Tight arse Fuel Efficiency

I recalled a research paper of the aerodynamics of the tyre mounted on the bonnet. I had a quick search and found it here Wheel on Bonnet = Aerodynamic improvement

I have tried very basic testing with 80 and 100 over 50 km. By measuring amount of fuel used. Obviously this is so rough a method the results would be barely worth mentioning. However the difference was definitely there. Also it needs to be done on as flat a stretch as poss. and it is very hard to maintain 80 kph even in a 300tdi.

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Originally Posted by JDNSW

I don't have a 300Tdi (Isuzu), but none of these diesel engines have much change in specific fuel consumption with engine speed, so the main variable is the drag from various sources on the vehicle.

For a relatively un-aerodynamic vehicle like this, the major source of drag over about 60kph is aerodynamic drag. This is proportional to the square of airspeed, so increases pretty rapidly above this speed. Accessories etc such as roof racks, oversize tyres etc increase the amount of drag, but not the way in which it varies with speed.

At first glance, this suggests that around 60 would be the best economy speed, but remember that this is the speed at which aerodynamics become most important - at that speed other factors are still important. Perhaps the most important is that travelling at that speed you tend to use the brakes and acceleration a lot more, and can't use momentum to go over even slight rises without putting the foot down. What this means is that the advantages of driving slower diminish as you drop back towards about 60.

In light of this, 90 is probably a reasonable compromise for most people. Certainly, this is what I have found.

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Not a Tdi Defender but my EV Landy has about the same aerodynamic profile , the decrease in efficiency increased exponentially above 50kmh , its clearly evident on my instant kwh/100km readout .

Below 40kmh its almost a flat line but higher efficiencies are evident with higher tyre pressures, free wheel hubs disengaged , and AT's instead of MT's (about 5%) . Above 80km simple things like folding the mirrors in, closing the windows make a quantifiable difference . If you have a roof rack, get rid of it.

Biggest difference of all is a tailwind / headwind about 40% gain / loss respectively .

I would conclude similar efficiency gains no matter the fuel source.

I think you would be right. I can see an opportunity for you to measure, tabulate, and publish, differences for example, from free wheel hubs, tyre pressure, type of mirror etc. Preferably done on a dead calm day on the same bit of road!

Good food for thought. Higher tyre pressures. Have AT's. Wing mirrors( legal question??). Roof rack definite no. Pity coz it is a hallmark of the marque. Tail wind/head wind. Sometimes one has to go in a certain direction.....

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Originally Posted by goingbush

Not a Tdi Defender but my EV Landy has about the same aerodynamic profile , the decrease in efficiency increased exponentially above 50kmh , its clearly evident on my instant kwh/100km readout .

Below 40kmh its almost a flat line but higher efficiencies are evident with higher tyre pressures, free wheel hubs disengaged , and AT's instead of MT's (about 5%) . Above 80km simple things like folding the mirrors in, closing the windows make a quantifiable difference . If you have a roof rack, get rid of it.

Biggest difference of all is a tailwind / headwind about 40% gain / loss respectively .

I would conclude similar efficiency gains no matter the fuel source.

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Not sure the fuel measurement method is accurate enough to produce publishable data....

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Originally Posted by JDNSW

I think you would be right. I can see an opportunity for you to measure, tabulate, and publish, differences for example, from free wheel hubs, tyre pressure, type of mirror etc. Preferably done on a dead calm day on the same bit of road!

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I think that in goingbush's electric Lightweight the measurement probably is accurate enough! Several goes at it and seeing how much vriance there is would also be a good measure of the probably accuracy.

Lowest consumption I ever got from a 110 Defender was 0 l/100km. Was trying to reach Holbrook to refuel but ran out 5km short and was flat towed by mate thereby achieving the 0 l/100km - my mate didn't do so good though. :whistling:

Realistically though, the best I ever got was 8.7 l/100km from a 300 Tdi Disco, travelling at not over 100 KPH from Stanthorpe to Sussex Inlet including a few days driving around Sydney.
Usual average was around 10 l/100km.

8.7 would be highly desirable. 10.5/11 is my norm

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Originally Posted by Xtreme

Lowest consumption I ever got from a 110 Defender was 0 l/100km. Was trying to reach Holbrook to refuel but ran out 5km short and was flat towed by mate thereby achieving the 0 l/100km - my mate didn't do so good though. :whistling:

Realistically though, the best I ever got was 8.7 l/100km from a 300 Tdi Disco, travelling at not over 100 KPH from Stanthorpe to Sussex Inlet including a few days driving around Sydney.
Usual average was around 10 l/100km.

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Sounds about right.