Lousy computer readings are bogus

How are you making your calculations?

 

Maybe a little of both. It's hard to get a consistant fill, you can vary by a gallon easily. Over 4 or 5 tanks it should settle down.

On my car, the computer overestimates mileage by about 3.25 MPG, or about 6.9% on average. I've tracked it for the last 25 tanks. Weighted averages are 47.06 average mpg calculated, 50.30 displayed mpg. Others have had different results. It depends alot on the enviroment you drive in and how close to tolerence your fuel system is.

 

Mine varies too, but is not consistent.

I believe that the bladder has a lot to do with it...but I also think that the MFD calculations are not perfect and most likely do not account for all factors.

When you are talking about high MPG even then smallest deviation can cause the numbers to skew pretty quickly.

 

There are a ton of threads just like this, and I read all of them and still don't get it. How do people come to believe they know how much gas has actually been used? The capacity of the tank is always in flux; you have no constant upon which to base your calculation, in my mind.

 

I'd prefer the car show numbers lower than expected rather than higher.

Say it has reported I've used my 10 gallons, but then I could only get 9.5 to fit. That'd be OK with me.

People like the poster, and me most of the time, who would be told 10 gallons but then needed 10.5 to be full, aren't exactly happy. I just never bothered to post about it.

And, umm, when you TRY, you can fill the tank to the point where the gas is still sitting there looking at you when you put the cap back on. That makes for a reference FULL point to determine what you added next time, when you do the same thing consistently.

Yes, there are potential downsides, like all life on Earth coming to an end if you spill a drop on the pavement and the car falling apart when some part becomes saturated. It's not like I haven't heard them, it's just that I ignore them...

 

<div class='quotetop'>QUOTE(sl7vk @ Jul 20 2006, 11:04 PM) [snapback]289799[/snapback]</div>

[sarcasm]You're right! Take the car back to the dealer immediately! Better yet, get rid of that debil car with the POS MFD![/sarcasm]

Just drive the car, keep track of everything, and learn. Get back to me when you reach the 5k mark (you'll have some useful FE data by then).

 

you never know if you used all the gas in the tank because of that infernal bladder. you could be adding a gallon or 2 to your actual gas used, which would throw the numbers off significantly. the gas tank is a mystery.

long term tracking reveals a very minimal difference between mfd and calculated mpg over years.

 

So, can someone explain to me how it's possible for the computer to be wrong? It presumably gets the "miles driven" number from the same place that you get it when doing your manual calculation (i.e. the odometer) My understanding is that it gets its "gas used" numbers from measuring the amount of fuel the injectors use. Is it possible that this number is *so* much out of whack?

Even for narf, who is doing it in the right way (i.e averaging over several tanks), I'm wondering if the fuel bladder is gradually expanding. It's just very interesting to me that the computer could be off by that much; I mean, it has to calculate how much fuel it should send to the injectors anyway (i.e. to keep the ICE running in the right way), so shouldn't the information it passes on for the MFD calculation be right?

Clearly, the amount of fuel that can go into the tank with the bladder system is HIGHLY variable, but when someone averages over several tanks (i.e. narf), I wouldn't expect it to be off by as much as it is.

<div class='quotetop'>QUOTE(sl7vk @ Jul 21 2006, 10:14 AM) [snapback]289918[/snapback]</div>

It's not just "stretching." Different fuel pumps respond to the bladder differently. In other words, fuel pumps that aren't properly calibrated (and it appears that few are) will have their automatic shutoff triggered early. *How* early seems to vary from pump to pump. Several people here top off to get fuel up to the very tippy top. However, one of my favorite factoids is that ONE CUP of spilled gas releases as many hydrocarbons into the atmosphere as your Prius does in 8000 miles. It's things like this that stop me from topping off.

 

<div class='quotetop'>QUOTE(sl7vk @ Jul 21 2006, 09:14 AM) [snapback]289918[/snapback]</div>

Realistically, I'd lean more toward the pumps you used to do the two fillups being out of calibration; i.e., telling you you bought more gas than was delivered. Whether it's an intentional rip-off is another matter.

 

<div class='quotetop'>QUOTE(McShemp @ Jul 21 2006, 09:33 AM) [snapback]289965[/snapback]</div>

Gas pump calibration is a favorite topic of TV news shows during sweeps. What they typically find is one or two out of the 20 or so they check is actually out of tolerence. From the only info I could easily find on line, (the Arizona specs), out of tolerence is 1 ounce error in 5 gallons. That's pretty tight. So, I'm not sure I'd assume most of the pumps are cheating you.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 21 2006, 10:51 AM) [snapback]289939[/snapback]</div>

ONE injector. Possibly not the best one for the job, in your case...

 

I've had my measured top-offs tell me I was getting 65 mpg when the computer said about 48 and similarly, recently topped off with ~3.4 gallons and had only gone 101 miles since the previous top-off... that's all of 29 mpg and again the computer was telling me about 48.

I believe that if you track it over several tanks the variabiliy of the bladder becomes much smaller than the summed miles and the summed gallons.

