Probably so, sorry about that. That's the only way I have ever calculated milage and never believed the computer. Too each his own(respectfully) :confuse: .

In fact, trip computer (if calibrated well) is likely more accurate than your usual rudimentary miles/gallons method. Why? Because you really cannot trust the "gallons" reading from the pump. Yes, all pumps are periodically tested for accuracy, but pumps of gas stations click off at different thresholds. In my experiences, the differences can be as much as 1 gallon (fill-ups). In order for the miles/gallons method to be accurate, you have to fill up at the same station and, even better, the same pump between fill-ups.

I have a ScanGaugeII installed and calibrated (with 4.4% error adjustment). So far, it has been very accurate within 0.2 gallons at fill-up (or ~0.2mpg)

Just filled up the tank for the first time. 16 mpg in urban traffic, not bad. Will average out over the next fill ups using "odo B" to keep track of eventual improvements.

I know it is heresy, but comparing it to the Sienna minivan it replaces, the engine needs its revs to move.

I still don't see what difference filling up with 2.5 or 5 or 10 gallons in the tank has to do with calculating your mileage though.

It dilutes the error rate. Let's say for example the pump you're using sometimes adds an extra 1/10 of a gallon of gas more or less. Adding that 1/10 of a gallon to a 2.5 gal fillup will create a 4% error (.1 / 2.5 x 100), while that 1/10 gallon pump error will only account for a 1% error on a 10 gal fillup (.1 / 10.0 x 100). Let's say your pump error is 1/2 gal because it's flowing faster or slower, more is released on the click, you're holding the pump slightly differently, etc. That 1/2 gal difference will create a 20% error on a 2.5 gal fillup vs only a 5% error on a 10 gal fillup.

It's basic statistics. The larger the sample size (i.e. the quantity of the fillup) increases accuracy (eg MPG for that fillup).

That's also why scan gauge really isn't accurate for snapshot MPG ratings, since MPG is Miles Per Gallon and if the car only moved a few feet and scan gauge tried to calculate a Miles Per Gallon number based on a few feet of driving, it wouldn't be very accurate in comparison with calulating a MPG over a hundred mile range. So if Scan Gauge calculates that it took 0.023 gal to travel 6ft that would equal 20.24MPG (0.023 x 5280 / 6), but if Scan Gauge was 1/100 of a gallon off and the actual amount used was 0.033 instead of 0.023, then the MPG would actually be 29.04MPG, so by being 1/100th of a gallon off, the MPG inaccuracy was 43% off.

My daughter is in 6th grade. Her science project this year is MPG vs PSI. My CX9 GT-AWD played an important role in this. Here is the prelim result. On a stretch of 2-mile local road w/o light/stop sign. Each data point was obtained by averaging four data points (back and forth twice to counter slop and wind effect). Vehicle was maintained at target speed with cruise control.

As you can see the CX9 is well-capable of doing 27mpg provided that - run tires at 40 psi (MAX=51psi), it is a bit bumpy... - maintain speed at 35-40 whenever possible on local roads.

My daughter and I spent the entire afternoon for that data. BTW, the TPMS lights came on at 20psi (not 25psi or above). It went away when I pumped them up to 35psi (my typical) and CX9 reached 15mph from start.

Also, a PSI drop of 40/35 down to 20 costs you ~10% drop in MPG. The MPG was based on my ScanGaugeII, which was +7% compensated. I have calibrated it for almost 1-year at the pump. It has been very accurate now (+/- 0.05 gallon error out of 17-18 gallons)

Yes, but basic statistics depends critically on using correct data. The departments of weights and measures in the various states regulate the accuracy of gas pumps to be about 6 cubic inches per 5 gallons. Since there are 231 cubic inches to the gallon, that works out to a relative error of just 0.5% per gallon pumped, not 20%

If one is rounding off their numbers to the nearest 10^{th} of a gallon then one is just being lazy. The price you actually paid for the gas and the posted price per gallon are sufficient to get an appropriately accurate measure of the volume of gasoline that you pumped.

