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Accurate Fuel Guages - Bob H
Does any modern car have a really accurate fuel guage that can tell you, say, when you have 1 litre left in your tank?

My Golf(Mk4) is supposed to have 55L capacity with the low warning light coming on with 7L left. I can always put in 51-52L if I fill as soon as it comes on. On the other hand I once drove 60 miles with the light on, (with can in boot) without running out, at according to the computer 34mpg.

The figures don't add up.

Bob H
Re: Accurate Fuel Guages - Brian
Fuel gauges are pretty basic, they usually work by a float moving a contact up or down a resistor. The accuracy depends on the length of the resistance wire.
Most fuel tanks are installed under the car and are fairly flat i.e. wider than they are high. Assuming that your 55 litre tank is 20 cm high then in order to measure a litre the gauge would have to measure one 55th of 20cm, or about 4mm.
Added to that, unless the vehicle is dead level and stationary the fuel is slopping around.
IMHO, for the reasons above, you will be lucky to get better than 5 litres accuracy unless a lot of favourable factors come together at the same time.
Re: Accurate Fuel Guages - Mark (Brazil)
How about those cars, like my Saab 9000 a few years ago, which told you both the range you had left and how much petrol you have left.

It always felt fairly accurate to me, not that I ever tested it the hard way.
Re: Accurate Fuel Guages - David W
I use a calibrated bamboo cane for the tractor (100% accurate), if only more of life were that simple.

Re: Accurate Fuel Guages - Bob H
They had a computer on the previous model Nissan Primera that told you the range left but that was equally inaccurate. It registered zero when you had quite a bit of fuel left.

I had hoped that technology had moved on from the float/resistor principle on up market cars. I seem to remember a very sophisticated system used on Army tanks (Main Battle that is) like the Chieftan which was multi-fuel.

Does anyone know the principle a GP car uses to measure fuel left and transmit the info back to pits? Or does the telemetry just measure fuel flow rates from which they calculate consumption. It must be pretty accurate as they can estimate range left to 1 mile - or did Murray Walker get that wrong too!

Re: Accurate Fuel Guages - David W

We do have an expert on pink Chieftan tanks here, he'll be posting about 1am so you may have an answer in the morning.

(Just on the line M?)

Re: Accurate Fuel Guages - Ian Cook
I don't know about grand prix cars - never driven one, but I noticed on top gear last night that the testing for new car official fuel consumption is done by measuring emissions and not consumption.

I presume they know the carbon value of the fuel and have a formula for relating "carbon out" to "fuel in". The test apparently takes 20 minutes.
Re: Accurate Fuel Guages - Jonathan
Surely the most accurate way of measuring the amount of fuel used would be to weigh the car before and after.

There would be no inaccuracies because of fuel bubbling, and provided that the car was not consuming litres of oil, the weight difference would be solely down to the diesel. Diesel has a known density, which would allow an accurate consumption rate to be calculated, in much less than 20 minutes.

Re: Accurate Fuel Guages - Darcy Kitchin
As long as you didn't use the winscreen washers ;-)
Accurate Fuel Guages - David Lacey
In the Motor Trade we get a fair few complaints about fuel gauges. These 'gauges' are NOT an instrument, they are simply an 'indicator' of fuel remaining.

The reasoning behind leaving a fairly large reserve amount of fuel when the needle hits red is to prevent the lazy motorist from running out of fuel, which would lead to engine misfires/weak mixtures and we all know what effect that has upon catalytic convertors.....

Re: Accurate Fuel Guages - Richard Hall
A friend once gave me a lift in his Saab 9000. About 500 yards short of destination, it lost power and coasted to a halt - out of petrol. My friend was deeply shocked and disappointed - according to the trip computer, he still had another three miles worth of fuel left....
Re: Accurate Fuel Guages - Andrew Wills
my Cavalier CDX has trip computer, but I've never trusted its readings since early on in ownership when car ran out of petrol while "range" still indicated "103 miles left" - best to use commonsense, surely, and fill up after certain, regular/normal mileage covered?

