Five fillups

[Frank F]

After 5 fillups:

Distance - Average:

 

340 @ 45,4

210 @ 50.8

340 @ 40.7

336 @ 40.3

485 @ 47.4

 

For a weighted average (if my arithmetic is correct) of about 44.7.

 

I suspect the 50.8 was a case of not filling the tank enough. Since the third fill, I've been filling on slowest automatic pump setting, waiting till shut off, then waiting about 4 seconds, filling with hand, second shutoff, wait about 4 seconds, filling with hand, and getting 3rd shutoff.

 

[bdginmo]

It's not uncommon for me to get 50 mpg tanks especially in warm weather. When filling up I do one automatic shutoff and one manual shutoff. Like you I've seen too many times where the automatic shutoff does not fill the tank all the way. But, I also don't want to overfill or top-off either so one manual shutoff seems like a good compromise.

[Cobra348]

Are these numbers calculated from the receipt information or trip meter/car comp? The trip and car comps are off as much as 3 MPG from receipts many times. I filled up today for the closest - 4/10 MPG difference between receipt and comp.

 

Fill method I use: fill until shutoff, pause for slow count of 5 then about 1/4 open on the pump until shutoff. Repeat pause and 1/4 open one more time. No overflow occurs but also does not overfill.

Edited February 27, 2017 by Cobra348

[md13ffhguy]

I used to use a "fill method," but now I just let the pump call the shots. It's all the same in the end, and my data did not prove to be any more consistent.

[Frank F]

Are these numbers calculated from the receipt information or trip meter/car comp? The trip and car comps are off as much as 3 MPG from receipts many times. I filled up today for the closest - 4/10 MPG difference between receipt and comp.

 

Fill method I use: fill until shutoff, pause for slow count of 5 then about 1/4 open on the pump until shutoff. Repeat pause and 1/4 open one more time. No overflow occurs but also does not overfill.

These are calculated from the pump receipt. I.e miles / fuel quantity. I haven't been paying too much attention to the vehicle's display at fill-up time, although, the most recent (bottom) one, the vehicle said 47.1 mpg average.

 

I believe the two in the 40.x range were displaying >=45 mpg on vehicle display.

[Texasota]

After hand calculating mine for over 20,000 miles the trip computer was very close to 2.25 MPG optimistic. Others have mentioned that the odometer has some error (on the pessimistic side) which I have never tried to measure. This would suggest that the actual figure would be a little less than 2.25.

[Griswald]

How can you be certain that the vehicles calculations are "off"?

 

In the same line, how can you be sure that "your" calculations are 'right"?

 

Unless you add a calibrated external measuring wheel and a calibrated gasoline container, your numbers are guesses, just like the manufactures numbers.

[Texasota]

How can you be certain that the vehicles calculations are "off"?

 

In the same line, how can you be sure that "your" calculations are 'right"?

 

Unless you add a calibrated external measuring wheel and a calibrated gasoline container, your numbers are guesses, just like the manufactures numbers.

 

My calculations were simple. I just divided miles driven (from trip 1 computer ) by gallons to fill the tank. This was recorded in my spreadsheet along with the trip 1 computer's MPG claim. Granted there is a lot of variability in one tank (uneven ground, different pumps, one vs two clicks etc) but tracking this in a spreadsheet for ~20,000 miles I assume the cumulative result to be a reasonably accurate comparison.

 

I did not claim in my above post that my numbers were 100% accurate. I was simply comparing my spread sheet calculations to the trip computer's calculation and they differ by ~2.25 MPG. This is consistent with results posted by other forum members.

[ptjones]

You can use Fuelly.com to record your fillup info and display it on this Forum. My CMAX with OEM Michelin tires is off by +1.8% of Odometer so that works out to about +1 mpg it seems like all members have come up with similar amounts. Use portable GPS to figure out the difference. :)

 

Paul

[Griswald]

But you are still "guessing " at miles driven and gallons used, no matter what calculation you use.

[Frank F]

But you are still "guessing " at miles driven and gallons used, no matter what calculation you use.

I'm not following your logic. Are you saying the vehicle odometers and the gasoline pumps also "guess" at quantities? Or are you implying there's error in the measured quantities? If so, I'll assume you too are "guessing" at how much error.

 

I get the impression you're saying "We don't have any idea what fuel economy we are getting. It can say 44.7, but it could be 12, it could be 78, because all we are doing is guessing."