And, speaking of statistics, the "Law of Large Numbers" tells us that whatever errors you incur at each fill up will tend to the correct average over the course of time.

That's really nice! I was going to make a graph of your daughter's data to post but I just don't have the time right now (it's late). Maybe tomorrow - or someone else would care to plug it into Excel. (Hint, hint!)

Your PSI and MPG from Scan Gauge prove my point exactly. For the first four 2-mile stretches at 40PSI, Scan Gauge calculated between 24.7 to 27.3 MPG for the exact same distance, speed and conditions over a 2 mile length, so on a 2 mile stretch it's reading varies by 10%. If you would have driven 200 miles instead of 2, then the Scan Gauge would have been more accurate, and vice versa if you'd only have driven 0.2 miles.

It would be interesting (but very timeconsuming) to see measure the accuracy difference by only driving 0.2, 2, and 20 mile lengths.

"works out to a relative error of just 0.5% per gallon pumped, not 20%"

Correct, but that's only the "fuel actually pumped" vs "what's shown on the gas pump gauge accuracy"

What I'm talking about is the accuracy of person's ability to fill up the tank to the same level every time when they fill up their tank and then calculate their MPG. Even if the pump is 100% accurate, the point at which it clicks and shuts off varies a lot, depending on the speed of the fuel flowing into the tank, position of the nozzle, etc.

Your original question was why it made a difference if you calculated MPG on a 2 gallong vs 20 gallon fillup and that's what I was trying to explain. It's the inaccuracy of the fillup (not the pump) that creates the error, and that error (which relates to the Law Of Large Numbers) will create a larger MPG inaccuracy the less you pump.

Another way to think about it is to imagine if you drove one mile, filled up the tank and calculated MPG. The drive 100 miles, fill up the tank and calculate MPG. Which do you think will provide a more accurate result? Yes, it's an extreme example, but it shows that when trying to increase the accuracy of an MPG calculation it's better to drive more miles.

The four numbers at the first row represents the MPG collected from various speed (35, 40, 45, 50 mph). Each data point is an average of four trials (back and forth twice). The ScanGaugeII computes MPG every 1 second or so. I reset the trip computer at the start and observe the average MPG at the 2-mile end. There are other factors that cause variations more than the ScanGaugeII such as the wind resistance. (on the day of our test, it was pretty windy) I also employed the cruise control to avoid human foot pressure variation on gas pedal.

Your original question was why it made a difference...

Actually, that was Steve's question, not mine.

What I'm talking about is the accuracy of person's ability to fill up the tank to the same level every time ...

The information for doing it correctly is available based on what you pay and the price per gallon for your current fill up AND your previous one. Nevertheless, you have a point because most of us are not going to take the extra bookkeeping steps.

It is interesting how the theory works out. I have a spreadsheet with ~400 tanks over a decade on my minivan and I could delete a few of them and it wouldn't affect the lifetime mpg. At least not out to one decimal point.

Moderator Need help navigating? stever@edmunds.com - or send a private message by clicking on my name.

Al right. I have had my CX9 for over a year now. It has 46K km (around 28.5K miles) on the odometer. I have not reset the trip computer for approx. 25K km (around 15K miles). The average I have got based on the trip computer readings is 13.2L / 100 km (17.8 MPG).

our 09 we got back in Aug has about 4400 mi on it and the avg is 17.5 ...that w/one long trip between TX and MO and the rest of the time all here in the metromess.

Does anyone know why the 2011 is rated 2 miles per gallon better. Everything I have read says the 2010 and the 2011 are the same. I cant figure out how it is getting better mileage. Mazda lists the 2010 as 16 city and 22 hwy and the 2011 ,..17 city and 24 hwy. Anybody know whats going on ?

I wonder if it would be possible for the dealer to load the 2011 chip program on a 2010. I just bought a 2010 CX-9 GT this weekend. My wife will use it to commute 90 miles/day so every mpg counts!

