automobile fuel gauges

Factual Questions
1

Ok, I’m beyond the red zone on my fuel gauge. I know there’s only about one gallon left.

I have read this http://www.howstuffworks.com/fuel-gauge.htm

but I wonder…

Why can’t auto fuel gauges and tanks be made to be more accurate?

My fictional dream car, would have a fuel gauge that told you how much fuel is literally in the tank. For example: Let’s say the fuel tank holds 10.5 gallons maximum. I have not filled up in awhile. Instead of a stupid “needle” going from F to E, it would have a LED bar graph with LED number readout - e.g. Current Fuel Level: 1.250 gallons

One other feature of my fictional fuel gauge and tank would be that it’s “sway free.” (Needle/float gauges often tend to “sway” when going over hills, bumps, etc.)

I imagine that such a fuel tank would be more of a fuel “cylinder,” similar in concept to a tube of caulk. As fuel is consumed, an undetermined amount of air pressure is applied from behind a movable gasket in the “vacant” area. This would prevent “sway”

Is this possible? I would LOVE to have one of these. Too bad I can’t design and build one.

2

Actually at one time fuel gauges were incredibly accurate. I had a '71 Chrysler Newport that when the needle hit “E” I was literally out of gas! Car makers deliberately make their gauges inaccurate once the car gets below half a tank, this is so that people can be on “E” and still make it to a gas station (because they’re too stupid to stop when the gauge gets below a 1/4 of a tank). In a way, its a good thing because on some fuel injected cars if you run out of gas, you’re in major trouble. And while its not directly on the subject, the latest issue of Car and Driver has an article on why car makers use speedometers that are inaccurate. (It turns out they do so because its the law!)

3

I have ridden with my friend a couple of times, and he owns a Ford Expedition. (These things have just about anything you could think of for a vehicle).

Anyway, his fuel guage was an LED… I’m not too sure about how accurate it was or anything, but I know it told him what his miles per gallon was, and also told him how much farther he could drive before having to fill up again. So… you could probably look further into this… you might find your “dream car” afterall. : )

4

Just a WAG, but it’s probably cheaper to make a gauge that’s pretty good rather than REALLY REALLY good.

5

I learned the hard way that “E” doesn’t mean the same thing on all vehicles. The one and only time I have ever run out of fuel was a few months after I got my truck. From the car I had driven previously, I thought I knew that “E” meant I had about two gallons left, but on my truck it is a little less than one.

6

Yep, modern gauges often give a bit of fudge factor since people insist on driving close to the edge. There are a number of reasons why you should not drive around on an empty tank, apart from running out of gas.

Modern cars often utilize an electronic fuel pump embedded in the tank itself, they ‘push’ fuel to the engine rather than ‘pull’ as the mechanical fuel pumps mounted at the engine do.

The fuel in the tank acts to cool the fuel pump to a certain extent, which is basically an electronic motor. Too, a constantly near-empty tank is largely full of air, which contains moisture and condenses on the tank walls. Eventually, lots of crud and water builds up in the tank.

It’s best to keep the fuel level above the 1/2 mark, preferably full.
Why manufacturers haven’t included a “reserve” tank similar to motorcycles has always puzzled me, though.

7

When my car says “E”, you’d better damn well believe it’s “E”.

I’ve seen reserve tanks on an old MG (TF series?), but I do not think it was standard or common to see. My recollection from an ex-GM guy was that essentially, if you gave people a reserve tank, they just ran the main one empty, then tried to stretch the reserve one as well, and then ran empty a day later. So they figured why bother?

8

A precise, accurate gauge system would be too costly. There’s not strong enough demand for the extra cost.

The gauges do tend to vary from car to car. Think of any car’s fuel gauge as an approximation.

As mentioned, electric fuel pumps need to deal with liquid. Running dry fries them in short order. Letting your tank get too low may cost quite a bit more than just the inconvenience of being out of gas.

9

So what does it say? In the last three cars that I own, I’ve checked my speed against my GPS receiver, and the car’s speedometer has been right on the money, at least as well as I can make out a precise reading from my car’s speedometer.

10

I’ll have to dig it up, but basically it said that US speedos are more accurate than foreign ones because of the various laws (that are really convoluted and I can’t quite comprehend) involved. The Japanese are close behind the Americans and the German cars have the least accurate ones. All of them tend to read higher than what the car is actually going. US cars are about 1 MPH faster, the Japanese are 3 MPH faster and the Germans are 5 MPH faster. Part of the reason they do it is because of the variablility that comes into play with underinflated tires. Don’t remember the rest. Have to find the mag and post it tomorrow.

11

Causality is around the wrong way.

Motorcycles generally have a reserve capacity not because their fuel gauges are inaccurate but because most of them don’t have a fuel gauge. You keep track of when you need fuel by keeping an eye on the trip meter on your odometer (which you reset every time you fill up). But this isn’t terribly accurate and sometimes you just run out and need to flip over to reserve.

