KSV. replied:

It has very little to do to revs – major factor here is aerodynamic drag that apparently depends from speed in proportion to power 6 (if my memory serves me correctly). Thus speed around 80 km/hr should be most economical regarding of car. It is very easy to check on any modern car equipped with trip computer.

Cheers
Serg

takenbyaliens replied:

1. In my experience overdrive is always more fuel efficient. 2. You are running at fewer revs. even factoring in the extra drag at 100klicks. ( 1800 v 2100 ) 3. If there were a head or tail wind you would still be be reving lower in overdrive as opposed to not in overdrive.

zacc replied:

this is true but it will also depend on where throttle position is a the time of the revs. say at 80klm the revs might be at 2100 but the throttle might be at quarter open, then at 100klm the throttle might be at half even when revs are a 1800. this is where the wind resistance plays a part

porl replied:

so what is the lowest economical speed say for a 4 cyclinder diesel, not towing anything, to drive at in 5th gear without damaging 5th gear which I understand, could be wrong and am new to diesels, shouldn't be driven at a speed where the motor is labouring.

Mick15 replied:

yea, like in the above post, more throttle on less revs may use more fuel, generally i'd say it would pay not to labour the motor too much as that will require a lot of throttle.
Something else that comes into it (probably the main thing along with aerodynamic drag as mentioned above) is where the engines peak efficiency is at, im fairly sure most engines will have maximum volumetric efficiency right where they make the peak torque - same rpm, so if you cruised at or slightly above this rpm point you should be theoretically making the most efficient use of every litre of fuel.

robak (QLD) replied:

2 years ago we went to pebble beach from brisbane in a 2.7 L diesel terrano with a loaded roof rack. (about a 4-5 hour drive)

On the way there our speed was between 80-90km/h and averaged 8.6L per 100. On the way back we stuck to the speed limit and (100-110) and only averaged about 10L per 100.

R.

Nifty1 replied:

I think throttle position is a major influence (talking about a 2.9 litre turbo diesel here). Used to think I'd get best economy at 80 to 90 kph, but in fact it is better at 110! Even towing.

I bought a Scangauge which plugs into the wiring loom and it has been a bit of an eye-opener, since it shows instantaneous consumption as well as trip and tank consumption. I don't think it is completely accurate (too many variables) but there is no doubt that changed driving habits has improved my fuel economy by at least 10% and possibly 20%. Cruise control works better than my right foot too. Mind you, if someone else told me this I wouldn't believe them....

howie replied:

applying some aviation stuff.
i never linked the 2 before but i suppose that a vehicle should be driven at its 'minimum drag speed'
this will be slightly different for every vehicle as total drag comprises of induced & parasite drag.
after the minimum drag speed (80ish), the total drag increases dramatically as speed is increased, mainly due to an increase in a part of parasite drag, called 'form drag'.
form drag is governed by the 'speed squared law' and is basically the resistance by air to the vehicle moving.

phew! i knew those lessons would come in handy one day

so i suppose you should find out your vehicles min drag speed first and apply the highest gear possible.

Robin Miller replied:

You have more or less got it Ian but there is a extra factor
and this is the engines actual efficentcy at any give revs otherwise
we would keep getting better as speed dropped.

There is a sweet spot usually around 70-80 at which best overall occurs.

(Its good idea to have car modified so you can be in overdrive when you choose)

Jim from Best Off Road replied:

I'll have my say and duck for cover.

No-one should be travelling at 80 in 100 or 110 zone without bloody good reason. They hold up traffic and cause frustration.

They are both a menace and a danger on our roads.

Member - Doug T (FNQ) replied:

OK I'll explain it in a different way....
get a sheet of Ply , hold it upright and run flat out into the breeze for 500m ...your going to be stuffed the body will run out of fuel ,
Then try it with again ( after you recouperate ) and not with XXXX
, Do it at a brisk walk and you should go twice the distance.

geez I don't know if I understand that myself and NO i'm not trying it.

.

jeepthing replied:

My vehicle which is a Jeep Grand Cherokee Laredo CRD V6 3l Turbo diesel has an Electronic Vehicle Information Centre ( yes it's a fancy word) and included in that centre is an Average Fuel Economy display. The engine management system calibrates instant fuel usage which is displayed as an average in the instrument panel. It is very accurate as I have performed manual checks, fuel used against total mileage.

From the figures that this system provides the best fuel economy has a lot to do with headwinds, load of the vehicle and may be some other factors that I haven't considered.

As an example I can be travelling at 80k into a head wind with revs of 1800 and the average is 10l/100k. Now if I increase that speed to 90k the revs might increase to 1900 but the fuel economy actually get better and drops to 9.5l/100k because the engine is not working as hard. But if I increase it to 100k the usage will increase to 10l/100k.

