: accurate computer thinggy on dash

02-18-03, 12:44 PM
ok, i just got back from the gas station. The computer thinggy said that i used 13.2 gallons that time... but the pump said i needed 14.1. How accurate is that thing?

02-18-03, 02:26 PM
The gas pump at your station is calibrated and checked by the state in which it is located.
The computer "thingy" in your car is usually fairly accurate but that's it. I would never believe it to be dead-on-the-money.
It's like your gauges, they're only for general guidance. If you want gnats-ass accuracy, you need professional mechanical gauges, proper wiring, and properly installed senders.

02-18-03, 06:16 PM
yeah, but it was almost a gallon off

02-18-03, 06:30 PM
Take in mind that there could be about a gallon of fuel in all the lines. The computer figures it out by how much fuel the injectors have used, not fuel taken from the tank. So that would be where your extra gallon could have gone.

02-18-03, 06:55 PM
ok.. thanks :)

02-18-03, 07:06 PM
you got it

Allante North *
02-18-03, 07:26 PM
I always fill to the automatic cut-off and then fill to the top of the filler neck. i usually get about another gallon in it. If you do this too, it might be your solution.

02-18-03, 08:00 PM
Filling up to the filler neck is not a good idea. There are vents in there that can get plugged up, and cause the tank to collapse

02-18-03, 08:06 PM
In cold weather, you should NOT top-off the tank. The fuel is cold in your tank and you're adding fuel that is approx. 60 some degrees. Then as you drive the car, the fuel is recirculated AND WARMED. All this adds up to mucho thermal expansion. Don't be surprised if the fuel just starts peeing out the filler neck while running (I've seen it happen).

02-18-03, 08:09 PM
Not good for the paint.....

02-19-03, 10:05 AM
The other thing that comes to mind here is...When you fuel you don't always get the same amount in the tank unless you fill to the cut off and then top off. Pump rates vary and that will affect the cut off point if it pumps faster it'll cut off slightly sooner. I've found mine to be fairly accurate but it's more of a toy than something to be taken dead serious just like your instant/ average fuel economy. These things are usually fairly accurate but they're subject to a few outside variables.

Dead Sled
02-19-03, 11:40 AM
Also miles to empty is just a referance! My dads gone down to 18 miles to empy in his V10 superduty. Drove it into the grage then didn't start the next morning :D

02-19-03, 06:48 PM
They arent perfect, but not bad.

A friend of my parents had an aurora, and it said 20 miles to empty, and it ran out of gas on the freeway!!! Once it gets below 50, its time to fill up.

Dead Sled
02-19-03, 07:56 PM
what worse is when your gas guage is not accurate at all like in my truck. and you run out of gas on your way to the gas station but you think it is something much worse. DOH!

02-19-03, 07:58 PM
stupid guage, stupid technology, stupid everything!!!!!!!

02-19-03, 08:06 PM
the gauge in my s10 was great. it would take 150 miles to get 1/4 down... then 50 miles to get to empty. lol. gotta love that!

Dead Sled
02-19-03, 08:33 PM
My jeep goes all day and then some untill it gets to 1/4 tank then poof it gone

02-22-03, 12:00 PM
That's not uncommon among OEM gauges. The OEM has this thing about making gauges read what they want them to. Matter of fact, many OEM gauges are electronically damped. In other words, they make them read certain ways regardless of what the TRUE reading should be. Just because it makes the customer happier and less likely to take it in for repair.

02-22-03, 12:08 PM
Yeah, Lots of happy customers....Walking for gas!

Makes sense to me!

02-22-03, 03:28 PM
You remember when the GPz900 came out? Maybe Jeff does. Anyway, not long after the bike came out, a lot of customers complained about the bike running hot. The bike came with a wimpy little temp gauge and it usually read toward the top of the gauge. Kawasaki issued a service bulletin on the issue and when we got the repair kit in, you know what it was? How 'bout a resistor that we were supposed to solder in-line with the gauge to make it read a little lower!
Another case in point, several OEMs electronically dampen their gauges in order to have them read in a more confidence-inspiring area of the gauge. It's true dude, I s%#t you not.
That's why I ALWAYS tell people to take what their OEM gauges say with a grain of salt.

