Fuel economy question

[Ext User(The Red Krawler)]

G'day,

I've got a petrol Mercedes MB100 work van (cue laughter)....

At 100kph, it revs at about 3000rpm in 5th. On long freeway trips, I get
somewhere between 7 and 8k/litre. Around town its worse, obviously.

Just recently I travelled Bris -> Gold Coast several times a day for two
days in a row. During this time, I was cruising somewhere closer to 120kph
at best, 140kph at worst. When I filled up (twice... grr, guzzling petrol
vans) I noticed I was getting between 8 and 9k/litre.

How the hell does that work? The only thing I could think of is that at
~130kph the engine is at a rev range that is more efficient, or at least
being given less injector time, and thus uses less fuel despite the extra
rpm. My thinking would have been that at a higher speed I'd be using more
fuel, cause a van sure as hell isnt aerodynamic and it weighs a tonne (two,
actually).

Any thoughts?

[Ext User(Saus29)]

assuming your calculations are correct, maybe the fable about german engines
being designed to run at autobahn speeds could become true?

"The Red Krawler" <redkrawler@hotmail.com> wrote in message
news:ChYbf.11575$Hj2.4383@news-server.bigpond.net.au...
> G'day,
>
> I've got a petrol Mercedes MB100 work van (cue laughter)....
>
> At 100kph, it revs at about 3000rpm in 5th. On long freeway trips, I get
> somewhere between 7 and 8k/litre. Around town its worse, obviously.
>
> Just recently I travelled Bris -> Gold Coast several times a day for two
> days in a row. During this time, I was cruising somewhere closer to 120kph
> at best, 140kph at worst. When I filled up (twice... grr, guzzling petrol
> vans) I noticed I was getting between 8 and 9k/litre.
>
> How the hell does that work? The only thing I could think of is that at
> ~130kph the engine is at a rev range that is more efficient, or at least
> being given less injector time, and thus uses less fuel despite the extra
> rpm. My thinking would have been that at a higher speed I'd be using more
> fuel, cause a van sure as hell isnt aerodynamic and it weighs a tonne
> (two, actually).
>
> Any thoughts?
>
>
>

[Ext User(DalienX)]

The Red Krawler wrote:

> G'day,
>
> I've got a petrol Mercedes MB100 work van (cue laughter)....
>
> At 100kph, it revs at about 3000rpm in 5th. On long freeway trips, I
> get somewhere between 7 and 8k/litre. Around town its worse,
> obviously.
>
> Just recently I travelled Bris -> Gold Coast several times a day for
> two days in a row. During this time, I was cruising somewhere closer
> to 120kph at best, 140kph at worst. When I filled up (twice... grr,
> guzzling petrol vans) I noticed I was getting between 8 and 9k/litre.
>
> How the hell does that work? The only thing I could think of is that
> at ~130kph the engine is at a rev range that is more efficient, or at
> least being given less injector time, and thus uses less fuel despite
> the extra rpm. My thinking would have been that at a higher speed I'd
> be using more fuel, cause a van sure as hell isnt aerodynamic and it
> weighs a tonne (two, actually).
>
> Any thoughts?

Id say the vans torque curve is highest at around 3500.

[Ext User(TJ)]

"The Red Krawler" <redkrawler@hotmail.com> wrote in message
news:ChYbf.11575$Hj2.4383@news-server.bigpond.net.au...
> G'day,
>
> I've got a petrol Mercedes MB100 work van (cue laughter)....
>
> At 100kph, it revs at about 3000rpm in 5th. On long freeway trips, I get
> somewhere between 7 and 8k/litre. Around town its worse, obviously.
>
> Just recently I travelled Bris -> Gold Coast several times a day for two
> days in a row. During this time, I was cruising somewhere closer to 120kph
> at best, 140kph at worst. When I filled up (twice... grr, guzzling petrol
> vans) I noticed I was getting between 8 and 9k/litre.
>
> How the hell does that work? The only thing I could think of is that at
> ~130kph the engine is at a rev range that is more efficient, or at least
> being given less injector time, and thus uses less fuel despite the extra
> rpm. My thinking would have been that at a higher speed I'd be using more
> fuel, cause a van sure as hell isnt aerodynamic and it weighs a tonne
> (two, actually).
>
> Any thoughts?
>

That'll be right. The optimal speed for fuel economy should be around 90
kmph for most cars.

At high speed there's a big air drag factor from your van flat face. Also at
those speed your engine's reving way above 3000 rpm. If there a 6th gear it
to cruise at lower rpm it would help.

A V8 engine cruising at 100 kmph at 1500 rpm in 6th gear will surely be more
economical than yours.

[Ext User(The Red Krawler)]

> That'll be right. The optimal speed for fuel economy should be around 90
> kmph for most cars.

No, see thats the reason behind my post ... In this instance, its actually
the other way around.

130kph = 8.5k/litre
110kph = 7.5k/litre

What the hell? I do an extra 20kph and get BETTER fuel economy?

