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I have read older threads here that deal with MPG calculations and some things do not add up (to me) in what some consider milage calculations.

When I write down my milage and gas fill-up, most of the time it comes out within .25 to .5 mpg as the mpg register on the screen. I don't stop at the first click, usually the second and then round up to the next 5 or 10 cent figure. (ObsessiveCompulsive) However, I do always put the handle in the slowest fill rate and I know that affects the volume. (BTW, Our 2017 Rav4H Limited's mpg screen seems fairly accurate when I do this.)

However, I have read a few posts from some on other threads that they do not get what they feel like is accurate figures from Toyota’s calculations. Instead they divide gallons (or kilometers) into gallons or liters.

That is what puzzles me. How does one get accurate calculations from that - if the gas volume is based on the first click of the gas handle?


My father had a gas distribution company in the late 50’s through the ‘60s. I had a couple of relatives with SERVICE stations. Accurate Gas distribution through pumps was something I understood as a teenager growing up. And I know that even today, every pump delivers at a different rate, if even minutely. The gas flow rate on some is fast, and gas flow on others is slow. The number of vehicles being filled at the same time at a station affects the overall flow RATE.

Flow rate determines blowback which determines when the pump handle will click off, and stop the filling. There can be as much as 1.5 gallons difference in filling a tank with a slow fill versus a fast fill.

So, the pump flow rate and blow back determines when the handle cuts off, not the amount of gas the tank actually needs before it gets to the “neck”. Notices I did NOT say INTO the neck of the tank. And every station has a different pumping rate, depending on the number of vehicles being filled at that moment and even then individual pumps allow for a different pumping/flow rate. I lived in Japan for 26 years (7 years in Toyota City - Koromo-cho) and when Japan changed largely over to self serve in the early 2000’s, I noticed the same there. This seems to be universal. Because of my background, I can tell fast flow rate from slow flow rate simply by the sound.

Even using the same pump at the same station every time will not give one totally equal delivery rates every time. I have been to a few pumps that deliver faster on their slow rate - than some other stations do on their fast rate. This affects the handle cut-off.

What speed do you use to fill the tank? The fastest notch, the middle notch, the slowest notch to fill your tank?

I am puzzled how an accurate MPG can be obtained when an accurate measurement of the filling can not be obtained - IF one goes by the first “click-off” of the pump handle.

I have not seen this discussed anywhere and would like some opinions on this aspect, if you don’t mind.
 

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The neck is part of the volume of the tank, regardless of rate-of-delivery, so calculating mileage using the total amount of fuel should still be accurate, or so damn close it doesn't matter. (Pumps today measure the fuel as it leaves the well and makes an allowance for hose volume.)

IMHO you are over-thinking the whole thing.

Oh, and overfilling is a bad idea because it is possible to put gas into the evap. Overfilling is not necessary, and just a bad idea. You gain nothing.

If you do not believe your gas pump is honest, order an audit. Let us know that works out for you.
 

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Yeah, you are over-thinking this. To get an accurate MPG, you just keep a tally of the fuel going in and the miles or kilometers driven. It doesn't matter which click you stop at. (Except you do risk a problem as Pico said if you excessively top up and overfill.) The overall tally will adjust for the variables at the pump at each fill up. Give it about 10 fill-ups and it will all come out in the wash. The speed at which the fuel goes in means nothing, as does the click-stop over multiple fill-ups. Worrying about chasing an exact MPG on one tank is not realistic.

The computer gives a ball-park estimate and is useful as an approximation, and certainly gives valuable info in the short term. I personally think it will also "adjust" over several tanks, just as a manual tabulation will.
 

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Speaking more from other cars than just my RAV4 (since I've only had it for a short time) - I generally fill my tank at the slowest "set and forget" notch on the pump. After it stops I'll manually hold it one more time just to be sure it's actually full. I've had cars that would occasionally prematurely stop filling for one reason or another, so I like to check. But I only ever go to the second click. I also try to fill up at the same pump every as often as possible.

I would never base any calculations on just one fill-up. But it's probably safe to say that after 10 fill-ups you're probably eliminating a lot of the "noise" in the data, and certainly the more tanks you average over, the more meaningful your average.

I personally just use a spreadsheet in which I keep track of number of gallons, number of miles, and the REPORTED MPG from the trip computer for each tank. I can calculate my MPG and then compare the number against the REPORTED one and determine the estimated error rate for a single tank. But to really get a better sense of the average error rate, I keep a running total of all the data in order to be able to calculate the average MPG over, say, 10,000 miles. I can then look at what that calculated MPG is against the average of all my reported MPGs. Any minor fluctuations will really be evened out over that long of a distance / time frame.
 

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What RAVN4RED wrote. Calculating over a few fill-ups 'smooths', normalizes the idiosyncratic differences to find a real average.

Drive, be happy.
 

