your assumptions that this is a small variation or error. The good thing about this error is it will equalize over a number of consecutive calculations so if you are interested in lifetime average MPG (or even a 6-8 fill up average) then, I agree, the pump method is more accurate. But for a one tank average I maintain the Trip Computer is more consistently accurate.
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For ACCURATE mileage calculation (and I know this isn't "good", but I've yet to kill a vehicle doing this...), I top it off. Fuel sitting level in the fill neck where I can see it. No way to have a "varying fill level" like this. Then, I rely on the pump to be within that 0.3% range.
 
Unobtanium said:
For ACCURATE mileage calculation (and I know this isn't "good", but I've yet to kill a vehicle doing this...), I top it off. Fuel sitting level in the fill neck where I can see it. No way to have a "varying fill level" like this. Then, I rely on the pump to be within that 0.3% range.
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Among our current cars only Honda CR-V can easily see the gas sitting level to the neck, I simply can't see the fuel on VW Passat and BMW 528i even after the gas has overflown. After 17 years and 170K miles of doing this way on the CR-V - see the gas sitting level to the neck, the CEL is on. One of the possibility of the code is the expensive vapor canister connected to the gas tank is saturated and need to be replaced! Many people say this is caused by years of topping-offs while pumping gas.
 
yrwei52 said:
Among our current cars only Honda CR-V can easily see the gas sitting level to the neck, I simply can't see the fuel on VW Passat and BMW 528i even after the gas has overflown. After 17 years and 170K miles of doing this way on the CR-V - see the gas sitting level to the neck, the CEL is on. One of the possibility of the code is the expensive vapor canister connected to the gas tank is saturated and need to be replaced! Many people say this is caused by years of topping-offs while pumping gas.
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Never had that happen. Had cars over a decade old. Always topped off.
 
I top off my CX-5, and 90% of the time I use the same gas station at the same pump. I know my manual calculations are more accurate than what the computer shows. The computer used to be very close to what the manual calculation showed, within .5 mpg usually. But at the time I was using a gas station that hadn't yet switched to 10% ethanol; after they switched over the computer was still showing roughly the same fuel economy, but I was getting less actual mpg.
 
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I have kept track of mileage on my last 5 cars. On this one I added a column for the dash mpg readout, and average them both out. Over 39 fill ups, my pump mpg average is 28.0, while the dash mpg is 27.79. To me this is a negligible difference.
 
Goguinness said:
I have kept track of mileage on my last 5 cars. On this one I added a column for the dash mpg readout, and average them both out. Over 39 fill ups, my pump mpg average is 28.0, while the dash mpg is 27.79. To me this is a negligible difference.
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Exactly! And you will notice your pump average is slightly higher which could be caused by the pumps meter being calibrated to insure it's not under delivering (which is illegal).

If you look at your numbers you will likely see the standard deviation of the pump numbers is somewhat higher vs. the S.D. of the trip computer figures. This is explained by the fact that your tank was not filled to the exact same level each time.
 
Last time I filled up, I filled it until fuel could be seen in the filler neck. I did the same this time. I hand calculated 24.4mpg. The dash read-out said 23.4mpg. Average speed was 25mph. Distance traveled was 315 miles and change. Gas required was 12.90 gallons and change. Cost of the fill-up was $31.XX. Terrain was rolling hills mostly, in NW AR.
 
i had been averaging 25.8 mpg combined on my '16 GT AWD for the last 2 months. took it on a weekend roadtrip to Pennsylvania and got over 28 mpg without much effort.....80% highway driving REALLY helps mileage! :)
 
I average 25.6 mpg and drive 95 % highway miles. I drive around 70 mph to 75 mph with some hilly areas. Of course, if I slowed down I would get better gas mileage.
 
I get about 30 mpg but mine is FWD. my wife gets about 24.5 mpg with her AWD.
 
I did some traveling this week.