Re: Accurate Fuel Guages - John Kenyon
Bob H wrote:
> Does any modern car have a really accurate fuel guage that
> can tell you, say, when you have 1 litre left in your tank?
> My Golf(Mk4) is supposed to have 55L capacity with the low
> warning light coming on with 7L left. I can always put in
> 51-52L if I fill as soon as it comes on. On the other hand I
> once drove 60 miles with the light on, (with can in boot)
> without running out, at according to the computer 34mpg.

I used to interpret the low fuel light on my 306 to mean "You have enough
for one return trip to work" - 60 miles.

I always filled up when the light came on - and could brim the tank with
61 litres indicated on the pump. Not bad considering Peugeot reckon
the tank only holds 60 litres in total!

(The car runs on Diesel by the way - I never brim petrol fuelled vehicles
especially during the summer)

Re: Accurate Fuel Guages - Stuart B
All this talk gets me nervous as I came to work on the sniff of an oily rag this morning. According to the computer I can get home with 25 miles to spare. Whats the betting my bottle goes before I get too far, odds on I would say.
Re: Accurate Fuel Guages - David W
Give in to the urge Stuart. You know you need fuel, a local evening paper and a Mars bar.

Re: Accurate Fuel Guages - Richard Blackburn
When I was learning to drive, I remember my instructor telling me never to let the tank get nearly empty, because of the risk of pulling water or muck sitting on the bottom of the tank into the carb (not many cars had injectors then). It this still received wisdom?

At the time, I ignored his warning, and let the gauge drop to just on the 'E' mark. A few miles later, the car cut out - no petrol. I'd assumed that there would be a little leeway, but not so. Since then I've had a distrust of petrol gauges, and now I hardly ever let the tank get below a quarter full. On my latest car, the warning light was on when I collected it, and I worried (OK, I'm obsessive) all the way to the nearest petrol station (but got there OK).

At the other end of the scale I've also noticed that on both cars I currently drive you can fill the tank to the top and get a reading of over 'full'. The gauge then doesn't seem to move at all until you've done a substantial mileage (up to 50 or 60 miles in one car). Am I over filling?

Re: Accurate Fuel Guages - Darcy Kitchin
I've never understood this business of a nearly empty tank encouraging water and stuff to be sucked up into the works. The outlet for the tank is the same height above the bottom of the tank at all times, right? What possible difference can it make?

I have run all maner and age of cars and run most of them dry (except those with cats) so I know where the gauge needle is when the engine stops.

BTW on our Citroen AX with the faulty voltage regulator, the gauge "makes" a quarter of a tank of diesel between starting and when the engine starts charging.
Re: Accurate Fuel Guages - Andrew Wills
PS I have pal (ex-Met police) now in Devon who teaches youngsters .. and before pupil sets off, one lesson is: do you have enough fuel to complete your intended journey?

Re: Accurate Fuel Gauges - Don
It's simply that I can't let the misspelling of "gauge" continue unchallenged.
Re: Accurate Fuel Guages - Bob H
Muck in the bottom of a tank was a problem when they were metal. Now they are all plastic. However I had never thought about the point that Darcy Kitchen made regarding outlets.

Re: Accurate Fuel Guages - Colin M
The electric fuel pump in my old mini used to start ticking which gave me just enough warning to stop at the next available petrol station. In the 1970's they all closed at night and I remember bottling out and parking up outside the Bell Hotel in Thetford on my way home. Poshest hotel car park I'd ever slept in!