Probably not. Here's an update. Stick with stock and check tire pressure each month.

Just did the same! CX-9 GTAWD/MRB/RLG) for her!

Mazda says fuel saving improvements were a outcome of work done on a model’s AWD differential rigging carryout, engine optimisation for improved explosion control (during deceleration as well as at resting as well as reduction in delivery attrition.

Depending upon various, new 18 as well as 20 in. amalgamate wheels will set a 2011 Mazda CX-9 detached. The brand new alloys are lighter than prior to as well as come wrapped in tyres that suggest improved rolling insurgency (better fuel saving.

MY 2010 CX9 GT FWD after 2 weeks is ONLY GETTING 13.3 according to its computer, how accurate are these; my wife is freaking out; her BMW 530 got at least 16 in the city...these are not even close to the posted sticker values; wonder if anyone ever sued a car company over missing the mark; DO NOT LAUGH, Mazda got sued and lost years ago when the MPH on the RX8 did not come closeto what was advertised; my neighbor was in on the class action suit,; the service writer says some get better as the car breaks in, some get worse? :sick: dogdoc

Short drives give you 13mpg. That is perfectly normal if you know how EPA do the tests. My wife's Prius gets 33mpg all the time on school routes, which is about 2 miles away with several stop signs and lights.

Take the CX9 on a long trip, you will get 22 mpg AT LEAST!

I have had my Mazda CX9 AWD for more than two years now. I haven't reset the onboard computer for about 75-80K km. According to the computer, my average is 13.2l/100 km (21.4 MPG).

Engines take some time to break in. Don't be suprised that it may take up to 10K miles before peak mpg is achieved. Further, actual mileage is affected by length of trip, weather, engine temp, amount of alcohol in the gas, et al.

Further, if you have complaints - point them at the EPA. Mazda did not come up with the numbers, the EPA did.

This is not true. I have a brand new cx9 2011 sport and MPG is terrible. I'm finishing a chart right now (I will post in minutes new info) but I'm really dissapointed with the MPG advertised and real results.

On City with no traffic at all (at 02:00am) I made 17.5MPG (driving 23km= 14.3 miles)

Today morning with heavy heavy heavy traffic I made 8.9 MPG!!! (26.4 L/100Km - 3.78 Km/L). This last values are horrible numbers.

In the next weeks I will have a highway driving (310 miles) so I will check the MPG consumption in highway.

Some weird stuff I see on the new CX9 2011 is I can go at 40miles x hour and the gearbox is in 6th gear. I read in the past 6th gear is only at highway but seems the new cx9 have a new programming (stupid) because MPG is horrible.

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4http://www.carspace.com/pirogue/Albums/pirogue%27s%20Album/Oct01_0002.jpg/page/p- hoto.html#pic

not sure how to use url function to make address smaller.

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0 · Like LOL3,572computerin the 2009 CX-9.- Spam
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0 · Like LOL1,093In order for the miles/gallons method to be accurate, you have to fill up at the same station and, even better, the same pump between fill-ups.

I have a ScanGaugeII installed and calibrated (with 4.4% error adjustment). So far, it has been very accurate within 0.2 gallons at fill-up (or ~0.2mpg)

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0 · Like LOL370I know it is heresy, but comparing it to the Sienna minivan it replaces, the engine needs its revs to move.

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0 · Like LOL2,997I still don't see what difference filling up with 2.5 or 5 or 10 gallons in the tank has to do with calculating your mileage though.It dilutes the error rate. Let's say for example the pump you're using sometimes adds an extra 1/10 of a gallon of gas more or less. Adding that 1/10 of a gallon to a 2.5 gal fillup will create a 4% error (.1 / 2.5 x 100), while that 1/10 gallon pump error will only account for a 1% error on a 10 gal fillup (.1 / 10.0 x 100). Let's say your pump error is 1/2 gal because it's flowing faster or slower, more is released on the click, you're holding the pump slightly differently, etc. That 1/2 gal difference will create a 20% error on a 2.5 gal fillup vs only a 5% error on a 10 gal fillup.