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I personally just use a spreadsheet in which I keep track of number of gallons, number of miles, and the REPORTED MPG from the trip computer for each tank. I can calculate my MPG and then compare the number against the REPORTED one and determine the estimated error rate for a single tank.
What a bunch of trouble for an insignificant gain or loss. Insignificant!
.
 

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Discussion Starter #7
The neck is part of the volume of the tank, regardless of rate-of-delivery, so calculating mileage using the total amount of fuel should still be accurate, or so damn close it doesn't matter. (Pumps today measure the fuel as it leaves the well and makes an allowance for hose volume.)

IMHO you are over-thinking the whole thing.

Oh, and overfilling is a bad idea because it is possible to put gas into the evap. Overfilling is not necessary, and just a bad idea. You gain nothing.

If you do not believe your gas pump is honest, order an audit. Let us know that works out for you.
So, what is "overfilling"?

Basically if there is no set level, there can be no accurate measurement of MPG. Right?
 

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Discussion Starter #8
Speaking more from other cars than just my RAV4 (since I've only had it for a short time) - I generally fill my tank at the slowest "set and forget" notch on the pump. After it stops I'll manually hold it one more time just to be sure it's actually full. I've had cars that would occasionally prematurely stop filling for one reason or another, so I like to check. But I only ever go to the second click. I also try to fill up at the same pump every as often as possible.

I would never base any calculations on just one fill-up. But it's probably safe to say that after 10 fill-ups you're probably eliminating a lot of the "noise" in the data, and certainly the more tanks you average over, the more meaningful your average.

. . .
Your fill up method has been mine also. However, I have seen several quotes from people (on this forum) that stop on the first click, period. That stumps me because I have experienced pumps that cut off at 8 or 9 gallons (Very Fast flow rate) when I know the tank is near empty and should hold 12 or so gallons.. I say this from experience on previous a Prius 2010, my 2009 Camry Hybrid and getting used to my wife's Rav4.

When stopping on the first click, the variation of amounts will give way off base MPG, IMO. Averages over 10 fill ups I can see as giving a fairly good average, but not on a single fill-up.

Filling up at the same station is not possible for me or my wife. We do have a local station that we use at home (small town, 8 stations) but we do fill up in a large city 30 miles north due to 20 - 25 cents a gallon cheaper, or another town south (Wally world) with gas 20 cents a gallon cheaper. I do not work in the town I live, so fill up at the same pump consistently is difficult.
 

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Many states and counties have inspectors who actually test fuel pumps to make certain that they deliver a consistent VOLUME of fuel regardless of the flow rate. If a driver is concerned that their gas tank is not full at first pump click off simply wait for maybe 10 seconds to allow any trapped air in the tank to escape and then run the pump until the nozzle clicks off again, then stop. Overfilling can cause problems, as other posters have noted. Fueling a vehicle shouldn't need to be a using an eyedropper to cram in that last drop of fuel experience and doing so would have a negligible effect upon mileage calculations.
 

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Lee, I agree! IMO there is no such thing as overfilling - unless gas is running out on the ground that is.

I've always topped my cars off for probably 40+ years, sometimes until I can see gas in the filler neck. Makes for less stops for gas and more accurate mpg calculations. Others may swear by the first-click method but I often get as much as another two gallons in, sometimes as much as the tank's listed capacity. What I never do is drive 100ft and park, my house being four miles from the nearest station I frequent.

And my answer in advance to the dire "overfilling" warnings of damaging something in the evap system is I've never had a problem with any of the likely 30 or more cars I've owned.
 

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What a bunch of trouble for an insignificant gain or loss. Insignificant!
.
Well hey... we're all on this forum because we have an interest in our cars in one way or another, right? Some of us happen to enjoy tracking our mileage, that's all... :smile
 

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...When stopping on the first click, the variation of amounts will give way off base MPG, IMO. Averages over 10 fill ups I can see as giving a fairly good average, but not on a single fill-up.

Filling up at the same station is not possible for me...
Okay, I see what you are trying to do: be consistent with how much you fill each time. Yes, the click-stop will likely vary from pump to pump and other variables. Topping it up each fill would be more accurate for the one tank.

But you are missing the point about just tallying the amount of fuel you put in. It is not an "average" over the proverbial 10 fill-ups, it is the actual fuel used. It wouldn't matter if you put in 7 gallons each time or 11.2 gallons, as long as you start with an approximately full tank and end with an approximately full tank, and total it all up and divide it into the distance. The more fuel-ups and the greater the distance, the more accurate the MPG. (Thus the value of the the website Fuelly.com where owners can do just that. After 20k-miles, I feel pretty confident I know what the MPG within a gnats azz, and can spot outliers.)

