Car computer gas mileage

toyota
camryhybrid

#1

I have a 2009 Toyota camry hybrid. It has considerable electronics including a pretty useless NAV (cant do anything while driving). But my car computer ALWAYS reports a higher gas mileage than I calculate when I purchase gas and fill up the tank. I have spoken to the dealer several times, and they “took it for a ride” and pronounced the computer fit. But after 5 years and many gasoline purchases, I can say

with EVERY purchase of gas, the calculated mileage is LOWER than the reported mileage. EVERY ONE.

If this were a random thing owing to different fill levels, then I would expect that the calculated mileage would be higher than the computer’s calculation. But never have I calculated a higher mileage based upon my purchase.

So I have to wonder if this is a conspiracy by Toyota to make their product look better than it actually is.

I have gotten NO help from Toyota, either. In fact, it seems that they just do not understand the issue.


#2

???

If the mileage is off by a consistent %, then it’s perfectly serviceable…just factor jn the 10% (or whatever) to get your actual mileage.

The mileage computer is a “nice to have” vs. “need to have” technology, and Toyota will not perform a warranty repair just to calibrate it. Nor should they.


#3

“my car computer ALWAYS reports a higher gas mileage than I calculate when I purchase gas and fill up the tank.”

By what amount does it report a higher gas mileage?
Is it something along the lines of less than 1 mpg, or is it more like 3-4 mpg?

When I got my current car, I was skeptical about accuracy of the gas mileage read-out, but after checking it several times, I found that it reported gas mileage that was consistently .4 mpg optimistic, and that is close enough for me.

I don’t consider an error of less than 1/2 mpg to be…conspiracy worthy…but others may differ.


#4
But my car computer ALWAYS reports a higher gas mileage than I calculate when I purchase gas and fill up the tank.

They are notoriously always wrong. The don’t use very sophisticated metering to determine gas usage and mileage. It’s calculated based on RPM’s miles. The calculation get’s thrown off a lot if you drive anything but flat straight roads.


#5

it is wrong by a variable amount (0.5-6 mpg), which might be consistent with MikeInHH comment.

So I cannot use the mean joe “factor.”

My point…the car cost a lot of $$ and is touted as a high gas mileage vehicle. The computer typically reports 33-37 but I get 31-33. I am guessing that most people just go by what’s on the dashboard and do not follow their purchases, which is why Toyota gets away with this. But for a lot of money, I expect my accoutrements in the vehicle to work correctly. The mileage calculator is not even close.


#6

To me, consistency is more important the accuracy in the computer mileage readout. I keep tabs on the mileage to give me an idea of the health of the engine. If there is a big drop in mileage, I look for the problem. Before we had these computer readouts, I would calculate the mileage in my head by rounding the number of gallons to fill the tank to the nearest gallon and then adjusting the distance traveled to compensate. I’m sure that the computer readout is probably more accurate, but I am not obsessed with calculating my mileage to the nearest .00000001 mpg.
The first vehicle I owned with a computer gasoline mileage indicator was a Ford Aerostar Eddie Bauer model. The mileage indicator always read higher when our son, who was a teenage driver used the Aerostar. He didn’t know how to reset the readout and probably didn’t even know it what it meant. After I rode with him, I figured out why. He had a light touch on the accelerator and drove without heavy acceleration or abrupt stops. He learned his lessons well in driver education.


#7

Does trip computer do anything correct? Can u fill tank, drive 2-3 days, consult computer “fuel used” and than fill tank again and verify amount used? If u know for sure u used 4.5 gal, and than refill, u should put in 4.5 gal.


#8

“My point…the car cost a lot of $$ and is touted as a high gas mileage vehicle.”

You should also consider that the touted gas mileage is calculated based on driving in perfect conditions at the right speed. I’ve never owned a car that has lived up to its claimed mileage but I don’t drive in a perfect world.

If the computer calculated mileage is within .5 mpg of your calculation, it’s much better than I would expect. As others have written, consistency is more important than perfect accuracy. In my opinion, it should be thought of as a tool to show you when you are optimizing your fuel consumption rather than something that is 100% accurate or a conspiracy by the car manufacturer.

Your mileage may vary… :slight_smile:


#9

We seem to have had this discussion numerous times. I calculate mileage by using the oldfshioned way and use actual fuel used and actual distance driven. Anyone with a grade 3 education can do this. Vehicles that show the instantaneous as well as the elapsed miles per gallon cannot do so very accurately; it’s a convenience feature to help sell cars.