It's basic statistics. The larger the sample size (i.e. the quantity of the fillup) increases accuracy (eg MPG for that fillup).

That's also why scan gauge really isn't accurate for snapshot MPG ratings, since MPG is Miles Per Gallon and if the car only moved a few feet and scan gauge tried to calculate a Miles Per Gallon number based on a few feet of driving, it wouldn't be very accurate in comparison with calulating a MPG over a hundred mile range. So if Scan Gauge calculates that it took 0.023 gal to travel 6ft that would equal 20.24MPG (0.023 x 5280 / 6), but if Scan Gauge was 1/100 of a gallon off and the actual amount used was 0.033 instead of 0.023, then the MPG would actually be 29.04MPG, so by being 1/100th of a gallon off, the MPG inaccuracy was 43% off.

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0 · Like LOL1,093My CX9 GT-AWD played an important role in this. Here is the prelim result.

On a stretch of 2-mile local road w/o light/stop sign.

Each data point was obtained by averaging four data points (back and forth twice to counter slop and wind effect). Vehicle was maintained at target speed with cruise control.

35 40 45 50

---------------------------------------------

40 27.3 26.4 25.4 24.7

35 26.7 26.4 24.8 24.7

30 26.4 24.8 24.5 24.7

25 25.6 24.4 23.7 23.5

20 24.3 22.8 22.8 21.4

--------------------------------------------

^

PSI (all four tires) - warm

As you can see the CX9 is well-capable of doing 27mpg provided that

- run tires at 40 psi (MAX=51psi), it is a bit bumpy...

- maintain speed at 35-40 whenever possible on local roads.

My daughter and I spent the entire afternoon for that data.

BTW, the TPMS lights came on at 20psi (not 25psi or above).

It went away when I pumped them up to 35psi (my typical) and CX9 reached

15mph from start.

Also, a PSI drop of 40/35 down to 20 costs you ~10% drop in MPG.

The MPG was based on my ScanGaugeII, which was +7% compensated.

I have calibrated it for almost 1-year at the pump. It has been very accurate

now (+/- 0.05 gallon error out of 17-18 gallons)

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0 · Like LOL10,110It's basic statistics.Yes, but basic statistics depends critically on using correct data. The departments of weights and measures in the various states regulate the accuracy of gas pumps to be about 6 cubic inches per 5 gallons. Since there are 231 cubic inches to the gallon, that works out to a relative error of just 0.5% per gallon pumped, not 20%

If one is rounding off their numbers to the nearest 10

^{th}of a gallon then one is just being lazy. The price you actually paid for the gas and the posted price per gallon are sufficient to get an appropriately accurate measure of the volume of gasoline that you pumped.And, speaking of statistics, the "Law of Large Numbers" tells us that whatever errors you incur at each fill up will tend to the correct average over the course of time.

tidester, hostSUVs and Smart Shopper

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0 · Like LOL10,110tidester, hostSUVs and Smart Shopper

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0 · Like LOL2,997It would be interesting (but very timeconsuming) to see measure the accuracy difference by only driving 0.2, 2, and 20 mile lengths.

40 27.3 26.4 25.4 24.7

35 26.7 26.4 24.8 24.7

30 26.4 24.8 24.5 24.7

25 25.6 24.4 23.7 23.5

20 24.3 22.8 22.8 21.4

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0 · Like LOL2,997Correct, but that's only the "fuel actually pumped" vs "what's shown on the gas pump gauge accuracy"

What I'm talking about is the accuracy of person's ability to fill up the tank to the same level every time when they fill up their tank and then calculate their MPG. Even if the pump is 100% accurate, the point at which it clicks and shuts off varies a lot, depending on the speed of the fuel flowing into the tank, position of the nozzle, etc.