As for the practice of topping up, the warning in the owners manual is that 'spilling fuel during fuel-up can cause the exhaust system to operate abnormally or damage the fuel system components.' Some members here have reported this problem. As long as you are careful not to overdo it, you should be okay. But why? just keep a running tally and chill at the first click. For short term grins, the computer is going to be fairly close.
 
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So, what is "overfilling"?

Basically if there is no set level, there can be no accurate measurement of MPG. Right?
The gas you pump is what goes into your tank including the neck. What is so hard to understand? The mileage gauge is an approximate. Forget it. Use miles and gallons. Simple, no?

Divide the miles traveled by the gallons you put into the tank. The result will be your average Miles Per Gallon.
 

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Discussion Starter #14
The gas you pump is what goes into your tank including the neck. What is so hard to understand? The mileage gauge is an approximate. Forget it. Use miles and gallons. Simple, no?

Divide the miles traveled by the gallons you put into the tank. The result will be your average Miles Per Gallon.
Yes, miles divided by gallons is the average; but you are missing the point. Stopping on the recommending "first click" can give you 8 gallons or it can give you 11 gallons, depending on the flow rate, in tank back splash and pump handle sensitivity. So using the gas amount shown on the pump at the pump handle click-off will not give an accurate average!
 

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I use fuelly.com to track my mileage and after 10 000 km, I must say that the computer is very accurate!
 

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With gas pumps in much of the West as I've experienced one has a difficult time refueling until one can see fuel in the filler. Filler nozzles are equipped with devices which shut off fuel flow if there isn't a tight seal against the fuel tank filler cap flange, to prevent air pollution as the fumes from the vehicle gas tank are drawn into a condensing chamber. One can try to cheat by physically holding the sealer device to simulate sealing, but hat isn't simple and even if that works one generally ends up with a strong gasoline smell on the holding hand.
 

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Gas pumps in the West generally will not allow filling until one can see fuel in the tank's filler due to a mandatory sealing air pollution control mechanism which means that the filler nozzle always must be in contact with the vehicle's tank flier flange. If contact is broken the filler nozzle is switched off. Sometimes it can be defeated but even if successful the user ends up with a hand which smells strongly of gasoline.
 

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I do not understand. What is so important to know the exact mileage. What difference does it make if one is off by a gallon or so. What difference. And if one keep a spreadsheet on this, for what purpose. It is just an estimate, as is the car computer--just an estimate. But if you want a precise number, need to make sure the exact, exact, amount is filled into the tank. Still do not know what purpose this all serves.
 

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I do not understand. What is so important to know the exact mileage. What difference does it make if one is off by a gallon or so. What difference. And if one keep a spreadsheet on this, for what purpose. It is just an estimate, as is the car computer--just an estimate. But if you want a precise number, need to make sure the exact, exact, amount is filled into the tank. Still do not know what purpose this all serves.
jaybb,

When I bought my (my wife's) Rav4H, I came on this forum and began reading older posts and on some of the past 2 years, I saw owners figures posted on 38 - 40 MPG (most of it on the EV mode) I saw several people referring to their calculations and there were several posts that dealt with (almost as an absolute) that one should not fill past the first click. This is not about one post, but over several posts and several people. My experiences are quite different, and if anyone mentioned filling past the first click of the pump handle, or complained of 29 to 31 mpg, there were legalists who would blast the questioning owner as not knowing how to do things.

My questions deal with the fact that MPG measurements by the first click (as mentioned in past THREADS here) do not seem to me to be accurate representations of true MPG. AND some of the references were that the RAV4H gauge was not as accurate. Perhaps if would have been better for me to have questioned this in the past posts, but those posts/threads were 6 months or a year old.
 

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jaybb,

When I bought my (my wife's) Rav4H, I came on this forum and began reading older posts and on some of the past 2 years, I saw owners figures posted on 38 - 40 MPG (most of it on the EV mode) I saw several people referring to their calculations and there were several posts that dealt with (almost as an absolute) that one should not fill past the first click. This is not about one post, but over several posts and several people. My experiences are quite different, and if anyone mentioned filling past the first click of the pump handle, or complained of 29 to 31 mpg, there were legalists who would blast the questioning owner as not knowing how to do things.

My questions deal with the fact that MPG measurements by the first click (as mentioned in past THREADS here) do not seem to me to be accurate representations of true MPG. AND some of the references were that the RAV4H gauge was not as accurate. Perhaps if would have been better for me to have questioned this in the past posts, but those posts/threads were 6 months or a year old.
So, are we saying that if one pumps past the first click, which I never do, the mileage drops from 38mpg to 29mpg. Seems like that drop in mileage is hard to be believed based on filling beyond the first click. If I am reading this correctly, adding a tiny bit (maybe 1/2 gal at most) of gas beyond the first click results in an average drop in mileage of about 10mpg. I would never conclude that. I would think that by adding that small amount of gas past the first click (say 1/2 gal) has no real affect on the mileage--even if it was to mostly evaporate or drain off.
 
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