12

Hey All,

I have a few things to say on the topic of fuel gauges.

But first I will respond to the OP topic. Fuel gauges are inaccurate cause they are cheaper to make.

  1. When I first got my new fuel injected truck I was a milage freak. I would write down the mileage on every gas receipt then go home and enter the data into a spread sheet. Over the years I stopped doing that, and after a hard drive crash, I don’t even have the data anymore, but one thing I learned was that my fuel guage is very inaccurate, especially as the truck aged. I know I have a 13.5 gallon fuel tank, but when the needle is on E, it only takes 11.5 gallons to fill up. When I first bought the truck, when the needle was on E, it would take 12.3 gallons to fill it up.

  2. At one job I worked at a co-worker had the only optimistic fuel guage I have ever seen, on a 65 Chevy Nova. When he turned off the car, the needle went over to full. When he turned the car on, the needle would start from full and move to the level in the tank. I think they stopped doing it that way cause it is better to have a broken guage on E, than on F.

  3. I’ve noticed that my fuel guage on the truck moves faster on the last quarter tank than on the 3/4 before it. From Full to 3/4 full it is a linear movement, as fuel is used, the needle moves. On the last quarter it is almost exponential, as fuel is used the needle moves faster and faster.

  4. On my wife’s new jeep, there is a tone and warning light that come one when you hit 1/4 of a tank.

  5. You could buy a trip computer and install it on your car that will give you instantaneous mpg and distance left etc.

  6. You could study the mpg of your vehicle, know how many gallons you put in and then know how far you could go. That’s what I do and I basically ignore the guage.

  7. For earthquake preparedness in CA, you shouldn’t let your tank run lower than 1/2 as you want to have gas in your car after an earthquake, same for winter driving. You don’t want to get stuck in a blizzard with no fuel to keep the motor/heater working.

  8. Guys seem to think that E means, “Eh, I’ve still got some gas left” (stolen from Rita Rudner)

  9. When will this list end, you’re thinking.

  10. Whew, made it to ten!

Enjoy,
-Sandwriter

13

The reason the fuel gauge is non-linear is because the fuel tank isn’t perfectly square and/or perfectly horizontal. Therefore, if fuel is used at a constant rate, the float level changes at a non-constant rate. To make it perfectly accurate, you’d have to calibrate each fuel gauge assembly for each fuel tank type. I suspect they just make a handful of different models to accomodate tank shapes and such, and then just accept the remaining ‘small’ errors. And the guage face itself is probably oriented so that the needle bottoms out somewhere below empty from a normal viewing position so that any errors there are won’t add up to make someone think that they still have gas when they don’t.

The other posssibility is that the guage IS oriented so that the needle is exactly opposite the ‘E’, but when viewing down on the gauge from a normal seating position it looks like the needle goes below the E because of parallax error.

14

Causality? Not at all. I know why motorcycles have reserve tanks, I was merely pointing out a mechanical type reserve of some kind on automobile tanks would be a handy feature.

15

Sorry, causality is the wrong word.

What I meant was: Motorcycles have a reserve capacity because they don’t have fuel guages. With cars, as you do have a reasonably accurate fuel guage, a reserve is redundant.

The sort of person who runs out of fuel even when they have a fuel guage to warn them that that is going to happen, is still going to do so if they have a reserve.

16

I’m new to this board, but wow, there sure are some smart people here! I’m a car nut myself, so I can vouch for the comments of some other folks here:

Tedster is right about the fuel pump being immersed in gasoline; it helps cool the pump. If you run your car low on gas, no, it won’t break, but it might shorten the pump’s life.

To elaborate on what Sam Stone said, imagine this: Your fuel tank has a float inside, which, of course, floats. If the gas tank is 10" tall, and the float is sitting at 5", the gauge will probably read half-full… this is all and well if your tank is shaped like a square or a cylinder… but most cars have unusual tank shapes; imagine if it were shaped like a triangle or something; the gauge wouldn’t be accurate any more.

Regarding what LilShieste said, I used to own a 1985 Ford Crown Victoria, which had the optional “Tripminder Computer”, which I later determined to be very accurate. Among other things, it had a setting that showed me how many gallons I had burned…I could merely compare that with the capacity of my tank, to determine how much I had left.

From I think 1985 to 1993 or so, the Cadillac DeVilles showed exactly how many gallons were in the tank, on a digital display on the dash; I don’t know how accurate it was.

If you want to truly know how much gas is in the car you currently own, here’s THE way do it. I’ll try to give all the details.

Fill the tank up all the way. Write down how many miles are on your car, or reset the trip meter. Drive as you normally would, until it gets low. Fill it up again, and note how many miles you’ve driven, and how many gallons of gasoline you bought.