02-22-03, 04:38 PM
thats incredible!

02-22-03, 06:51 PM
I do remember that! At the time, Of course We all said it was further proof thjat rice was crap! Later on we realized that it was just a stupid idea and that was that! I always tell guys that think that their OEM guages are hot sh*t that they're just fancy idiot lights!......They (of course) love that!

02-22-03, 07:15 PM
its like the ford oil guages. as long as the pressure is above 5 PSI its right in the middle of the Normal on the gauge.

02-22-03, 07:26 PM
I think the ones that they make sure are right, or at least more conservative are the gas guage and the speedo. If a lot of people were running out of gas with the guage above empty, there would be a lot of people pissed.

02-22-03, 10:52 PM
Speedo? I've seen more than my share of speedos that weren't even close!

02-22-03, 11:57 PM
Well i am saying more or less within 10 years, do you think that there are still speedos that arent right?

02-23-03, 02:09 AM
Absolutely....But they're getting better in most cases!

02-23-03, 10:22 AM
Do you think even on higher scale cars, like my Q or caddys? I would have to think a $50k car new wouldnt have a bad instrument. Maybe not

This is directly from my owners manual: "The [fuel] guage indicates the APPROXIMATE fuel level in the tank.

I guess that means that nissan admits it.

02-23-03, 11:32 AM
OK Wes, Heres the deal. I have certified police speedometer on my bike, I have it calibrated every year. My motorcycle sits high enough to see inside most cars that I'm around if I choose to look. I've looked at a lot of speedometers when I've been running next to cars particularly at night when I can read the digital ones without even trying. I've seen variances of 4-5 miles an hour to be very common and have seen as much as 10 miles an hour. That's one reason that cops will usually not even stop you in most cases until you're going 10 M.P.H. over the limit. That's what I'm talking about!

02-23-03, 01:48 PM
Are they usually over or under???

02-23-03, 03:08 PM
It varies from vehicle to vehicle. As expected they get more accurate in the higher end cars!

02-23-03, 03:15 PM
do you think all the other guages are correct. I think the most accurate would be the tach, IMO.

02-23-03, 03:35 PM
The tach's are notoriously inaccurate....The OEM guages in most cars are not real good. Read the previous posts in this thread a little more carefully and you'll get the rest of the story. It's pretty much all been covered.

Dead Sled
02-23-03, 09:04 PM
A while back in Car and driver they did a guage comparison between various price level of cars and it turned out that most of the cheaper ones had more accurate spedos. with variances between 2 off and 15 off!

02-23-03, 09:53 PM
So how would you go about finding out if yours is correct? Or is there no reasonable way to find out.

02-24-03, 02:20 AM
Speedo shop.... Have it checked and calibrated if needed.....That my friends is what is commonly refered to as a NO BRAINER!!

02-24-03, 09:16 AM
Remember back a few years when magazines actually listed the "speedometer error" on all cars while they tested them? You don't see that much anymore because they are more accurate these days.
Unfortunately, most gauges are there more to fiil up room in the dash than to accurately display data.
Hell, when was the last time you saw a fuel gauge have a proper linear track when following the fuel quantity? How 'bout NEVER!!
Why do you think OEM gauges seem to stay on FULL forever? It makes customers feel like their cars are getting great fuel mileage. It's all about "perception" in marketing.

02-24-03, 11:42 AM
The gas gauge usually isn't necessarily dead-on. But the gallons used in the DIC should be right on the money. That's not a mechanical measurement -- that's information from the computer about how much gasoline has passed through the injectors. The computer knows precisely how much gasoline the car has used!

I'd trust the gallons used on the DIC over either the gauge or the station. I've heard stories about stations using two measuring methods -- one for day-to-day use and one for when the inspectors come around. I don't know if they're true or not, but the only think I'd trust is how much gas the car says you've used (not how much is left).

By the way, the reason the gauge stays on full longer than usual is the shape of the arc the float travels as the gas goes down. It's all about degrees of rotation. The first few gallons, the pivot of the arm doesn't rotate much. But the last few gallons, it rotates around much more. Try it and see.

