Do You Ignore The Fuel Gauge?

[CO2SLAVE]

I dont know if anyone else find this a problem but I just think the blocky digital fuel gauge on my Auris HSD is completely pointless !

The low fuel warning comes up on the dash when you have about 120 miles left in the tank( depending on your mpg), then after a few more miles the last block on the indicator flashes, so after that you have ready no idea from the gauge.

So if you think you are spending too much time in petrol stations then here is a little tip , I realise that probably most of the old hands on this site are aware of this but it might help a few newbies.

The fuel tank in the Auris is 10 gallons ( 45litres)

every time you fill up you need to do the following;

1 .Reset trip A or B to zero

2 .Reset the average mpg readout to Zero as well.

3.Completely ignore the stupid blocky fuel gauge

then calculate your range as follow;

lets say you have done 400 miles on a tank and the average mpg readout is showing 56 mpg.

Calculate your range as follow 10 ( gallons) x 56mpg = 560 miles theoretical range then subtract 40miles because the mpg readout tends to be 4 mpg optimistic , then subtract another 20 miles just to be safe , so 560 - 40 - 20 = 500miles on that tank before you absolutely need to fill up.

simple stuff but I find it to be the only way of knowing for sure. If I filled up every time the low fuel warning light came on I would be filling up about 10 times more per year than I need to!

[Grumpy Cabbie]

It appears to be a Toyota thing, probably caused by our litigious friends in America. It is often said on the Prius (on which the Auris hsd is heavily based) that there are a couple gallons after the range gets to 0 miles. Easy way to test is run the car to 20 miles or lower if you dare and then fill the car up totally. Make a note how many litres that is and then note the difference between that figure and the official tank size.

4.5 litres equals one UK gallon.

Personally I'm not a fan of running the tank low as you might suck up some gunk or undesirables from the bottom of the tank. You might also cause condensation issues in the tank if you keen the petrol levels low all the time.

[Terry953]

I dont know if anyone else find this a problem but I just think the blocky digital fuel gauge on my Auris HSD is completely pointless !

What a lot of pratting about !!!

Just fill up when there are two bars left - planty of juice to get you to your favourite petrol station.

[CO2SLAVE]

Yes I have done the 0 miles test some time ago , I think I put 5 or 6 litres in . It was at that point that I realised that the fuel gauge was completely useless.

Not sure about the condensation issue because when I release the fuel cap it makes a vacuum noise as the air pressure equalizes so the tank doesnt breathe that well ,

I think allows for expansion more than contraction.

[CO2SLAVE]

Hi GC,

Yes I have done the 0 miles test some time ago , I think I put 5 or 6 litres in . It was at that point that I realised that the fuel gauge was completely useless.

Not sure about the condensation issue because when I release the fuel cap it makes a vacuum noise as the air pressure equalizes so the tank doesnt breathe that well ,

I think allows for expansion more than contraction.

Terry953 ( old hand )your choice mate you probably dont do the miles I do

but I did say "I realise that probably most of the old hands on this site are aware of this but it might help a few newbies."

[monya]

Yes the fuel gauge does show empty when it isn't but I don't want to run near empty for reasons mentioned by others so am happy to fill up on 1 bar. You're not using any more fuel so who cares?

[johalareewi]

so am happy to fill up on 1 bar. You're not using any more fuel so who cares?

As you drive, you are always carrying around the weight of your unused fuel.

For the ultimate in mpg, you should have a filling station at each end of your journey and put just enough fuel in the tank to make the trip. That way you keep the unused fuel weight down to a minimum. Obviously this isn't practical for everyday life, but technically, keeping your tank full up does use more fuel.

[timberwolf]

Apparently a litre of petrol weighs between 0.71 and .77 Kg. To keep the maths easy if we say a litre of petrol is 0.75 Kg and the petrol tank is on average roughly half full i.e. approximately 20 litres, I think that would be an extra 15 Kg of weight. A full tank of petrol, 45 litres, should weigh 33.75 Kg. If my maths is right?

It doesn't sound like much but I don't have any idea how much extra fuel would be used for each extra Kg carried.

[CO2SLAVE]

Yes the fuel gauge does show empty when it isn't but I don't want to run near empty for reasons mentioned by others so am happy to fill up on 1 bar. You're not using any more fuel so who cares?

My motive was not to try to encourage people to fill up when you are running on fumes , it was simply to take the guesswork out of the rather inaccurate fuel gauge

Since my daily commute is about 100 miles it helps me when I am pressed for time. On several occasions a bit of mental arithmetic it has saved me from having to fill up when I really need to get either home or to work, and you don't get that unpleasant feeling when you think you may run out of fuel when actually you have a gallon in the tank!

[johalareewi]

Apparently a litre of petrol weighs between 0.71 and .77 Kg. To keep the maths easy if we say a litre of petrol is 0.75 Kg and the petrol tank is on average roughly half full i.e. approximately 20 litres.