I also notice that sometimes after a top off I go about 50 miles and the guess gauge drops 1 bar from full, but other times I can go 120 miles, which has to be at least two gallons, and the guess gauge still shows a perfectly full tank.

What I do know with absolute certainty is that mpg is more than twice what my old Sebring convertible used to give me (~18 city, ~30 highway) and probably closer to three times the old milage.

Also, the Toyota electrical system is not showing any of the "eccentricities" the Chrysler had. I once hit a water puddle a little hard in the Sebring when it was all of two years old and the next day a water-logged sensor made the engine almost impossible to start. I had to change my mind about driving in to work and drove to the dealer instead.

 

It is impossible to do accurate comparison calculations due to the 'sack' in the gas tank. (I have ranted extensively about this thing (yes I understand its purpose but the rest of the wrold doen't have to suffer its mysteries))

Over about 165 full tanks of fuel I am still showing almost a 1 mpg discrepency between the two calculations. Between 43 and 44 mpg. (62000 miles in 18 months woo hoo)

 

<div class='quotetop'>QUOTE(narf @ Jul 21 2006, 08:47 AM) [snapback]289972[/snapback]</div>

I'll bet they'd find a big difference in California. For many years now, we have had no enforcement of weights and measures. The state government decided a long time ago that it wasn't "cost effective" for them to actually measure pump accuracy and fine stations for cheating. So they now ask stations to check their accuracy themselves. Yep, the fox IS guarding the hen-house in California. On my other car, I've put "22 gallons" into a 20 gallon tank. You decide who's right...

 

As others suggested, keep track of it over time.

Each time I fill up, I record my odometer reading, fuel added and what the MFD reads.

Over a few fill ups, it should even out. Sometimes mfd will be lower, sometimes higher. Try tracking it all in a spreadsheet and you'll see better results.

 

<div class='quotetop'>QUOTE(Maytrix @ Jul 21 2006, 04:39 PM) [snapback]290149[/snapback]</div>

That's right, over the first 10k miles the computer is just over 1% optomistic on my car. I've given up on tank to tank calculations b/c they vary so widely.

 

<div class='quotetop'>QUOTE(Devil's Advocate @ Jul 21 2006, 12:23 PM) [snapback]289990[/snapback]</div>

Agree. I'm up to only seven tanks so far, and the numbers are starting to indicate things.

MPG and Avg are based on hand calculations from the odometer & gallons indicated at the pump.
Tgals is Total gallons pumped so far.
MFD is reported straight from the screen.
CGals is based on the number of gallons the MFD seems to think the car used (Miles/MFD).
TCGals is total gallons the MFD thinks the car's used so far.
CAvg is is (Odo/TCGals).

Odo__Miles__Gallons_Tgals___MPG__Avg__MFD__Cgals__TCgals__CAvg
_321__321___7.005__7.005__45.82_45.82_46.2__6.948___6.948__46.20
_789__468__10.468_17.473__44.71_45.16_45.9_10.196__17.144__46.02
1057__268___6.297_23.770__42.56_44.47_43.5__6.161__23.305__45.35
1428__371___7.598_31.368__48.83_45.52_45.1__8.226__31.531__45.29
1699__271___6.599_37.967__41.07_44.75_45.9__5.904__37.435__45.38
2125__426___7.502_45.469__56.78_46.74_45.9__9.281__46.716__45.49
2480__355___9.674_55.143__36.70_44.97_46.1__7.701__54.417__45.57

Of note:
1. Compare Gallons to Cgals and you'll see the MFD usually thinks I use less gas than I do, but looking at tanks 4 & 6, it seems obvious that I wasn't able to completely fill the tank, especially tank six, since tank seven got REALLY bad numbers. Tank seven, BTW, is where the pump stopped after 3.5 gals, giving me only half a tank, and I had to drive to another pump and add another 8.5 gals. Compare Tgals (55.143) to TCgals (54.417) and you'll see it all seems to average out in the end.
2. My lifetime average is just under 45mpg; the MFD says it's 45.57. Not too much of a difference!

I'm just about ready to switch to the MFD's numbers rather than my own. I think I trust it more.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Jul 21 2006, 10:51 AM) [snapback]289939[/snapback]</div>

There can be couple things that contribute. How is the comp calculating the MPG? Is it using total miles and total gallons, or averaging the immediate results on the energy screen? If going by the energy screen numbers, does it record a data point at the frequency the screen renews or sooner? Depending on the methodology, there can be room for some fudging there.

We know it measures gas used at one cyclinder and times that number by 4. It is possible that the cyclinders aren't getting the exact same amount of gas each. There are also three basic grades of equipment functionality for an instrument; consumer, industrial, and research. Research is the highest. Though, sometimes, it might just be a way of getting someone to pay $4000 for a $1000 blender. Consumer is the lowest and cheapest. I doubt Toyota would pony up the cash for a high accuracy flowmeter to put in a consumer vehicle. So the comp, for whatever reason, may(likely) not be getting the true, real world numbers to begin with.