Petrol pump accuracy

BrianD3

I drive high mileage and am tight with money so naturally pay a lot of attention to how much fuel I'm using in the car and notice any discrepancies and inconsistencies. I often think that either the on board computer in my car or the pumps themselves are quite poor when it comes to accuracy and consistency.

Basically if I purchase 20 euros of fuel several times at the same price per litre the mileage I will get (according to the OBC) till the low fuel light comes on can vary by as much as 25-33%. Sometimes I get more than I'd expect, sometimes less. I can get different results from the same pump in the same garage on two different days. A few times i found a "good" pump that every time I used it would seemingly dispense more than I expected but this trend rarely lasted more than a few days.

BTW the mileage calclations that the OBC does are based on average MPG since the last reset. My average MPG is very steady as I only reset the OBC every 10k miles or so.

Anyone else notice this? Or have any insight into how petrol pumps work (or fuel gauges and OBCs for that matter) How accurate they are? I see stickers from Metrology authorities on many pumps but what sort of accuracy/error do they deem acceptable.

maidhc

A fuel guage in older cars is just a potentiometer with a ballcock onto the movable arm. Pretty crude affair. The current is higher when the tank is full so the needle moves higher on the dash. The newer cars are probably much fancier though!

I normally don't fill my car up at the pumps (we buy diesel 1100L at a time), but once when I did my 52L tank took 56L. Interesting considering the tank could not have been completely dry.

peasant

There is another factor in your calculation ...the fuel warning light.

This coming on could either be governed by your OCB or simply by a sensor in the tank (which is more likely)

Ever noticed how the light comes on when you accelerate/break, or in lh corners/rh corners only to go off again afterwards?

In that case that would suggest that the light is "switched" by some sort of sensor in the tank and reacts to the fuel sloshing around. The more you "slosh", the less precise it is ...which in turn might explain the variances in shown mpg / miles left on the OCB.

No fraud or trickery involved in that case

BrianD3

peasant wrote:

There is another factor in your calculation ...the fuel warning light.

This coming on could either be governed by your OCB or simply by a sensor in the tank (which is more likely)

Ever noticed how the light comes on when you accelerate/break, or in lh corners/rh corners only to go off again afterwards?

In that case that would suggest that the light is "switched" by some sort of sensor in the tank and reacts to the fuel sloshing around. The more you "slosh", the less precise it is ...which in turn might explain the variances in shown mpg / miles left on the OCB.

The way the light works on my car is closely related to the OBC. If I'm getting 44.8 mpg (which is what I normally get over a long period) the light comes on and stays on without exception at 67 miles remaining. This works out at 1.5 gallons. If I'm getting lower economy the light comes on with fewer miles remaining which makes sense if it always comes on with 1.5 gallons left. I don't get any data once the light comes on as the OBC stops doing calculations and just displays "

---

" characters (this is normal beahviour for Renault OBCs)

I do tend to study the OBC a lot Also I pay little or no attention to the petrol gauge in the car. I woudl hope that the OBC is more accurate and sensitive than an analogue gauge.

VeVeX

BrianD3 wrote:

I do tend to study the OBC a lot Also I pay little or no attention to the petrol gauge in the car. I woudl hope that the OBC is more accurate and sensitive than an analogue gauge.

Yea, but wheres the OBC getting the fuel level information from? Analogue dials are used to within 1mph on most speedometers which are considered accurate.

Seth Brundle

The temperature outside also has a bearing. On a cool morning you will get more than on a hot sunny day!

ongarite

kbannon wrote:

The temperature outside also has a bearing. On a cool morning you will get more than on a hot sunny day!

True just like the eposide of Top Gear where Clarkson drove the A8 diesel from London to Edinburgh and back on full tank. He filled up early on a cold morning to get as much fuel in as possible.

Car Mad

in my car the the dash says i have more fuel when turning right.so if i turn right all day i wont run out

unkel

Car Mad wrote:

in my car the the dash says i have more fuel when turning right.so if i turn right all day i wont run out

LOL

I think the general point is that whatever the car tells you is a lot less reliable than what the (calibrated) pump tells you

petrolhead

Don't know if you have heard of this motorcard but you can ask the filling station to record the odometer reading of your car each time you use it. If your looking to measure consumption the more data you have the better!

