Just how full is a 'full' tank?

Your petrol gauge is almost certainly pessimistic.

No.

There's only one way to find out... two actually. One involves a very small camera, the other involves trying it.

We're not going to know because:

(a) we don't know what car it is. (b) we don't know how you drive.

Fuel lights and gauges are deliberately designed such that people don't run out.

It is a little frustrating. Our '99 Accord has a quoted 65 litre tank, the light is supposed to come on with 9 litres left but if I fill it right up when it illuminates I only get 51 litres in.

But I don't want to run out.

Three - drain the tank, and measure how much comes out after the guage reads zero. (Avoid setting yourself on fire.)

"Stuart Lyon" wrote

This isn't good.

Ideally you should never let the fuel warning light illuminate. As much as anything else, the more petrol there is in your fuel tank, the less room there is for water vapour..

Vince

And the lower you run the tank the more likely you are to pick up the sediment that congregates at the bottom over time to clog up all those little pipes and rarely-changed filters.

DervMan ( snipped-for-privacy@ntlworld.com) gurgled happily, sounding much like they were saying :

My XM has a nominal 85 litre tank. My record is 91 litres on the pump.

I think the idea of muck in fuel tanks is something left over in motoring folk law from the time when mechanical pumps delivered just the fuel the engine used.

My car is 1991, took my pump out last year [1], fairly sure no one had been there before. Tank was pristine sparkly clean. The "sock" on the pump inlet was very mucky as was a filter/baffle to the swill pot that the pump dropped into. A second car of 1989 vintage was the same. Modern system's have full flow all the time and return unused fuel to the tank causing any muck to get stirred up and pulled into the in tank filters.

[1] Problem was actually the alarm running the 5amp circuit though a 2amp relay. Nice and crispy with very dry joints.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Then lean on ass end of your car and shake it sideways to "burp" the tank. Top up careful and burp it again, until you have filled the neck brim full and you can see fuel in the filler spout. You will get another 2 litres or so in.

No it means your fuel warning light comes on way too early.

That would be pushing it, 40 would be reasonable, 60 if you start from brim full.

You don't wan't to run it dry, the fuel pump runs at high speed and is lubricated by the fuel.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Maybe so. But the 1991 Volvo I took the tank off had pinhole corrosion on the top and small rusty flakes floating round inside...

I would guess the majority of the sediment comes in through the filler nozzle with the petrol. Has every single car got a fine enough inlet filter?

Managing to fill with more than the tank capacity is a strange one! The wife mangaged to put 63 litres into her 60 litre tank, and it's not as if she pushed it to the petrol pump! We thought it'd broke the petrol gauge as it didn't move down from full for the first 100 or so miles - when normally it'd have gone down by a quarter of a tank.

Some vey helpful posts.

It seems as though a tank with a stated capacity of ,say, 60 litres will hold approx 62 litres if filled to the brim (2 litres being the volume of the pipe between nozzle and tank).

Stuart.

Go to the shop and buy a life? Seriously, petrol stations are so plentiful in this country that it is a non issue.

And piss a gallon out on the first half dozen corners.

I've always brimmed the tank on every car I've owned because I keep records of the fuel consumption and need identical fill points on every tankful to work this out from. It varies with the car but you can usually get at least a couple more litres in after the pump first cuts off automatically.

Chances are you could get 54 or 55 litres in if you brimmed it which means you've got about 5 or 6 left when you fill up. Certainly not enough to do another 80 miles. More like 40 and you'd be running on fumes by then. That would tie in with warning lights coming on when there are about 2 gallons (9 litres) left in the tank on most cars. Enough for 60/70 miles or so which will always cover you to the next service station on a motorway with a decent safety margin left over.

Dragging up rubbish from the bottom of the tank if its old.

You can probably get more in, i get another 4 litres after that.

Doubt it. My tank is a 70 litre capacity. I get 60 litres in it when i fill up at the first orange light. I run out of fuel after 25 miles of orange light and only get 63 litres in it lol. J

There can be a significant difference caused by how level the surface is that you're parked on when filling.

Depending on the position of the filler pipe entry to the tank, a slight slope in either the forwards or sideways direction can increase or decrease the effective "air space" at the top of the tank by quite a few litres.

My fuel consumption spreadsheet calculates averages over multiple fillings (as well as the current one) to counter this effect.

John

And what the outdoor temperature is.

And what altitude you're at.

As someone who has the maths to actually do those sorts of calculation (and has done them) I'd be interested to hear what difference you think temperature and altitude make to fuel tank capacity.

Only on truck diesel tanks and old cars. Modern (post 1989 maybe earlier) cars have a one way valve in the cap, it can breath in to reduce vacuum but not blow out. This prevents HC emissions to the atmosphere from the tank.

But yes you there is a possible problem, cars with these one way valve filler caps also have carbon canisters on the vent line. If the car is not driven far enough and is then parked arse end facing the sun on a surfaced car park, then reflected heat from the sun can expand the fuel up the vent line and soak the carbon canister with fuel. Engine will run badly just off idle as once the purge valve on the canister opens it sucks excess fuel into the engine when it should just breath vapour. Reports from California indicate this problem is so bad the only cure is a new canister. They don't think it will dry out in any time frame worth suffering the poor running.

My bad I did assume he was going to drive the car. I did assume it was modern and well maintained. I also assumed he was in cold wet UK.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Yes it's true. When warmer, the fuel tank will have expanded slightly.

Assuming the fuel is kept at a cool temperature underground then we have a constant here.

Are we going to notice the difference?

That's true as well. Are we going to make the difference?

This may be stretching the realms of possibilities here, but I'd wager that we'd notice if the car were level or not rather more than the ambient air temperature.