calculated vs. computer MPG - Please post your results

Flag that tank, and see what happens on the next tank. Assuming you pump until it shuts off, maybe you got a pump with a touchy shut-off mechanism, it wasn't quite full? If the next tank is not so good, and markedly lower than what the car's telling you, that'd be a tip-off.

 

For best repeatability & refueling accuracy:

1) always set gas pump handle on "slowest" setting or handle-step.
2) STOP pumping fuel when the gas pump handle clicks OFF.
3) Do NOT attempt to "fill-to-the-brim" (so you don't flood the carbon "vapor" canister).

 

Just got my best tank ever -

534.8 miles / 9.337 gal = 57.3 MPG

indicated = 60.6 MPG
Avg speed = 25 MPH

I've made gains in MPG recently by scanning ahead of the car in front of me in heavy, commuter traffic, anywhere really. I'll look to adjacent lanes left, right for brake lights and to see flow of traffic. End up using brakes less and get higher MPG.

 

Do you increase following distance too? I find that invaluable: anytime there's slow down ahead, I can just coast up to it, "eat" the gap, and as often as not avoid having to touch the brakes. Yes drivers hop into that space from time to time, I just say so what, makes life easier for all parties.

 

Forgive me if this has been covered already (I haven't read the entire thread), but how can anyone be so precise about their own calculations as so much of the stuff I'm seeing here? (Gallons and MPGs expressed to as much as 3 decimal places!)

Unless you fill up at the same pump everytime, how can you know the pump is "clicking" in the same spot? And maybe even the same pump won't respond the same way twice?

Are any two "full" tanks exactly the same?

 

Yes, I follow about 4 seconds back, more than most on freeway, and coast up. The 4 second habit came about after an 18" crack in my windshield in April (honestly, I don't recall the rock hit that caused it). I looked at 3 aftermarket windshields and everyone had the rearview mirror mount tab at least 1" lower down the glass (outside of the shaded 3rd visor (non electrochrome mirror)) ...

I allowed shop to put in PGW aftermarket glass seeing it also had mirror tab out of place, but the quality was not as good as Toyota. It was annoying to look through.
Battled some more and finally got approved for Toyota glass, but that is expensive for the insurance co, about $1,200 job total opposed to less than $500 for aftermarket glass. toyota windshield alone costs about $1,000 ...

So, I keep my distance on the highway. Quite disturbing how the aftermarket windshields differed from the Toyota glass.

 

Well, the gas receipts give 3 decimal places, like 9.337 tonight. So, that's what I feed into Fuelly - Track and Compare your MPG

Car only gives one decimal, so I only quote one decimal for MPG

 

No, they aren't. As I said in another thread, my manually calculated mileage has differed from the CONS display by as little a 1.8% and as much as 9.9% over the 30 fill-ups I've done since we bought the car in July. Most of the fill-ups were done on a 7,400 mile trip, but it differed by 4.5% - 6.4% for the 5 times I filled up at the same station here and also differed using the same pump twice 4.5%-5.8%. I always use the lowest speed setting and stop when the pump shuts off.

I only use the CONS display to gauge how good or bad I'm doing over various points along a route, like when the speed limit rises to 75 and 80 mph or when I encounter a lot of hills. Unlike many, I don't alter my driving much to gain mpg. I've always watched traffic ahead and slowed for traffic lights, etc. Driving in Phoenix for the past 20+ years, I've learned not to hurry too much just to get to the next traffic light, especially when taking an unfamiliar route. I was using Fuelly, but now I just keep track in my own spreadsheet where I can see how much I'm saving in fuel costs over the van we used to drive, just to satisfy my curiosity. We'll hit our first 10,000 miles in a week or so and the minimum savings so far are over $650 and probably closer to $800+ if we had driven the van over the same terrain, etc., on our trip.

 

Yeah, that makes sense to do that, of course. I'm just saying that all this fixation on the differences between the computer MPG and what is calculated manually seems somewhat futile to me since how much gas you've actually used between fillups is an estimate at best. I don't doubt that the computer probably reads higher, but it's probably a more consistent measure within itself.

 

No two tank fillups are precisely the same, obviously. But if you use consistant technique, they'll be close. If, for example, you underfill the first tank, it'll balance out with the next fill up.

 

"it will balance out with the next fill up"?? How do you know that? It makes sense that over the course of many fillups one could presume things would average out. But over two or three? I don't see the logic in that.

Even assuming that the pump reads out an accurate measure of what it actually put into the tank, I see no reason to believe that any two pumps would "click" at the same time or even that any one pump would "click" at the same time twice. Even assuming one uses "consistant technique" when filling, you're still trusting the pumps to operate in a consistent manner.

And with a tank as small as the Prius tank--fill ups usually under 10 gallons---and a car that gets around 50 MPG, a difference of even a few tenths of a gallons would make a big difference in computed MPG. And for all anyone knows, the differences could be much more than that. Who's to say that Pump A isn't "clicking" a half gallon earlier than Pump B?

 

1) Lookup the meaning of "Law of Averages" along with "...sufficient number of data points..."