Your original question was why it made a difference if you calculated MPG on a 2 gallong vs 20 gallon fillup and that's what I was trying to explain. It's the inaccuracy of the fillup (not the pump) that creates the error, and that error (which relates to the Law Of Large Numbers) will create a larger MPG inaccuracy the less you pump.

Another way to think about it is to imagine if you drove one mile, filled up the tank and calculated MPG. The drive 100 miles, fill up the tank and calculate MPG. Which do you think will provide a more accurate result? Yes, it's an extreme example, but it shows that when trying to increase the accuracy of an MPG calculation it's better to drive more miles.

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0 · Like LOL1,093The four numbers at the first row represents the MPG collected from various speed (35, 40, 45, 50 mph). Each data point is an average of four trials (back and forth twice). The ScanGaugeII computes MPG every 1 second or so. I reset the trip computer at the start and observe the average MPG at the 2-mile end. There are other factors that cause variations more than the ScanGaugeII such as the wind resistance. (on the day of our test, it was pretty windy) I also employed the cruise control to avoid human foot pressure variation on gas pedal.

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0 · Like LOL10,110Your original question was why it made a difference...Actually, that was Steve's question, not mine.

What I'm talking about is the accuracy of person's ability to fill up the tank to the same level every time ...The information for doing it correctly is available based on what you pay and the price per gallon for your current fill up AND your previous one. Nevertheless, you have a point because most of us are not going to take the extra bookkeeping steps.

tidester, hostSUVs and Smart Shopper

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0 · Like LOL40,441It's the inaccuracy of the fillupWell, ok, but I live in a perfect world and Tides will vouch that my mpg spreadsheets are accurate out to 9 decimal points. :shades:

ModeratorNeed help navigating? stever@edmunds.com - or send a private message by clicking on my name.

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0 · Like LOL10,110Tides will vouch that my mpg spreadsheets are accurate out to 9 decimal points.Absolutely! But you never did tell me how you measured your fill ups to the nearest molecule or distance travelled to the nearest millimeter! :P

tidester, hostSUVs and Smart Shopper

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0 · Like LOL2,997My basic point was that point and short distances measurements aren't as accurate as those measured over longer distances.

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0 · Like LOL40,441It is interesting how the theory works out. I have a spreadsheet with ~400 tanks over a decade on my minivan and I could delete a few of them and it wouldn't affect the lifetime mpg. At least not out to one decimal point.

ModeratorNeed help navigating? stever@edmunds.com - or send a private message by clicking on my name.

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0 · Like LOL8,372Makes you wonder why it wasn't done to the 2010...

Regards,

OW

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0 · Like LOL8,372Just did the same! CX-9 GTAWD/MRB/RLG) for her!

Mazda says fuel saving improvements were a outcome of work done on a model’s AWD differential rigging carryout, engine optimisation for improved explosion control (during deceleration as well as at resting as well as reduction in delivery attrition.

Depending upon various, new 18 as well as 20 in. amalgamate wheels will set a 2011 Mazda CX-9 detached. The brand new alloys are lighter than prior to as well as come wrapped in tyres that suggest improved rolling insurgency (better fuel saving.

Regards,

OW

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0 · Like LOL1,093That is perfectly normal if you know how EPA do the tests.

My wife's Prius gets 33mpg all the time on school routes, which is about 2 miles away with several stop signs and lights.

Take the CX9 on a long trip, you will get 22 mpg AT LEAST!

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0 · Like LOL7,796Further, if you have complaints - point them at the EPA. Mazda did not come up with the numbers, the EPA did.

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0 · Like LOL6Today morning with heavy heavy heavy traffic I made 8.9 MPG!!! (26.4 L/100Km - 3.78 Km/L). This last values are horrible numbers.

In the next weeks I will have a highway driving (310 miles) so I will check the MPG consumption in highway.

Some weird stuff I see on the new CX9 2011 is I can go at 40miles x hour and the gearbox is in 6th gear. I read in the past 6th gear is only at highway but seems the new cx9 have a new programming (stupid) because MPG is horrible.

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