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  Reply # 2093473 19-Sep-2018 21:02
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Mark:

 

$20 got me only 7.5 litres,

 

 

At that, $200 wouldn't even fill my tank (95 litres).

 

 


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  Reply # 2093519 19-Sep-2018 22:21
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Coil:

 

driller2000:

 

Coil:

 

Interestingly my car uses less gas at 135 than 100. Drops down to around 7L per 100Km at 135 and 8.5L per 100KM at 100. I think it is due to the car covering more ground for fuel burnt and thats in the sweetspot for that chassis. 

 

 

Umm - i guess it could appear this way from a cursory review of speed, mileage and fuel use from your gauges.

 

But measured properly - and even though my physics is pretty ropey nowadays - i don't think what you are claiming is physically possible - ie. from an energy perspective / 1st law if thermodynamics.

 

What with kinetic energy being a product of v2 - this greater speed requires more input energy output from your engine = more fuel needed....

 

 

I think at the same time you need to consider the new velocity and the new rate of ground covered also gear ratios and RPM. My gearbox throws my car into 5th around 90KPH then overdrive in 5th around 110 and that makes a noticeable difference.

I rely on my economy gauge on the dash so it could be wrong... 

 

 

 

 

So what I see is the most economical is the slowest speed you can do in top gear. For the S4 that's 70km/h in 7th, 1200-1300 rpm. For the same speed in 6th, engine is turning faster, burning more fuel, even though the work output is the same. So more torque means you can use a higher gear for the same speed so is more fuel efficient. 





 

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  Reply # 2093521 19-Sep-2018 22:26
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kryptonjohn:

 

It's amazing how much difference a 1.6km change in elevation makes over a 331km drive. On the weekend the drive from Auckland to the top of the Bruce Rd at Mt Ruapehu I averaged 7.8l/100km. On the drive back home I averaged 6.5l/100km! Similar traffic conditions each way - perhaps less traffic driving down meant higher average speed and worse economy though. 

 

 

Interesting. I notice that my car (any car) is about 10% more efficient driving Turangi to Wellington than Wellington to Turangi, observed over many trips. It could be the prevailing northerly, but I don't think so because I see it is southerlies too. I can only think it's the shape of the hills - more coasting heading south. 

 

Oh, and the Audi S4 trip computer tells me it's done about 10.9 L/100km over the last 2500km, but I think it reads about 10% low. Averages 10L/100km on open roads, can get it down to 9 if I'm careful. Up to double that with round-town Wellington traffic. 





 

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  Reply # 2093530 19-Sep-2018 23:09
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TinyTim:

 

kryptonjohn:

 

It's amazing how much difference a 1.6km change in elevation makes over a 331km drive. On the weekend the drive from Auckland to the top of the Bruce Rd at Mt Ruapehu I averaged 7.8l/100km. On the drive back home I averaged 6.5l/100km! Similar traffic conditions each way - perhaps less traffic driving down meant higher average speed and worse economy though. 

 

 

Interesting. I notice that my car (any car) is about 10% more efficient driving Turangi to Wellington than Wellington to Turangi, observed over many trips. It could be the prevailing northerly, but I don't think so because I see it is southerlies too. I can only think it's the shape of the hills - more coasting heading south. 

 

 

Turangi is 360m above Wellington at sea level - not super high but it means there's a 720m difference between the two trips which will contribute something.

 

 

 

 


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  Reply # 2093694 20-Sep-2018 11:04
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i've just signed up for fuelly too, having recently learnt my passat 3.2 OBC is lying to me. I have a largely rural commute (32km) and was quite happy with the 9.4L/100km my computer told me, until I did some math whilst driving and realized it was wrong, it is actually closer to 11.4l/100km, bled her dry recently too and topped out at the $150 limit that mobil imposes.

 

 


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  Reply # 2093983 20-Sep-2018 17:46
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kryptonjohn:

 

TinyTim:

 

kryptonjohn:

 

It's amazing how much difference a 1.6km change in elevation makes over a 331km drive. On the weekend the drive from Auckland to the top of the Bruce Rd at Mt Ruapehu I averaged 7.8l/100km. On the drive back home I averaged 6.5l/100km! Similar traffic conditions each way - perhaps less traffic driving down meant higher average speed and worse economy though. 