2) Which do you think is more accurate, a gas-pump which is calibrated (at least) annually to 3-decimal places, ie: X.YYY, or a 1-decimal place estimation number from an ECU that is never calibrated?

3) And, remember, the ONLY common value between YOUR hand-calculated MPG and the cars CONS estimation is the distance (MILES) collected on the odometer (to 2-decimal places internally from wheel speed sensor).

 

Snark aside, I already said one could presume things would average out over many fillups. But some people are saying things like "it will balance out with the next fillup". Since when is TWO a "sufficient number of data points"? Or are YOU the one who needs to look it up?

As I already said, I trust the pump reading to be reasonably accurate. What I DON'T trust is that the mechanism that triggers the "click" will operate consistently between pumps or even between fillups from the same pump as this is just the result of a vacuum tube reaching a certain level of fill. Are these pumps all built the same way? Of course not. There's no requirement that the shut off mechanism be calibrated to anything.

All I said regarding the ECU is that I would expect it to be more consistent within its own readings. If it says I got 2% better MPG over this 500 miles than the last, I'd expect that to be reasonably accurate. Because it would know how much gas actually passed through the system over that 500 miles. But between hand-measured readings? To what degree can those measurements be trusted if you have no idea if the pumps filled the tank to the same level each time?

As I said, I think it's pretty clear from all the data collected that the ECU gives a higher-than-actual MPG reading. But in all honesty, you'd probably be better off just lopping a couple MPG off the reading and going with that as opposed to trying to hand-calculate your MPG when you don't know how much is actually in your "full" tank.

Unless you want to top off until you can actually see the gas perhaps. But then that causes other issues with MPGs. But then at least you'd know for sure that there was the exact same amount of gas inside the tank each time.

 

I wasn't addressing you specifically, but rather, directing the 'doubting' Thomas's who were arguing against what you wrote to "...where to find the answers."

...and, I do know the difference between "Law of Averages" (samples) and the "Law of Large Numbers" (probability).

 

Rather than being about a law of averages, it's more about that over time you're using different figures to determine calculations. If over time you've driven 25,000 miles and purchased 500 gallons of gas then determining you've averaged 50MPG over that period would be pretty accurate. Because whatever is left in the top or bottom of the tank at the beginning and the end becomes irrelevant. It either isn't in the calculations at all, or is so minute as to have virtually no effect on the calculation.

But for measuring MPG over individual small trips or tanks of gas then it's a different calculation. Because while I can trust that I put 9.677 gallons of gas into my car when the pump clicked off, what I CAN'T trust is that that same figure can be used to tell me how much gas was consumed since the prior fillup. I might as well be determining how many miles were travelled by adding up the distances posted on the highway signs between cities.

 

-2015 persona

-Tank #1, filled by dealership.

-just over 10 gallons to fill er up.

- 47.5 mpg indicated.

-45.01 mpg calculated.

So about 5% off..

And btw this was about 2 weeks of my 18 mile each way commute into Philly from the burbs taking it easy using CC and pulse n gliding, tires at 40/38, Winter blend gas sucks.

 

So far I've had 2 fill-ups (technically 3, but one was only a partial fill-up, so I combined it with the full fill-up after it).

First (since leaving dealership):
Calculated MPG: 49.06
Indicated MPG: 51.4
Error: 4.55%
Notes: Driving ~10-20 miles a day, local suburban roads. I was working part-time with the occasional trip to Target or my grandmother's house.

Second:
Calculated MPG: 48.96
Indicated MPG: 52.8
Error: 7.27%
Notes: Got a new job! Covers my new commute into NY and back every day (about 70mi round trip) with extra 20-40 miles on the weekend. Mostly highway driving with some stop and go due to traffic (not quite as "ugh" when I know I'm not wasting tons of gas).

I found it interesting that my indicated mileage went up while my calculated mileage remained about the same, or decreased by a tiny bit. I wonder if it's just a quality of the data or if the colder weather has had something to do with it. We'll see.

 

Granted, it's a generic "rule-of-thumb" approach, but mentally subtracting 2-MPG from the dashboard displayed Consumption-MPG number yields an immediate "reality check" MPG in realtime as you drive, which at fillup time, typically holds true.

 

2015 Prius Persona, computer MPG reads typically about +3 mpg higher; I have only had the car a month, but am averaging 49.85mpg in real life, computer typically reads 53mpg or so.

I had a MINI Countryman where the computer was always way off, high sometimes, low others. I take computer MPG read outs with a grain of salt and concentrate more (on how my tank is doing) by monitoring the fuel level vs. the mileage for that tank.

Craig
Beaufort, SC

 

I bet a LOT of people aren't aware of this but one explanation for variability between when the pump clicks off has to do with the temperature of the fuel in the tank. If the car is stone cold, you can stuff more fuel into the tank before it clicks off, giving the perception of worse fuel economy and the reverse situation also true. So if you fill up your tank after a long drive, the hot fuel and vapor that is still in the tank will have the tank more pressurized so when the pump detects the vapor pressure limit in the tank from the nozzle, it clicks off.