Car Mad

petrolhead wrote:

Don't know if you have heard of this motorcard but you can ask the filling station to record the odometer reading of your car each time you use it. If your looking to measure consumption the more data you have the better!

very interesting.does that only work at statoil stations coz there like some of the most expensive in the country.good old tesco for me.love to know how it works out tho

maidhc

petrolhead wrote:

Don't know if you have heard of this motorcard but you can ask the filling station to record the odometer reading of your car each time you use it. If your looking to measure consumption the more data you have the better!

Bit early for reading ads, but to go off topic a little it works out far cheaper for people to buy diesel in bulk.

If you do happen to live in the countryside, and drive a diesel, it nearly always works out around 2/3c cheaper to get a thousand or more litres delivered to your house. (e.g. last week we paid 88c for car diesel, before then we had 105c diesel for the past 6mths). Diesel is a stable fuel, and is no more dangerous than kerosene which many people will keep outside their back door anyway.

You can of course get caught out if you fill the tank and prices fall dramatically, but if you play the game right it can work out at a substantial saving.

You also get far cleaner diesel, and there is no way any forecourt can screw you with a poorly calibrated pump. Only problem is coming out with the grand or more at the outset!

dubstub

As well as previous posters comments about the fuel gauge not being very accurate, it seems that you are not starting from the same starting point every time:

BrianD3 wrote:

Basically if I purchase 20 euros of fuel several times at the same price per litre the mileage I will get (according to the OBC) till the low fuel light comes on can vary by as much as 25-33%.

Am I right in thinking that once the fuel warning light comes on you drive to a petrol station and put €20 in and then measure the milage till the fuel warning light comes on again?
If that is the case, you need to account for fuel used while getting to the nearest petrol station.

BrianD3

dubstub wrote:

Am I right in thinking that once the fuel warning light comes on you drive to a petrol station and put €20 in and then measure the milage till the fuel warning light comes on again?
If that is the case, you need to account for fuel used while getting to the nearest petrol station.

yes but I add more very soon after the light comes on. I'd usually time it so that when the light comes on I have 0-5 miles left to the station. I'd take account of any small extra mileage driving to the station anyway

Also I don't always run it down to the light. For instance I might add 20 quid when the OBC says I have 100 miles remaining. Now the OBC tells me that I have 300 miles left. (seems like I got too much fuel, not complaining )

2 days later I have driven approx 200 miles and am again at 100 miles left accordng to OBC. Once again I add 20 quid @ the same price per litre but this time the OBC tells me that I have 250 miles left (seems like I got about the right amount of fuel this time)

Moanin

The way you drive would also be a factor?

dubstub

With all respect, I'd imagine that the inevitable inaccuracies involved in your measurement - when the warning light goes on, how far you drive to a petrol station, differences in MPG between refills (you mention that your average MPG is steady but you only reset every 10k, so the difference in one €20 refill may not affect the average at all) - would more than account for the difference you are seeing. The chances of a change in the calibration of the same pump in the same station over a number of days would be slim, I would have thought.

It BeeMee

It also depends on how the OBC calculates your "remaining" capacity.
I know VAG computers base it on your last 80 miles or so driving. Therefore if you've been doing a few short journeys where you've been using a lot of fuel, the OBC it'll estimate that your remaining capacity is less.

BrianD3

It BeeMee wrote:

It also depends on how the OBC calculates your "remaining" capacity.
I know VAG computers base it on your last 80 miles or so driving. Therefore if you've been doing a few short journeys where you've been using a lot of fuel, the OBC it'll estimate that your remaining capacity is less.

Well if that is the case then it would certainly make a difference. I assumed that the OBC in my car uses the average mpg figure since the last reset to predict the miles remaining. My OBC does not display an instantaneous or recent mpg but I suppose the function could still be there and used for a "hidden" calculation.

If that were the case though the "miles remaining" could increase as you drove along if the recent economy improved even though the overall avaerage might not have changed. I find that the miles remaining does not increase for me unless the average MPG changes which makes me think that it does in fact use the average for the miles remaining.

Idleater

BrianD3 wrote:

I assumed that the OBC in my car uses the average mpg figure since the last reset to predict the miles remaining.

This is probably incorrect, because what you do in the past 20km for example can be completely different to what you have done over the past 1000 km.

On the bike, I get in or around 200km per refil - ie, the warning light goes on, I stop soonish. Sometimes I get to 210, other times I get to 190, but I always put roughly the same liters of fuel back in when I stop.

BrianD3 wrote:

If that were the case though the "miles remaining" could increase as you drove along if the recent economy improved even though the overall avaerage might not have changed.

It may, increase, but it would increase much slower than it would conversly decrease if you were doing less mpg. Effectively, it would not increase, it would just decrease slower.

L.