It doesn't sound like much but I don't have any idea how much extra fuel would be used for each extra Kg carried.

Let's have a rough go to get a feel of the numbers involved...

To move 1Kg, 1 metre takes 1 Joule of energy.

So for 20Kg of fuel (half a tank), that is 20 Joules to move 1 metre.

If moving at 45mph, that is about 20 metres per second.

So your 20Kg of fuel is moving 20 metres in 1 second = 400 Joules per second at 45mph.

Say 1 litre of petrol has about 36 MJ of energy (rough guess to make the maths easy).

So you can move 20Kg of fuel at 45mph for 36000000 / 400 = 90000 seconds on 1 litre of fuel.

90000 seconds = 25 hours.

[Opifex]

Let's have a rough go to get a feel of the numbers involved...

To move 1Kg, 1 metre takes 1 Joule of energy.

So for 20Kg of fuel (half a tank), that is 20 Joules to move 1 metre.

.......................

Ermm .. I think we need a physicist here.

As I remember it from a long time ago... this definition is only true if the 1Kg is moving vertically. If moving horizontally at constant speed then, ignoring friction, I don't think any energy is used, no matter how far it is moved.

[Hybrid Harbour]

In response to the original question, No I do not ignore the fuel gauge, on my Auris when it gets to 1 flashing bar the warning gives just 30 miles range, so time to fill up.

If the OP wants to try and go another 120 miles then they are either very brave or very stupid, why take the risk?

[johalareewi]

Ermm .. I think we need a physicist here.

As I remember it from a long time ago... this definition is only true if the 1Kg is moving vertically. If moving horizontally at constant speed then, ignoring friction, I don't think any energy is used, no matter how far it is moved.

Whoops. You are right!

Back to the drawing board...

Need to get the mass of fuel up to speed of 20 m/s from rest so using

KE = 0.5 * Kg x v^2

= 0.5 20 x 20 x 20 = 4000 Joules to get up to speed of 45mph.

So it isn't really that easy to work out a rough and ready figure for how much extra fuel will be used transporting the fuel itself. But the more accelerating you do, the more fuel you use up.

[sjrainsford]

I just fill up when its low. Maybe when its flashing, or one blob or maybe two blobs. Sometimes I may even fill it up when its showing half a tank!

I certainly dont bother wasting time doing calculations or getting as much as I possibly can out of a tank.

[timberwolf]

Ermm .. I think we need a physicist here.

As I remember it from a long time ago... this definition is only true if the 1Kg is moving vertically. If moving horizontally at constant speed then, ignoring friction, I don't think any energy is used, no matter how far it is moved.

Whoops. You are right!

Back to the drawing board...

Need to get the mass of fuel up to speed of 20 m/s from rest so using

KE = 0.5 * Kg x v^2

= 0.5 20 x 20 x 20 = 4000 Joules to get up to speed of 45mph.

So it isn't really that easy to work out a rough and ready figure for how much extra fuel will be used transporting the fuel itself. But the more accelerating you do, the more fuel you use up.

I've been trying to understand what your calculation means but I haven't got very far.

Coincidently, I worked out that I've lost 15 Kg weight in the last year, well actually in the first 6 months of last year. I haven't noticed any increase in the mpg on the Prius. So the extra energy required to move around an extra 15 Kg must be very small and easily hidden by other factors such as temperature.

Anyone notice any difference in mpg based on the number of passengers?

[Opifex]

Certainly with a full car I need more accelerator to get the same performance so I assume that means more fuel.

[CO2SLAVE]

In response to the original question, No I do not ignore the fuel gauge, on my Auris when it gets to 1 flashing bar the warning gives just 30 miles range, so time to fill up.

If the OP wants to try and go another 120 miles then they are either very brave or very stupid, why take the risk?

The whole reason for starting the thread is to negate the need to be brave (or stoopid) because if you do the simple maths ,you can then make an informed choice wether you need fuel today or tomorrow or maybe next week! It s just a simple way of know how much fuel is in the tank without relying on the inaccurate fuel gauge and range indicator, and lets be honest sometimes you have to go out of your way to fill up - which of course is wasted fuel is it not?

So to be clear if this is no use to you then ignore it , on the other hand if you find this useful then let me know

[Hybrid Harbour]

OK thanks, I am just letting you know that I choose to ignore your post from now on and rely on my accurate fuel gauge.

[timberwolf]

Certainly with a full car I need more accelerator to get the same performance so I assume that means more fuel.

You would think so, but is it enough to show a difference with the mpg?

BTW If we say an adult weighs 65 Kg. That would be about 4 times the 15 kg for half a tank of petrol...

The whole reason for starting the thread is to negate the need to be brave (or stoopid) because if you do the simple maths ...

I don't know why you are being such a hard time over this. I guess the problem may be to do with the basic mental arithmetic that you have covered ... it must really scare some people!

[CO2SLAVE]

You maybe onto something there...

[GreenJuice]

You would think so, but is it enough to show a difference with the mpg?