Re: Accurate Fuel Guages - Reverend Pickled Onion
My Pug 406 has a trip computer which gives the range left. it varies depending on how you drive - if you climb a lot of hills fast the range drops off dramatically but "makes" fuel on the way down the other side. it seems pretty accurate and it would be stupid not to fill up as soon as you can once the low fuel light comes on.
Re: Accurate Fuel Guages - Carole Adams
It's easy enough to sort this one out. When my red light is on in the morning, on my (not that steep) drive, I just drive onto a nice flat bit of road, switch off, restart and hey presto - half a tank!
Re: Accurate Fuel Guages - Brian
Why, oh why, don't cars have a dual height outlet from their tanks with a simple changeover tap like practically all motorcycles.
This has two advantages, firstly the normal pull-off point is a little higher in the tank so that any water or muck has less chance of clogging the filter (I understand that diesel may absorb water from the atmosphere, hence the drain taps on their filters). Secondly, despite the almost universal lack of fuel gauges on 'bikes it is difficult to actually run completely out on a 'bike.
Re: Accurate Fuel Guages - THe Growler
An interesting point has been made by someone in this thread that when they brim their tank, the needle reads above "Full" and takes 30 miles or more before it starts to creep back. Every car/truck/petrol or diesel I have ever owned, rented, borrowed or otherwise driven regardless of make or country made/driven in, has had this feature. The converse seems to be needle progression actually accelerates once it gets below half-full.

Can someone throw any light on this? Not that it matters in the least, but since so many motoring brains I am beginning sincerely to respect here are able to debate fuel gauges (even if not all can spell the word) at such length and in the context of interesting backgrounds from their personal experience (sleeping outside pubs :-)), surely there must be some views on why fuel gauges behave in this fashion?

Personally, I tank the damn thing up at a quarter full, whatever. The reason I do this no one is going to believe when I explain why, especially when I say it happened in Afghanistan - in those days alas a rather happier country than now. The following story however is 100% true. My companion and I were driving from London to Kathmandu, Nepal. We left Herat, in W, Afghanistan, for Kandahar, in the south. The AA maps we had (this was 1965) showed a mainly dirt highway for the 300 plus miles, dotted with various towns. This was born out by the then few available accounts of Westerners who had made the trip before us.

I must tell you Afghanistan is seriously inhospitable desert, and I have driven through many a desert in my time. Companion pointed out should we get fuel at Herat (we showed about half a tank). Since it was 4 a.m. (to beat the heat) waking up the local gas station for low grade stuff from the USSR which burnt with a tarry smell (anyone remember that from motoring in the Eastern Bloc pre Berlin Wall?) didn't seem appetising, yours truly says oh let's not bother, there are plenty of towns shown on the road, anyway we've got 4 galls spare in the jerry.

Well, we drove and drove, the sun came up, and the heat climbed into the 40's, as it does in those parts in May. The Bedford CA Van boiled a few times but nothing unusual. Weeks before when crossing Iran we had got used to driving with the red temp light on all day, it didn't seem to matter. Mile after mile of very narrow but unexpectedly pleasantly smooth concrete 2-lane unrolled beneath the wheels, with not another vehicle anywhere. A blisteringly hot wind blew. Meanwhile the fuel needle slipped lower and lower, and none of the advertised towns or indeed so much a hut, appeared over the shimmering horizon. Yours truly meanwhile pontificating that tanks always held more than you thought they did, etc etc. When the jerry's reserve had been all but used up such assertions began to take on a somewhat less convinced note. After some hours and probably 250 miles, the inevitable happened and the van spluttered to a halt. Now what? Hadn't seen another vehicle in hours, most of our water had gone into the radiator and we began to realise we were in fairly serious doo-doo. As the book said, we stayed with the vehicle, and didn't go walkabout. After 8 hours and just as the sun went down, a car appeared. It was as luck would have it a USAID vehicle driven by an American, based at the town of Bost, which the map said was nearby. Turned out it was 2 miles nearby (!). In those days the USSR and the Americans competed for influence in this strategic country via peacful projects...... What had happened was Russian aid had built this nice new road, which totally bypassed all the map's towns and villages, hence why we hadn't seen any. No gas stations however had been built. THe American said the Afghans hated the new road anyway, it didn't have any tea-shops or places to eat, so they stayed on the old tracks with their rickety Russian trucks.

From that day to this I have never played mental one upmanship with a gas gauge, nor passed a gas station on a low tank and said never mind there'll be another one in a minute.

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