 

 

Interesting. I notice that my car (any car) is about 10% more efficient driving Turangi to Wellington than Wellington to Turangi, observed over many trips. It could be the prevailing northerly, but I don't think so because I see it is southerlies too. I can only think it's the shape of the hills - more coasting heading south. 

 

 

Turangi is 360m above Wellington at sea level - not super high but it means there's a 720m difference between the two trips which will contribute something.

 

 

It will contribute something - but it couldn't be near 10%. The total elevation gain over all the hills in either direction is more than 3000m for a start.





 

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  Reply # 2094029 20-Sep-2018 18:46
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I don't see the up and downs being relevant as they'd cancel out either way, apart from the net difference?

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  Reply # 2094110 20-Sep-2018 20:11
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kryptonjohn: I don't see the up and downs being relevant as they'd cancel out either way, apart from the net difference?

 

Well... if we take my previous car as an example - 7.7L/100km heading north and 7.0 heading south (approx). If we round the distance down to 300km, that's 21L plus an extra 2L heading north. An extra 2L to climb 300m (or maybe 1L climbing and saving 1L going down) sounds a bit much! My house is 100m high, up a 1km road; if I used 1/3 L each time I went up that hill going home I'd certainly be looking at moving to the flat!

 

 

 

 





 

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  Reply # 2094166 20-Sep-2018 22:10
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TinyTim:

 

So what I see is the most economical is the slowest speed you can do in top gear. For the S4 that's 70km/h in 7th, 1200-1300 rpm. For the same speed in 6th, engine is turning faster, burning more fuel, even though the work output is the same. So more torque means you can use a higher gear for the same speed so is more fuel efficient. 

 

 

 

 

Last comment from me I promise:

 

I Was Addressing A Comment:

 

"Interestingly my car uses less gas at 135 than 100. Drops down to around 7L per 100Km at 135 and 8.5L per 100KM at 100. I think it is due to the car covering more ground for fuel burnt and thats in the sweetspot for that chassis. "

 

-----------------------

 

Sorry but what was written above is impossible from an energy conservation POV - given that the kinetic energy of said vehicle will be approx 80% more @ 135 kmh vs 100 km/h due to v2.

 

The energy to create this increased kinetic energy comes from the potential energy of the fuel - and more energy is therefore required for 100kmh vs 135kmh - therefore more fuel is required.

 

It is that simple.

 

 

 

That's it from me  :P)

 

 


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  Reply # 2094186 20-Sep-2018 22:26
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driller2000:

 

TinyTim:

 

So what I see is the most economical is the slowest speed you can do in top gear. For the S4 that's 70km/h in 7th, 1200-1300 rpm. For the same speed in 6th, engine is turning faster, burning more fuel, even though the work output is the same. So more torque means you can use a higher gear for the same speed so is more fuel efficient. 

 

 

 

 

Last comment from me I promise:

 

I Was Addressing A Comment:

 

"Interestingly my car uses less gas at 135 than 100. Drops down to around 7L per 100Km at 135 and 8.5L per 100KM at 100. I think it is due to the car covering more ground for fuel burnt and thats in the sweetspot for that chassis. "

 

-----------------------

 

Sorry but what was written above is impossible from an energy conservation POV - given that the kinetic energy of said vehicle will be approx 80% more @ 135 kmh vs 100 km/h due to v2.

 

The energy to create this increased kinetic energy comes from the potential energy of the fuel - and more energy is therefore required for 100kmh vs 135kmh - therefore more fuel is required.

 

It is that simple.

 

 

 

That's it from me  :P)

 

 

Probably what's more important is friction which increases exponentially with speed.





 

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  Reply # 2094247 21-Sep-2018 08:38
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TinyTim:

 

Probably what's more important is friction which increases exponentially with speed.

 

 

Agreed - the extra kinetic energy is minimal compared to the energy available in fuel, and you only need to add this kinetic energy once by accelerating, 

 

But, air drag is also proportional to square of speed, so a car going at 135kph has 1.8 times as much wind drag as one going at 100kph. It seems extremely unlikely that the change in specific fuel consumption (see here for an explanation of what that is) due to a gear change between these speeds would compensate for this increased drag.


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