I don't know why you are being given such a hard time over this. I guess the problem may be to do with the basic mental arithmetic that you have covered ... it must really scare some people!

The extra weight would be an independent variable. As you suggest, it might be that other factors like temperature, driving style, etc. would overwhelm its effect but there is no denying that it is an independent factor contributing to fuel consumption and moreso a factor which can be reduced if one chooses to do so.

In other situations, there is less choice: for example the 30+ kg solar roof *must* be sold with 15" wheels in order to meet the claimed FE and emissions specifications.

CO2SLAVE being given a hard time might also be to do with the implication that the gauge is 'inaccurate'. If one didn't know (or care) that we have a 10 gallon tank, then it doesn't matter if the gauge shows empty after 8 gallons with a 2 gallon 'reserve'!

I don't ignore the car's fuel gauge because ironically I use its 'other' gauge. Scangauge reads off the car's computer, measuring the fuel used and calculating fuel remaining, 'distance to empty' or 'time to empty'. (The figures are of course exactly as posted by CO2SLAVE). I fill the same time as I did on my previous Gen 2, after using around 41 litres; which is when scangauge says I have about 60 miles left and about 50 miles after the last 'pip' starts flashing on my current Gen 3.

There is no guessing or worry. The more you apply it, the more confidence you have; the less you do it, the less confidence you have. If you like the idea of going consistently well over 500 miles between fill-ups, like I did with my old Gen 2, sooner or later you'll be adopting a method like this. If you are happy with more frequent fill-ups, you don't need it!

[timberwolf]

Yep, the sun roof is a good point, although I am not any closer to knowing by how much the mpg drops for a given weight.

I had assumed that knowing that a car's fuel gauge was inaccurate was obvious?

[johalareewi]

Yep, the sun roof is a good point, although I am not any closer to knowing by how much the mpg drops for a given weight.

It is not just the extra weight that is the factor, it is also how fast you move the extra weight.

You could get a very rough idea by finding out the weight of your Prius plus all the bits (and you) and a full tank of fuel. You know the mpg for moving this weight about and you have a rough idea of the average weight of unused fuel. So an approximation of the 'mpg' to move the fuel could be fuel weight divided by total weight times the total mpg.

Say the total weight of the Prius etc was 1000 Kg and your unused fuel is about 20Kg and you average 60mpg.

So 20 / 1000 * 60 = 1.2 mpg is a rough worst case guess at what the extra fuel has added to your mpg.

It is only a very rough figure because fuel usage is not just affected by weight but also air resistance. The air resistance depends on drag coefficient, surface area, speed, and air density. Note that weight doesn't factor here and it is the cube of the speed that is the main factor. So going faster has a massive impact on fuel consumption (with or without a full tank of fuel :) ).

Handy link if you are interested in the maths...

http://en.wikipedia...._in_automobiles

[Sagitar]

I am not any closer to knowing by how much the mpg drops for a given weight.

I'm afraid there is no simple dependent variable that connects them. If you increase the weight of the vehicle, you will certainly see an increase in the amount of fuel consumed, but by how much, will depend upon how the vehicle is driven and upon some other variables external to the vehicle.

The energy generated within the vehicle is used to overcome a number of variable energy losses. It isn't difficult to list a few of these.

1 - the power required to achieve acceleration. Here we are back to Newton's laws. If the vehicle is running at constant speed then the weight makes no difference.

2 - the power required to overcome wind resistance. This is non-linear. If you want to go faster then the power requirement goes up very quickly. It is dependent upon the external configuration of the vehicle and upon some external factors such as wind speed and air temperature, but the weight of the vehicle does not affect it.

3 - rolling resistance. The power required to overcome friction at the road surface. It is related linearly to weight but the coefficient of friction for a vehicle is very low (around 0.01 for a normal tyre on concrete) so significant changes of weight make little difference. The value of the coefficient of friction will vary with the nature of the road surface, the composition and structure of the tyre and the level of inflation of the tyre. Any one of these factors may make a bigger change than a small change in the weight of the vehicle.

4 - losses in the power train. This is all the stuff like bearing friction and pumping losses that reduce the efficiency of the power train. Some of the losses will relate linearly to speed but others are non-linear and rise rapidly with speed. There are also acceleration components. This is one of the reasons why infinitely variable gearboxes attempt to keep engine speed constant, at a speed that gives the best balance between power generation and power train losses. There are external factors such as working temperature and the use of power other than to propel the vehicle that affect these losses.

I am sure there must be research figures available but I haven't looked for them. Intuitively I would judge that changing the weight of the vehicle marginally would have a very small effect compared with some of the other factors. Unless you drive with the consistency of a robot and in completely unchanging circumstances, it would seem to me that the change in consumption attributable to a small weight change would be well hidden in the much larger changes arising from other things, but it will undoubtedly be there.

[Sagitar]

Sorry to be repetitious. I didn't see the earlier post until I loaded mine.