Divide the miles you drove, by the gallons. For example, if you drove 200 miles and had to buy 10 gallons of gas, that means your car gets 20 miles to the gallon.

Refer to your owner’s manual, and find out how many gallons your tank holds. Multiply that times your MPG (miles-per-gallon). For example, if your car gets 20mpg, and has a 20-gallon tank, it will drive for 400 miles on a full tank.

Once you’ve determined all this, simply reset your trip meter at every fillup, and refill when you’re near that 400 miles, or whatever your car happens to be.

Ignore the fuel gauge, as they’re nearly useless.

17

Be aware if using Chris’ method of determining the mileage left before empty that not every single gallon of fuel is actually usable in the tank. If you let it get too low the fuel pump wont be able to pick it up, especially if you are going up or down (or across) steep inclines.

18

Chris’s method also assumes that your tank is filled to the same level each time you fill up. I have noticed that different pump handles shut off at different levels. You can compensate for this by always using the same gas pump to fill your vehicle and/or topping off till you see the gas in the fill tube. Using the same pump also compensates for the level of the concrete or asphalt pad next to the pump. Different levels affect how much fuel you can pump into your car.

I know all this affects your milage when my mileage would vary 1-1.5 mpg between fill-ups. I then started paying attention to the above details, but still, I was only able to narrow the error down to 1 mpg per fill-up.

What happens is this, you think you fill up your tank, but you are really .5 gallons shy of a full tank, so when you calculate your milage, you get a higher mpg than you thought, i.e. 300 miles on 11 gallons = 27.27 mpg, but if you are shy .5 gallons, you get 300 miles on 10.5 gallons or 28.57, an artificially high mpg. Then the next time you fill your tank, you fill it to the top, and now you have pumped in .5 gallons that you didn’t use, but still calculate in your milage that will give you an artificially low reading, i.e. 300 miles on 11 gallons = 27.27 mpg, but you pumped in an extra .5 gallons, so you calculate 300 miles on 11.5 gallons = 26.08 mpg, an artificially low reading.

Beyond trying to fill the tank up to the same level every time, you also have changes in mpg due to driving conditions. On the freeway on a long trip, my mpg would go up 4-5 mpg. Mpg is also affected by how you drive. If you are speed racer, you can expect a drop of about 4-5 mpg. If you are dare devil and start drafting off of bigger vehicles (honest officer, I wasn’t tailgating, I was drafting) you can get an increase of 2-3 mpg, although I don’t recomend doing it.

Another detail concerning mpg is ambient temperature. In the summer I noticed an increase in 1-2 mpg over the winter.

It may seem a bit confusing, and you may think I’m crazy for going into so much detail and I agree. However, whenever you see a commercial for a new engine lubricant that increases your gas milage by 10%, that 10% could just be ‘noise’ in your mpg calculations based on fuel tank filling and driving style.

When my mpg average dropped below a certain level, compensated for all the above factors, I would go in and get a tune-up.

Of course all that was back when I was single, now that I’m married, I fill up every time I go to Cost-Co or when the needle is at 1/4 tank. I don’t have time for all the above anymore. :slight_smile:

Enjoy,
-Sandwriter

p.s. If you see a guy rocking his pick-up back and forth a bit then pumping in a bit more gas, it might be me!

19

I used to have an old (87) BMW that a trip computer, and it would show how many miles I had left (based on a rolling average of fuel efficiency, which it would show also). It was pretty cool, but I never ran it out of gas, so I don’t know if also had a reserve.

Mercedes don’t have F, 1/2 and E, but 1/1 (!), 1/2, R. I’ve heard that the R either stands for “Refill” or “Reserve”, with “Reserve” being more probable. More accurate terminology than E.

20

I used to drive a Cavalier Z24 with a digital dashboard. Unfortunately hardly anything worked properly. The fuel gauge was a bar of little vertical lines like the life-meter on a Megaman game, and it never seemed to read properly. It wouldn’t start to go down from the “full” position until I got down to half a tank (e.g. if it read that it was 3/4 full, I actually only had a quarter tank left, and when it got down to about half I was running on empty). I learned that the hard way by running out of gas with the gauge still showing most of its bars, the first time I ever drove the damn thing. The only accurate gauge on the whole damn dashboard was the speedometer, which matched up exactly with the digital speed-checks I occasionally passed on the highway.

Anyway, I gave up on looking at the gas gauge at all, and just reset my trip odometer every time I filled up, and erred on the side of caution by stopping for gas every time I got to between 300 and 350. (I could’ve run about 430 miles on a tank, but I’d rather have a very large margin of error just in case.) If you get into the habit of leaving a large enough margin, it won’t really matter if your mileage varies by a few points or if you have a tenth of a gallon less gas in your tank because you used the wrong pump.