Fuel economy example
- Thread starter mooreaccurate72
- Start date
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- RDX Aspec Adv.
I drove from near Joplin, to near St Louis and back today, to collect my SBS. Total average was 26.9mpg. My girlfriend drove maybe 150 or so of the miles, and I'm sure that helped. I believe it was sitting on 26.4 or 26.5 when she took over.
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- RDX Aspec Adv.
10 mile trips really aren't the best measure. The EPA has always been too kind to this engine, as you can see the mileage rating is downgraded and downgraded again.
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- RDX Aspec Adv.
But what's the cumulative margin of error vs. that of the computer? It would be interesting to see a lab comparison, but I feel that either way the error is lost in the rounding.
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- State of Jefferson
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- 2017 CX-5 GT AWD+
Excellent question. Can't do it in a lab, tho; this is necessarily a real world experiment. We talk about mileage in terms of full miles/gallon-- room for a lot of rounding there.
The margin of error of a gas pump is .05 gal/fill-up, max, and should tend to cancel out over time, assuming the pumps are ACCURATE, which we cannot assume. The margin of error in the computer depends on the precision of the fuel metering, and the quantity reported for mileage calculations may be less precise than the actual metering. Does anyone know the value of that precision?
In any case, and every case, the greater the sample, the lower the rounding error. That's a given.
You can test this anecdotally, if you care enough. Reset the computer and unlink your it from trip A. Just record your fill-ups over an EXTENDED period of time/miles. Months/thousands. If enough people do it accurately and long enough, we might be able to come to a tentative conclusion about how they compare. But we still won't know which is more accurate!
I'd volunteer, but I don't care that much.
But those of you who DO record your mileage, please, step right up, and report back with your results... later.
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Here goes:
Bought early September 2016. Trip mileage "A" reset at every fill up to get a reading to enter into Fuelly. For example (off top of my head), 389 miles since last fill up, 42 litres of diesel pumped. Fuelly MPG average over 16,029 is 37.5 MPG. Mazda display average 42.3 MPG (which has never been reset from new) Note UK gallons, no idea what US gallons would be.
Make of this what you will.
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- 2021 CX-9 Sig
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- 2021 CX-5 GT
s
I record with Fuelly and will try it out for 10 fill ups. I am pretty anal about the way I fill up - same station, same pump and same method so this should be interesting.
Excellent question. Can't do it in a lab, tho; this is necessarily a real world experiment. We talk about mileage in terms of full miles/gallon-- room for a lot of rounding there.
The margin of error of a gas pump is .05 gal/fill-up, max, and should tend to cancel out over time, assuming the pumps are ACCURATE, which we cannot assume. The margin of error in the computer depends on the precision of the fuel metering, and the quantity reported for mileage calculations may be less precise than the actual metering. Does anyone know the value of that precision?
In any case, and every case, the greater the sample, the lower the rounding error. That's a given.
You can test this anecdotally, if you care enough. Reset the computer and unlink your it from trip A. Just record your fill-ups over an EXTENDED period of time/miles. Months/thousands. If enough people do it accurately and long enough, we might be able to come to a tentative conclusion about how they compare. But we still won't know which is more accurate!
I'd volunteer, but I don't care that much.
But those of you who DO record your mileage, please, step right up, and report back with your results... later.
I record with Fuelly and will try it out for 10 fill ups. I am pretty anal about the way I fill up - same station, same pump and same method so this should be interesting.
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- RDX Aspec Adv.
But what about temperature and the effect of it as well as specific flow rate due to condition of the filtration screens and the level of aeration of the fuel that would affect cut-off, and...?
Sweet lord. Just use the computer in the vehicle unless you have real reason to suspect that it's lying to you (you changed tire sizes or diff gearing or something and didn't re-tune, etc.)
I baby it till the blue light goes off. I have not seen a drastic effect as yours. Winter mpg sucks primarily due to the time it takes for CX5 to clean its windshield. Otherwise both were within 2 mpg.
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- 2021 CX-9 Sig
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- 2021 CX-5 GT
Yeah, more luck than anything. When I pull into the station, I generally take the first pump, which is almost always open. But even that does not guarantee that everything will be normal. One time, the flow rate was very slow - took almost 15 minutes to fill up- and the amount I added was crazy high.
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- NE Indiana
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- '16 CX-5 GT FWD
My dad always kept track of fuel mileage as an easy way to keep an eye on engine performance (this was in the days of carburetors, distributors and points and way before OBD and such). So I've been tracking fuel mileage in my own cars since the early '70's. The first vehicle in our household to have any live fuel economy display is my wife's '10 Nissan cube. I've tracked it from the start and quickly realized the car computer overstates the average mileage by 2.0 - 2.25 mpg. Over 290 fuel-ups now and it's pretty consistent but the bias is there.
Because of all of this I've tracked my CX-5's mileage from day 1 also. Every fill-up I reset Trip A, Avg. mpg and Avg. speed and record the previous values along with # of gallons used from the pump. After 84 fuel-ups over 31,274 miles the average from my car display is 32.9 mpg while my calculated average based on odometer reading and # gals. pumped is 33.6 mpg. It's a difference of 0.7 mpg, just a bit over 2%. So in this case the on-board computer is understating the mpg but not nearly as much as my Nissan overstates it.
I like the calculated figure since it's slightly larger, of course, but also it's the "real" number as far as what fuel it took to replace what I used in my previous travels. Whether the gas station is giving me the exact # of gallons indicated on the pump, I don't know. Probably not. But it's all I have to go by, so I'll use it. YMMV.
Because of all of this I've tracked my CX-5's mileage from day 1 also. Every fill-up I reset Trip A, Avg. mpg and Avg. speed and record the previous values along with # of gallons used from the pump. After 84 fuel-ups over 31,274 miles the average from my car display is 32.9 mpg while my calculated average based on odometer reading and # gals. pumped is 33.6 mpg. It's a difference of 0.7 mpg, just a bit over 2%. So in this case the on-board computer is understating the mpg but not nearly as much as my Nissan overstates it.
I like the calculated figure since it's slightly larger, of course, but also it's the "real" number as far as what fuel it took to replace what I used in my previous travels. Whether the gas station is giving me the exact # of gallons indicated on the pump, I don't know. Probably not. But it's all I have to go by, so I'll use it. YMMV.