Fuel consumption displays

Years ago I had a Vauxhall Cavalier (petrol) with a trip computer. While carrying a spare can of fuel I tried to run it to empty. The tank was supposed to be 59 litres capacity. After the trip computer showed "Zero miles range" I drove for a further 30 miles, then filled up; the tank took 57 litres. There may have been two litres remaining, so possibly a further 15 miles range.

Talking to others about this give the general view that "Zero range" means about a gallon of fuel remaining.

I think the trip computer counts fuel used from the electronics driving the injectors. This seems to be surprisingly accurate.

More recently a diesel Vectra from November 2012 to Juy 2015 averaged 54 mpg (total miles divided by total fuel, logged in a notebook) whereas its trip computer showed 55mpg long term average. Possibly the difference is accounted for by changes in the tyre diameter as the tread wears.

It's going to depend on the design of the car and even an individual one. Making a simple device like a fuel level sender 100% accurate isn't going to be possible. The tank dimensions would have to be incredibly accurate too.

Best one I had was a BMW with twin float units. If you ran it to zero miles, it took the stated tank capacity to fill.

But as long as it doesn't run out while saying you have some miles left, OK, I'd say. Unless by a silly amount.

Dunno. i don't think all cars calculate it in the same way. My previous car (Volvo V70) estimated the remaining distance based on the amount of fuel in the tank and the rate of consumption over the most recent few miles. Since its MPG was much better on a motorway than around town, the estimated remaining distance often *increased* as I drove down the motorway, even though the fuel level was obviously going down.

My current car (Hyundai Tucson) appears to assume a fixed MPG. At any rate, the miles remaining only ever goes *down*.

I think, of the two, I was more inclined to believe the Volvo.

My Fiesta does that. I was taken aback until I thought about it.

It also has a readout for instantaneous consumption, which switches to flow rate when stationary. The flow rate doubles with A/C running.

Chris

My 57 plate diesel Zafira calculates the range based on driving style. This often increases in the same way as your Volvo when on motorways or other steady speed stretches. Hard acceleration drops the predicted remaining distance quite drastically.

Mine also changes the distance while warming up. Uses more fuel when cold.

One which assumed a constant MPG would be useless. The first one I had on a carb car had a sort of turbine in the fuel feed to measure how much fuel was being used. Injection is so much easier - you just measure how long a single injector is open. Might be different on posh diesels, though.

But their miles and their kilometers are the same as ours... that's when they acknowledge the existence of 'klicks', as they call them.

Starting from a full tank, Yes. But putting a gallon in once it has been at zero for a while does not get a reliable figure.

newshound used his keyboard to write :

They rely on the tank level sensor(s), which due to the float(s), can never measure the bottom 1" or so of fuel when they hit bottom. Likewise the tank pump, cannot pump fuel to the very bottom. The net result is that the mileage remaining in the tank is at best a guess the nearer in gets to the bottom. I rarely let mine run down that far -why take the risk?

My car has two pumps, one in the tank one under the bonnet and has a saddle shaped tank, with two floats. It uses a verturi system to pump from one side to the pump pickup side. If the in tank pump fails, because the transfer stops, it can run out of fuel with it still showing 1/4 tank. My fuel gauge uses a calculation, based on the two float levels, to work out how much fuel is left in the tank.

Harry Bloomfield was thinking very hard :

I forgot to mention, my fuel gauge/fuel warning light, dips quite dramatically when parked up, maybe to badger you to add fuel. When you restart the gauge shows much less than when you parked it and the warning light might also show. As you drive off, the gauge can rise and the light go off.

It's 1.2 US gallon = 1 imp gallon, so +20% (not 10%) and 0.833 going the other way.

Was it the old Beetle that had a "reserve tank" or tap, maybe like a proper (British) motorcycle? Perhaps it was the Renault Dauphine.

Rover P6. Don't know about the others.

My kitcar has two fuel pumps, one on a short pick up and a changeover switch. It was easier and cheaper for me to do than a changeover valve.

SteveW

The Triumph Herald had the pickup tube on a sloped part of the tank wall. If you ran out of petrol, then you turned the fitting 90 deg. and there was now another half-gallon or so in reach of the end of the tube.

very useful.

Fitting the tank from an estate version, which then provided a level floor, gave an extra 10 gallons or so of capacity. It was best to make sure that the spare wheel was not still in its storage space underneath, though.

it was the VW had a reserve tap as it did not have a fuel gauge i managed to run out of petrol twice in one day

Except that those who are so short of money they run with a near empty tank will run out regardless of a reserve or not.

In which case, they are beyond help.