From Bentonville, AR to San Antonio, I averaged 25.1mpg highway, travelling at 65 to 80mph.
From Dallas to Bentonville,AR, I averaged about 27mpg, travelling at 65-80mph. (The reason I started it at Dallas was because I sat idling for a bit while I ate in Austin, and didn't want to skew things, so I re-set it at Dallas-ish)

Thus far, this is the ONLY vehicle to significantly under-perform in the mileage department when driven by me on highway trips. Vehicles owned/tested to within +-1mpg of their EPA ratings:

Infiniti G20 A4 = 31mpg
370Z manual = 26.5mpg
C6 Z06 corvette= 26mpg
1995 Trans Am automatic= 25mpg
2001 WS.6 6-speed= 27mpg
Mazda CX-5= A dismal 25-28mpg. Fully 2-5mpg less than the EPA rates it at.
Grand Jeep Cherokee (WK) HEMI/4WD= 18mpg
Mustang 5.0 with 4.10's, 10.5:1 compression, X303 cam, TKO500 swap, full exhaust, MSD 6AL ignition and 575DEMON carb....Seriously? You think I cared? I recall it being around 16-18, to be honest though.

Am I disappointed that I own an AWD SUV that gets that mileage? NO! Am I disappointed that Mazda's EPA listed rating is complete horse crap? Yes.

Now you can go and blame me for driving 75mph or say it's my fault, but NO OTHER VEHICLE I HAVE OWNED has needed such an excuse for not achieving it's advertised mileage---or exceeding it, as you will note a few of my vehicles did. Some significantly.
 
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Come visit us here!!

Unobtanium said:
I did some traveling this week.

From Bentonville, AR to San Antonio, I averaged 25.1mpg highway, travelling at 65 to 80mph.
From Dallas to Bentonville,AR, I averaged about 27mpg, travelling at 65-80mph. (The reason I started it at Dallas was because I sat idling for a bit while I ate in Austin, and didn't want to skew things, so I re-set it at Dallas-ish)

Thus far, this is the ONLY vehicle to significantly under-perform in the mileage department when driven by me on highway trips. Vehicles owned/tested to within +-1mpg of their EPA ratings:

Infiniti G20 A4 = 31mpg
370Z manual = 26.5mpg
C6 Z06 corvette= 26mpg
1995 Trans Am automatic= 25mpg
2001 WS.6 6-speed= 27mpg
Mazda CX-5= A dismal 25-28mpg. Fully 2-5mpg less than the EPA rates it at.
Grand Jeep Cherokee (WK) HEMI/4WD= 18mpg
Mustang 5.0 with 4.10's, 10.5:1 compression, X303 cam, TKO500 swap, full exhaust, MSD 6AL ignition and 575DEMON carb....Seriously? You think I cared? I recall it being around 16-18, to be honest though.

From what I remember about the troopers in Texas...Tough place to get nailed speeding...especially with out of state plates on

Now they have just or are about to increase the speed limit in Nevada (ON RURAL HIGHWAYS) from 75 to 85 mph! We are talking about seeing NADA for miles and MILES!! NO GAS STATIONS EITHER!
Troopers use small planes and Hilos for enforcement.
Will have to give it a run after summer is over along with the 100+ temps! LOL HIGHWAY I-80 east/west will get those limits about 30 miles EAST of Reno :)

Me thinks that all EPA ratings are horse crap....
--------------------------------------------------------------------------------------------------------------------------------
Am I disappointed that I own an AWD SUV that gets that mileage? NO! Am I disappointed that Mazda's EPA listed rating is complete horse crap? Yes.

Now you can go and blame me for driving 75mph or say it's my fault, but NO OTHER VEHICLE I HAVE OWNED has needed such an excuse for not achieving it's advertised mileage---or exceeding it, as you will note a few of my vehicles did. Some significantly.
Click to expand...
 
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I don't have any problem getting the advertised 30 highway / 24 city, though I suspect I will never get 30 when driving 80. This is with the 2.5 and AWD. The CX-5 as a CUV simply has more wind drag as a result of a larger & higher frontal area.
On average, it is about the same as my previous Honda Accord, despite being a heavier vehicle with AWD.
 
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Unobtanium said:
I did some traveling this week.

From Bentonville, AR to San Antonio, I averaged 25.1mpg highway, travelling at 65 to 80mph.
From Dallas to Bentonville,AR, I averaged about 27mpg, travelling at 65-80mph. (The reason I started it at Dallas was because I sat idling for a bit while I ate in Austin, and didn't want to skew things, so I re-set it at Dallas-ish)

Thus far, this is the ONLY vehicle to significantly under-perform in the mileage department when driven by me on highway trips.
Click to expand...

Go for a day drive in AR hill country at 45-50mph. Fill up when you get home. You will get 32mpg.
 
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