Expensive jet aircraft have accurate fuel flow meters and fuel tank measurering equipment as well, and for good reasons. There are no filling stations in the sky and bad weather, like strong headwinds, can drastically increase fuel consumption.


#10

Are you resetting the MPG meter after each fill up? If you’re not, you’re just seeing the cumulative MPG since you last (if ever) reset it. So while you’re manually calculating the MPG for your last specific tank, the MPG readout is going to tell you what you average MPG has been over the past 90 or however many tanks you used since the last time you reset it. If that’s the case then it’s going to display a different number vs. what you calculate manually.


#11

I was under the impression that OBDII vehicles calculated MPG by looking at pulsewidth X number of injections. Basically, total amount of time injetors are “on.”

I had a Scangauge that worked on this principle; you entered “actual” full burn and it would then apply a correction.


#12

If you really want accurate measurement of gasoline mileage, then you should do what Consumer Reports did on its car tests. CR had a measured container of gasoline that was hooked directly to the fuel pump. The gasoline container was in the cabin of the car. A fifth wheel was attached to the back bumper that accurately measured the distance. The car was driven until the fuel in the container ran out and the mileage calculated to at least the nearest 1/10 of a mile per gallon. This was back in the days when cars had carburetors and the fuel pump was under the hood. The fifth wheel was also used to check the accuracy of the speedometer and odometer.
From my training in statistics, I am satisfied with a 5% margin of error on my measurement of miles per gallon.


#13
I was under the impression that OBDII vehicles calculated MPG by looking at pulsewidth X number of injections. Basically, total amount of time injetors are "on."

And you are wrong. And even with that method it can be very inaccurate. A clean injector is going to dispense more gas then an injector that’s even slightly.


#14

Uh, no kidding mikey. A partially-clogged injectior would still be off by a consistent amount, though…like a rifle that shoots consistently 3" right of target, it can be accounted for.

So, inform us, what metric is used for OBDII mileage calculations? Or are you withholding that info for some deep psychological reason? Because–on its face–it would appear you chimed in with a snappy put-down just for some childish opportunity to “score.” (And, in fairness, I did preface my comment by “under the impression,” indicating less-than-certain confidence.)

Your recurring flaw is that you talk about subjects that you lack ANY first-hand knowledge while grossly over-representing your degree of confidence in your opinion.

Since you did NOT preface YOUR comment thusly (I.e. “I suspect you would be wrong”) you must have solid information proving me wrong. I say present it! Failing that, you’re just talking out of a cavity not generally reserved for that purpose.


#15

Since this is estimated mileage, oxygenated winter gasoline will show lower actual mileage than during the summer. Yet, the estimated mileage should be the same throughout the year. Maybe some of the difference is related to the gas blend.


#16

You gas mileage calculations from fill up to fill up are only accurate if your odometer is accurate.
You can check your odometer pretty well in NY State on I-90 with the mile markers.
Don’t try it in Ohio though, they seem to space them where ever they land when they kick them out of the truck.


#17

UPDATE: Read what .GM’s Roger Clark says about how onboard mileage works.

Q…to da E…to da D.

I stand behind my earlier post.

@MikeInNH:


#18
A partially-clogged injectior would still be off by a consistent amount, though...like a rifle that shoots consistently 3" right of target, it can be accounted for.

Ummm…No…You’re making the assumption that the partially clogged injector will stay in that same state the whole time. It may become more clogged and even unclogged. So it WON’T be consistent. Nice try though.


#19

Interesting article meanjoe75fan. That is how I had understood it to work as well for my GM vehicles. I expect each manufacturer has their own version perhaps even significantly different than the GM design.

I applaud Roger Clark for being the only one to respond to the inquiry but his assertion that it’s “dead nuts accurate” simply can’t hold up to the design limitations;

The manufacturing variation in everything related to the odometer measuring the distance travelled. This includes the constant reduction in tire circumference as they wear.

The variation in energy content of the fuel being consumed from tank to tank.

The variation in the delivery of fuel being measured (flow, pressure etc)

And so on. It cannot be dead nuts accurate with all of this variation and considering some will tolerance stack up to either extreme means they will be off considerably more than others. This is in effect an offset error. The dynamic variations will be noise about the offset so expecting relative consistency with an offset correction may not be possible either…


#20

Actually I think it can be made to be a lot more accurate. Less then 1%. Currently 5-10% seems to be the norm.

Odometer measuring has the potential to be extremely accurate also. When a vehicle is running at lets say 2000 rpm’s…the gas consumption for THAT SPECIFIC vehicle is known. If you monitor the rpm’s say 100 times/sec you can calculate the gas consumption fairly accurately.