[Ext User(crazy ron bakir)]

On Tue, 08 Nov 2005 07:16:50 GMT, "The Red Krawler"
<redkrawler@hotmail.com> wrote:

>
>At 100kph, it revs at about 3000rpm in 5th. On long freeway trips, I get
>somewhere between 7 and 8k/litre. Around town its worse, obviously.
>
>Just recently I travelled Bris -> Gold Coast several times a day for two
>days in a row. During this time, I was cruising somewhere closer to 120kph
>at best, 140kph at worst. When I filled up (twice... grr, guzzling petrol
>vans) I noticed I was getting between 8 and 9k/litre.
>
>How the hell does that work?

at 120 km/h you must remember you are spending less time for the same
distance traveled. less time on the road means less fuel used in
total.

--

managing director and CEO
crazy ron's mobile phones
gold coast, qld.

[Ext User(The Red Krawler)]

> at 120 km/h you must remember you are spending less time for the same
> distance traveled. less time on the road means less fuel used in
> total.

I can see how that would affect k/litre.



*cough*

[Ext User(budgie)]

On Tue, 08 Nov 2005 11:54:07 GMT, "The Red Krawler" <redkrawler@hotmail.com>
wrote:

>> That'll be right. The optimal speed for fuel economy should be around 90
>> kmph for most cars.
>
>No, see thats the reason behind my post ... In this instance, its actually
>the other way around.
>
>130kph = 8.5k/litre
>110kph = 7.5k/litre
>
>What the hell? I do an extra 20kph and get BETTER fuel economy?

I can see it now ...
"officer, I was going that fast because I'm trying to save the ozone layer"

[Ext User(crazy ron bakir)]

On Tue, 08 Nov 2005 12:25:38 GMT, "The Red Krawler"
<redkrawler@hotmail.com> wrote:

>> at 120 km/h you must remember you are spending less time for the same
>> distance traveled. less time on the road means less fuel used in
>> total.
>
>I can see how that would affect k/litre.
>
of course

>
>*cough*
>
i'm scratching my balls but i don't relay that in my post


--

managing director and CEO
crazy ron's mobile phones
gold coast, qld.

[Ext User(crazy ron bakir)]

On Tue, 08 Nov 2005 20:50:10 +0800, budgie<me@privacy.net> wrote:
>>
>>What the hell? I do an extra 20kph and get BETTER fuel economy?
>
>I can see it now ...
>"officer, I was going that fast because I'm trying to save the ozone layer"

of course the officer would pick you up on confusing the greenhouse
effect for the ozone layer effect and haul your arse into jail :)

--

managing director and CEO
crazy ron's mobile phones
gold coast, qld.

[Ext User(Kev)]

crazy ron bakir wrote:
>
> On Tue, 08 Nov 2005 07:16:50 GMT, "The Red Krawler"
> <redkrawler@hotmail.com> wrote:
>
> >
> >At 100kph, it revs at about 3000rpm in 5th. On long freeway trips, I get
> >somewhere between 7 and 8k/litre. Around town its worse, obviously.
> >
> >Just recently I travelled Bris -> Gold Coast several times a day for two
> >days in a row. During this time, I was cruising somewhere closer to 120kph
> >at best, 140kph at worst. When I filled up (twice... grr, guzzling petrol
> >vans) I noticed I was getting between 8 and 9k/litre.
> >
> >How the hell does that work?
>
> at 120 km/h you must remember you are spending less time for the same
> distance traveled. less time on the road means less fuel used in
> total.
>


I'll remember that next time I'm almost out of fuel
drive really fast to get me there faster


Kev

[Ext User(D Walford)]

The Red Krawler wrote:
>>That'll be right. The optimal speed for fuel economy should be around 90
>>kmph for most cars.
>
>
> No, see thats the reason behind my post ... In this instance, its actually
> the other way around.
>
> 130kph = 8.5k/litre
> 110kph = 7.5k/litre
>
> What the hell? I do an extra 20kph and get BETTER fuel economy?
>
>
G'Day Krawlie, What was the wind direction on that day, a tail wind can
improve fuel economy especially in one of those vans.
Maybe its optimised for that speed, it is a European van intended for
use on roads that have higher speed limits than ours.


Daryl

[Ext User(Charlie)]

The Red Krawler wrote:

> Any thoughts?

vehicles waste a certain amount of energy just turning the engine over,
and they waste a certain amount forcing themselves through the air. The
point where these 2 waste energy levels cross (if drawn on a graph
versus speed) is the most efficient speed. Plenty of more detailed,
better explanations around the place ;)

Charlie

[Ext User(Charlie)]

DalienX wrote:

> Id say the vans torque curve is highest at around 3500.

torque curve only shows how efficient the engine is at squeezing in air
and fuel for it's capacity, not how efficient it is at burning the same
amount of air and fuel. A vehicle with a peak torque at 7000rpm won't
get the most economy at 7000rpm.

Charlie

[Ext User(stevesub)]

Our Troopy does 13.5l/100km at 100kph, 10.8l/100km at 90kph - big
difference. These figures were done on the Newell highway last month
over a tank of fuel per reading and using cruise control. The faster
you go, the more fuel you will use. Engine is reving faster, more air
resistance, bigger throttle openings, etc. Best speed for fuel
consumption on the flat is probably near 60kph in top gear but totally
impractical in modern traffic.

For economical driving, keep the tyres pumped to the manufactuers
pressure, keep the engine tuned and with a clean air filter, light
throttle openings, don't rev the engine, keep in high gears and keep
the speed down. Not very exciting but you still get there and it does
not take much longer on a journey, maybe 10% longer.

Stevesub