My reasoning for this is that the engine has got past that magic optimum rev/torque range for the conditions. When I am towing I very rarely exceed 90k and I have a "bloody good reason" for doing this I'm not going to tow nearly 3 tonne of boat or 2+ tonne of van any faster, come across a few accidents where a trailer has had a tyre blow out and seen vans lying destroyed on the side of the road, not a pretty site.

The great thing about the fuel management sytem means I can play around with the revs and speed until I get to the best rev range for the engine which also will be the most economical.

Anyway what I have gleaned from the figures this system produces is that there is no magic speed that will give you the best economy for all occasions it is a variable.

Steve from Top End Explorer Tours replied:

Have fun with this one.

I have a 100 series 03 model with a non Turbo 1hz motor it has done 13000 km.

I have another 100 series 00 model TD it has done 130000 km.

If I come home from Darwin, I fill up to the brim at the same pump, and with the 1hz motor sitting on 120 kph, I change from the sub tank at the Jabiru turn off.

If I use the TD same fuel same pump, and then sit on 140 kph I get to Jabiru with 1/2 a tank left.

The distance is 217.9 km, the sub tanks are the same size.

Cheers Steve.

lakedge replied:

My 2 bobs worth: Having a trip computer in the current model 3.0 TD Hilux gives me actual fuel figures which I follow with great interest under different conditions.
Best consumption is around town in 3rd or 4th at 50-70 km/hr - 7.6/7.8 l/100km (can't bring myself to sit on 80km/hr on an open road) with engine revs 1500-2400 (max torque band).
In my occasional freeway run Gosford to Sydney at 110 consumption drops to 7.9/8.0, or 8.2 to 8.3 if I push along with the other nutters on the morning commute at 120+.
Note that just prior to the 40000 km service could not get better than 8.4 under any conditions, but with new air filter consumption straight back to 7.8.
All these figures in a dual cab with glass canopy and all up weight of 2.1 tonnes.
I think Serg's and others' comments very valid - wind resistance over 80km/hr is critical. Also with my truck (or more correctly SHMBO's truck), lugging along in 5th under 80km/hr does not save fuel. It may, of course, be different with one of those older, larger, and less efficient engines. IMHO.
I will be interested to see the effect of a snorkel, as the dirty air filter demonstrated how critical airflow is on this engine. Just need permission to put one on...

John

stefan P (Penrith NSW) replied:

Its simple really!!!!!! put a hyclone in your car. They really work mine is installed under the loud pedal, thus stopping it from going more than half way down. Done wonders for my economy.

Hope this helps :):):)

Stefan

Member - Shane D (QLD) replied:

I'm driving a 2644 actross Mercedes Benz Truck for work and It has the fuel economy computer showing on the dash and found that in traffic lower revs = better economy.
By short shifting (change up sooner) I used 3-4 L/ 100 less, still driving at full throttle, This is of course a modern, Euro spec heavy truck Diesel, but has changed the way I drive my Toyota.
I used to think that it would be better for the fuel economy to change down sooner and utilise more horsepower in the lower gear, but try and maintain the same throttle position, if you know what I mean, (my mate who has a 4.5l petrol swears by this theory).
I get the best fuel economy around 85ks when the roof rack is loaded, the harder you push the more it uses.
I general stick to the limit, cos the cruiser has 4.3:1 diffs and the revs start getting up there, as well as the wind resistance and can be felt especially on freeways where there is a long constant gradual hill.
With my 96KW/285NM torque, it pretty much takes up most of the power, just to move the air, less air I have to push, the more power I get for the hill, for instance, If hit a hill at 110, It will start to die off and then settle as the speed washes off,
Point I'm making is that by increasing speed, I need more power, thus needing more fuel, regardless of what gear I'm in.

Shane
PS IMHO, If you have a "slippery" vehicle, lower revs will be better, If you drive something that isn't, IE tall, big tyres, racks, I think speed (wind resistance) will be the overiding factor.

Richo (SA) replied:

The fella who commented on the aviation theory was quite right.
Drag increases (parasite drag as there is no induced drag because we are not creating lift and flying) is directly relative to the square of the velocity (speed)

Therefore:
Drag at 80km/h is 80 squared or 6400 units (doesn't matter what units are)
Drag at 100km/h is 10000 units (56% higher!)
Drag at 110 km/h is 12100 units (89% higher than at 80 and 21% higher than at 100km/h)
Try 140km/h where the units are 19600 or triple the drag at 80km/h!!!!

Gearing does have a small effect of course but overall it is the drag (wind resistance) that has the most effect on economy.