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Accurate MPG Calculations Question

5K views 27 replies 10 participants last post by  rdgrimes 
#1 ·
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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#2 ·
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.
 
#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?
 
#13 ·
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.
 
#3 ·
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.
 
#4 ·
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.
 
#6 ·
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!
.
 
#9 ·
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.
 
#10 ·
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.
 
#16 ·
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.
 
#17 ·
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.
 
#18 ·
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.
 
#19 ·
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.
 
#21 ·
Ive had my 2017 RAV4 Hybrid for about 2 months now

When I fill up, I put in just a little bit past the first click. Maybe .15 of a gallon. I had one time where I wasnt looking & put in over .25, but then I saw gas overflowing, so I stopped. I wouldnt add too much after the first click. My mileage has been fairly consistent around 38.5 mpg, one tank a bit below & several tanks above, Ive been logging my fillups on fuelly. I drove my first tank on eco mode & switched to sport mode for the additional power & had a slight drop off & now driving back on eco.
 
#22 ·
If you keep a log of your fuel, such as the Fuelly.com website, it does not matter one bit how much is filled up at each stop!! Why is it hard for some to understand this concept.

The only advantage to topping off -- and risking fuel spill-- is to get farther on that one tank. Like if you were in Nevada in the middle of the night and needed to go 620 miles to the next open gas station. The extra .15 gallons would get you an additional 6 miles! But it doesn't change your MPG.

To recap: If you fill your tank to the first click each and every time, and calculate the fuel used over the distance, you will have an accurate read of your MPG. The little tweaks of over-filling mean nothing. The more fuel-ups, the more accurate the MPG--and takes into account minor discrepancies of each individual fuel-up.
 
#23 ·
Not so sure about that extra .15 gallon resulting in an additional 6 miles. How much more would one save by careful acceleration, braking, not having excess speed, air conditioning off vs window open, excess weight due to trunk loaded with excess weight items, car in perfect running condition (i.e. tune-up, tire inflation) . And beyond ones control is the road conditions, traffic. Look elsewhere first before dong that extra click for 6 miles.
 
#24 ·
Why do some people act so condescending of others?

To answer your response

1) Its pretty consistent when the pump clicks, but its not exact. I top off my tank to the nearest dollar or gallon. If you do look at my fuelly, my readings are very consistent. I had 1 tank where I partially filled my tank because of a promo on the price of gas, even then, my mpg was pretty much the same. I dont top off to extend my miles driven. If I wanted to do that, I could probably drive another 100 or so miles because theres still almost 4 gallons in the tank. Either way, its not going to be exact & Id rather err on the conservative side.

2) I overspilled because I was looking at the pump & didnt see that fuel was spilling out. It actually was my first tank & wasnt sure how much additional I could add to top it off. I found out real quick. And you are wrong about it not affecting the mileage. You divide miles driven by the amount of fuel added, it does make a difference, albeit slight. Filling to the first click every time will not give you the exact amount of fuel used every time. That first click would need to be constant every single fill up.

And by the way, more fill ups gives a better average its still not going to be perfect. In the end both of us are arriving at the same means give or take a few tenths of a gallon.
 
#25 ·
...And by the way, more fill ups gives a better average its still not going to be perfect. In the end both of us are arriving at the same means give or take a few tenths of a gallon....
I agree with some of what you say. But keeping a running total as you do on Fuelly is not an "average." It is a true running total that becomes more accurate over time because you have more fuel-ups and the small differences in various pump stoppage is negligible. And we do arrive at the same totals of fuel put in the tank over time--only you are adding an unnecessary step which could damage the system if you spill.

It is kind of fun to hit an exact number on the meter or for the purchase, but none of it affects the MPG by a few tenths or any other amount.
 
#26 ·
Im just happy with my 39 mpg that Im getting & more so far on this tank since I went back to eco mode.

I am confused about something though. I tried to switch to EV mode, as soon as I hit 40 mph it disabled itself. As Im still learning about the vehicle Im not sure why this happened. Is it because EV mode is strictly for local driving?
 
#27 ·
That answer is floating around on several other threads that are ongoing, so check them out. Good for you on your mileage! For the purposes of this discussion, I just try to stay in the bounds of the original question from the the person who started the post--that is how to get accurate MPG calcs.

From the little bit I know of the EV mode, it mostly prioritizes low-speed travel and is especially useful in stop and go traffic. But the ICE (Internal Combustion Engine) is there to keep the EV battery charged and provide power in a mixed way. For a variety of other reasons, like hiway speeds, the ICE (aka gas engine) takes priority. The EV kicks off fairly quickly by itself.

By the way, congrats on your hybrid! I'm still milking a couple more years from my 2009, but I do track all the posts from this generation as it will be my next purchase.
 
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