Edited March 1, 2017 by Frank F

[ptjones]

But you are still "guessing " at miles driven and gallons used, no matter what calculation you use.

I'm confused too. :shift: I have done a number of GPS tests up to 400 miles and coming up with the same 1.8% off. As far as filling up goes, that gets averaged out in Fuelly.com with multiple fillups. I have 247 fillups with 47.6 mpg on Fuelly and 48.6 mpg on my Life Time /smart Gauge. :) One mpg difference.

 

Paul

[Texasota]

But you are still "guessing " at miles driven and gallons used, no matter what calculation you use.

I am using the car's odometer reading of miles driven and the gas pump's digital reading of gallons pumped. Granted both have some relatively small amount of error but the data being used are not guesses.

 

I believe there are three key sources that contribute to error in the hand calculations and the MPG as reported by the trip computer:

1. The error in miles traveled as reported by my FFH's odometer
2. The error in gallons dispensed as reported by the gas pump's digital readout
3. The error in gallons used as reported by my FFH's fuel metering (used by the trip computer to report MPG)

There are also other sources of error such as inconsistent topping off of the fuel tank but this would become negligible over the course of ~20,000 miles. This is only speculation on my part, but I suspect the largest source of error is #3.

[Waldo]

But you are still "guessing " at miles driven and gallons used, no matter what calculation you use.

 

Not really, those are both required by the government to hold a certain level of accuracy. The MPG display in a car is not.

[Frank F]

So if we know the error / uncertainty range, then we could more accurately say something like "44.4 mpg ± 4.4 mpg" (10% inaccuracy), or "44.4 mpg ± 0.4 mpg" (1% inaccuracy).

I, like everyone else here am not buying the "we have absolutely no idea if your hybrid is getting better fuel economy than that Ford GT right next to you." -- not that anyone is implying it's that severe of "guessing."

From everything I've read on this site, the mpg postings from odometer and fuel receipts appear to be reasonable.

[Griswald]

All I am saying is that "your"number can be off just like the cars number.

[bdginmo]

Fuel pump metering is very accurate (required by law to be within 0.3%). I'd imagine the odometer is less accurate, but not unreasonably so; maybe on the order of a couple % at most. The odometer is probably biased high or low. You could use a GPS logger/tracker to get a pretty accurate reading to see what the error is on your odometer. It would be interesting if someone could do the experiment to quantify the error of both the computer and the manual calculation. I would, but honestly I'm just not terribly interested in this one.

[Texasota]

All I am saying is that "your" number can be off just like the cars number.

 

This is without question true. The trip 1 computer and the hand calculations are both using the car's odometer which means the odometer error is present in both MPG calculations. That leaves the two different methods of measuring fuel consumption to explain the discrepancy.

[md13ffhguy]

But you are still "guessing " at miles driven and gallons used, no matter what calculation you use.

That's always been my contention, too. However, I've decided that the pump (any pump) displays the most correct number. It's the number of gallons I've actually PAID for. ;)

[md13ffhguy]

All I am saying is that "your"number can be off just like the cars number.

Right, there's error and variance to account for, but as mentioned, it eventually averages out, or is at least minimized.

[bdginmo]

Yeah, given using the trip computer or pump receipts and odometer I'd imagine the later is the most accurate. One thing I do like about the trip computer is that it does not count the fuel used for remote starting. IMO that's the way it should be because remote starting is discretionary and has 0% correlation with the vehicle's capabilities so it shouldn't count against the mpg.

Edited March 2, 2017 by bdginmo

[ptjones]

As easy as it is to use GPS to check Odometer, I don't know why everyone hasn't check it. :shift: I found out there is a real noticeable difference between New and worn Tires. :)

 

Paul

[bdginmo]

As easy as it is to use GPS to check Odometer, I don't know why everyone hasn't check it. :shift: I found out there is a real noticeable difference between New and worn Tires. :)

 

Paul

 

Based on your signature you noticed a difference of 2.2% on at least one occasion. Do you have any other observations? Is it usually around 2%? How big of an effect does does tire wear have?

Edited March 2, 2017 by bdginmo

[ptjones]

 

Based on your signature you noticed a difference of 2.2% on at least one occasion. Do you have any other observations? Is it usually around 2%? How big of an effect does does tire wear have?

When they are brand new it's about 2% and when worn out about 1.6%. Right now it's 1.8%, I wonder if not all tires are the same size. :)

 

Paul