Car Question - Fuel Efficiency

Goldenyears

harveybobbles wrote: »

Just leave the computer alone and work it out manually. Brim the tank, note the mileage, drive, fill the tank, note the mileage, work the sums out

More acurate over a long distance - ie a full tank's worth.

Which "brim" would that be then ? In my case when the pump first cuts out, or after adding another 15% fuel when the tank really is brimmed. Or any intermediate fill. You need more than one fill to average out these differences.

Don't be too quick to condemn the fuel computer. The repeatability is good even if the bias isn't. Also if you have cruise control and a really flat road (but where ?? dry lake bed anyone ) you could measure the fuel consumption of your A/C compressor, instead of arguing about it, which btw has to be converted to eg. litres/hour to be used by others. % mpg doesn't translate well.

moonrakerz

anewman wrote: »

I doubt a heavy right foot in itself reduces overall mpg much, getting up to speed is only a very small part of the journey.

I am not talking about just "getting up to speed" - I am talking about the total driving technique.
Accelerating too hard, then having to brake because of this, is one of the main causes of excessive fuel consumption.

My son was complaining that he was getting less than 20mpg out of his (2.5L) van, I kept telling him it was the way he was driving it - but, oh no it wasn't he kept telling me !
I borrowed it for a few days and got over 30mpg out of it ................

Hammyman

anewman wrote: »

I doubt a heavy right foot in itself reduces overall mpg much, getting up to speed is only a very small part of the journey. Cruising in the highest appropriate gear and driving at the peak torque for the engine (instead of doing 80-90mph on the motorway) - and thrashing it to get to that speed, is likely to be still quite fuel efficient.

I used to do a trunk run from Hull to London 5 nights a week along with another driver. We both had the same wagon, the same load and took the same route, setting off at the same time.

I would arrive a couple of minutes after him. Whilst I was getting 10MPG, he was struggling to get over 6MPG. On a 450 mile round trip, that's a lot of diesel.

The difference in driving? He caned it up to the limiter in every gear and drove up to something before braking whereas I kept it in the powerband and would let off the accelerator a lot earlier and slow using engine braking.

cyclonebri1

Remember the Top Gear thing with Clarson on an economy run in a big Audi I think?

I just remember the comment, "whatever you do on one of these runs, just don't brake, ever".

The fuel computor is the best way to compare what differences various driving methods make as the brimming technique isn't suitable due to varying roads traffic conditions etc.

But if you actually want to know the accurate overall mpg it is the WTG.

Getting back to Top Gear, I think that Audi did over 70 miles after the tank read empty.

kaya

harveybobbles wrote: »

Just leave the computer alone and work it out manually. Brim the tank, note the mileage, drive, fill the tank, note the mileage, work the sums out

More acurate over a long distance - ie a full tank's worth.

disagree strongly, you cannot accuratly measure fuel consumption by fuel volume, it is stored below the ground at temps of a few deg C , fuel volume EXPANDS as the temp increases (which it will do as soon as you pull it from the tank under the ground and put it inot your fuel tank) , accurate fuel consumption can only be measured by weighing the fuel or checking the carbon given out by the exhaust, anything else is just a best guess average.
has your car got aftermarket alloy wheels or silly tyres fitted? changing the circumference of the wheel/tyre can put the calculations of your car's computer out

Pew_Pew_Pew_Lasers!

kaya wrote: »

you cannot accuratly measure fuel consumption by fuel volume

Really, that's odd, because I've been measuring fuel consumption by volume for years now, and quite successfully.

kaya

Pew_Pew_Pew_Lasers! wrote: »

Really, that's odd, because I've been measuring fuel consumption by volume for years now, and quite successfully.

lol, measuring it against what benchmark? the car computer or the "brim" method, i have spent tens of thousands of pounds at milbrook testing grounds to measure fuel consumption accuratly so its a subject i know an awful lot about, incidently my left leg is only 12 inches long if i measure it successfully using my home made ruler!

to put it in more simplistic terms if you put 1 liter of liquid in a 1l container and shut the lid when you heat it it will explode because volume of the liquid increases as you heat it, its the same with fuel, it expands with temp increases

Goldenyears

kaya wrote: »

lol, measuring it against what benchmark? the car computer or the "brim" method, i have spent tens of thousands of pounds at milbrook testing grounds to measure fuel consumption accuratly so its a subject i know an awful lot about, incidently my left leg is only 12 inches long if i measure it successfully using my home made ruler!

to put it in more simplistic terms if you put 1 liter of liquid in a 1l container and shut the lid when you heat it it will explode because volume of the liquid increases as you heat it, its the same with fuel, it expands with temp increases

Kaya, modern pump displays may use temperature correction to std temp.

However I agree with you that selling fuel by volume and not weight is an anomaly. How many people realise that half the increased economy of diesel (per litre) is because it's about 15% denser than petrol. Or if you add one particular octane improver to 95 RON petrol to bring it up to 99 RON you increase its density by 3-4%. Not all Litres are the same.

kaya

im not sure of the conversion factor but if you add 50 l of fuel to your car tank at say 2degC(the temp it is stored at underground), then after driving a hundred miles the temp of the fuel in your tank reaches ambient, say 18 degC it has increased in volume by x%, so in effect if you added a cold tank of fuel and drove nowhere by the time the fuel temp is ambient your car computer/brim method whould show you have more fuel in your tank than when you filled it up at the pump, as far as i know all fuel tank sender units still measure volume, i didnt know that pumps allow a temperature correction, not sure how they calculate that, but car computers, fuel gauges and "brim" methods dont compensate so are not at all accurate , unless its winter and the ambient temp is the same as the underground storage tank, powertrain at milbrook measure the carbon left unburnt in the exhaust to measure fuel consumption now, they used to go by weight

thor

Goldenyears wrote: »

Which "brim" would that be then ? In my case when the pump first cuts out, or after adding another 15% fuel when the tank really is brimmed. Or any intermediate fill.

This is exactly what I was trying to point out a couple of months ago. For some reason people seem to believe that when the fuel gauge indicates empty it is more accurate than any other point such as half or 3/4 full.