02-24-03, 11:47 AM
Originally posted by jadcock
I've heard stories about stations using two measuring methods -- one for day-to-day use and one for when the inspectors come around.
I'm not quite sure how that would work. The inspectors come around and actually check the pump calibration. They aren't looking at what you've pumped. The only way I can see to do what you're saying is to mechanically alter the meter itself and then swap it out everytime the inspectors come around. Problem is you don't know when they're coming.

02-24-03, 12:19 PM
That thing about the gas stations is a myth that's been floating around for years. The technology is such that it's virtually impossible to pull that scam off! And the penaltys are higher than anyone wants to deal with for the small benefit that they would reap!Other than that I sort of agree with jadcock about the differential between the guage readings.

02-24-03, 12:53 PM
Kat and KC, I agree that the thing about the pumps doesn't make total sense, but it got me thinking. I think nowadays, with the digital readouts, it'd be easier, but Katshot's right -- even with the digital readouts, there has to be a mechanical meter somewhere in the pump.

I've heard the version that with the digital pumps, there's a "master switch" in the station somewhere. When inspectors come around, they flip the switch and wha-la...everything's correct. Again, I'm sure it's not true...the penalties sure would be immense I would think.

02-24-03, 02:05 PM
I wouldnt think that they would do that. If lets say marathon did that, no one would ever go back to marathon again. Think about how many marathon stations there are.

02-24-03, 02:32 PM
Sure, if that "scandal" ever got out, the company would close its doors for sure. The point I was trying to make that the most accurate thing you have is the gallons used measurement, because it's not a mechanical measurement, but rather a combined total of pulse widths of the injectors if you will. The computer knows the time the injectors have been squirting fuel, since it was last reset, so it knows EXACTLY when a gallon has been used, etc. It's rounded to the tenth for simplicity, but I'm sure the number goes out many more decimal places.

I always make it a point to top it all the way up, since the auto shutoff things rarely work right on the cars I've owned. I never thought about the expansion, though...that's a great point. Interesting tidbit -- with the Nissan truck I just bought, full is FULL. You can't get another 25 cents of gasoline in there and it's coming back at you already. With the Cadillac, sometimes I can put an extra 2 bucks in there before it's really full.

02-24-03, 04:06 PM
well its not necesarily a small benefit.

lets say a station has 1000 customers in one day

each one has to fill a 15 gallon tank from 2 to 15 gallons (12 gallons each

if the pump is callibrated so its off and reads that for every gallon you put in, it reads 1.09 gallons on the pump.

if gas is $1.68 a gallon (like it is down the street here) that is $0.1512 per gallon extra they make, times 12 gallons is $1.8144 per customer. time 1000 customers: $1814.14.

in one week they would have: $12,700.80, in one month: $50,803.20, and in one year: $609,638.40

a small amount... right? :banghead:

02-24-03, 04:26 PM
That is a small amount compared to what they'd lose in penalties and lost business if they ever got caught doing it. :shhh:

02-24-03, 05:18 PM
My point exactly!

02-24-03, 08:22 PM
That wouldnt be a reasonable move for gas stations to do, since most of them are just franchises that are owned privately.

02-24-03, 10:52 PM
The quick money potential is there and I can see why some people would think it was a viable scheme. But it would still not be worth the risk to most dealers. I'm sure that there are a few ( Very Few ) dealers that might try stuff like that but I'd also be willing to bet that it's very rare if it happens at all!

Dead Sled
02-25-03, 12:12 AM
I dont think it would be worth the risk

02-25-03, 07:04 AM
Well some years ago some guy in NY was doing this. He was from Pakistan or India and he owed about 15 of these stores in an area of mostly his own people. He trained his workers to turn on the cheat pumps only when his own people were buying gas and turn off the cheat pumps when non-indian were gasing up. Made him a really rich guy. I think he ran off as soon as his cheat pumps were found. (ex-employee turned him in)

02-25-03, 08:47 PM
I am sure that it is more common in the rural areas, since the inspectors wouldnt be there as often.

Dead Sled
02-25-03, 10:53 PM
Its a yearly thing, so you could get away with it for quite a while :D

02-25-03, 11:34 PM
Weights and Measures out here will drop in on you unexpectedly at any time. And you'd better be in compliance!