Fuel economy vs speed

Submitted: Thursday, Apr 03, 2008 at 10:31
ThreadID: 56230 Views:3707 Replies:17 FollowUps:24
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I think it's safe to assume we have all heard that keeping to 80 kph will save heaps of fuel compared to doing 100 kph, and this is probably heavily related to the number of revs the engine is doing.
However, with an auto transmission, does this apply when at 80kph the car runs at 2100 revs, but at 100 kph, overdrive has cut in, and the engine drops to 1800 revs.
Higher speed now means less revs.
Based on no hills and no head or tail winds, are there other factors applied that would affect the fuel usage to make the original generic statement wrong, or does the benefit of the lower revs get counteracted by the increased wind resistance with the higher speed?
Cheers and thanks
Ian
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Reply By: KSV. - Thursday, Apr 03, 2008 at 10:38

Thursday, Apr 03, 2008 at 10:38
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
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Reply By: takenbyaliens - Thursday, Apr 03, 2008 at 10:39

Thursday, Apr 03, 2008 at 10:39
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.
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Reply By: zacc - Thursday, Apr 03, 2008 at 11:08

Thursday, Apr 03, 2008 at 11:08
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
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Reply By: porl - Thursday, Apr 03, 2008 at 11:28

Thursday, Apr 03, 2008 at 11:28
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.
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Follow Up By: KSV. - Thursday, Apr 03, 2008 at 11:31

Thursday, Apr 03, 2008 at 11:31
Where you are doing about 1100-1200 rpm
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Follow Up By: porl - Thursday, Apr 03, 2008 at 11:34

Thursday, Apr 03, 2008 at 11:34
Seriously ? I gotta try that then if that's real.

Though I was more thinking of 80km/hr in 5th gear. I was told don't buy one that even towed in 5th gear!

Have 2000 diesel manual hilux.
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Follow Up By: KSV. - Thursday, Apr 03, 2008 at 11:52

Thursday, Apr 03, 2008 at 11:52
I believe it going to be around 80 km/hr. They normally calibrate gearboxes in this way. I do not have tacho in my LC (still not enough time to finish this project!), but in all cars with tacho what I have before it was very much the case
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Reply By: Mick15 - Thursday, Apr 03, 2008 at 11:32

Thursday, Apr 03, 2008 at 11:32
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.
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Follow Up By: Member - shane (SA) - Thursday, Apr 03, 2008 at 18:27

Thursday, Apr 03, 2008 at 18:27
i know in my 4.2 td patrol if i run it in 5th the egt goes up with just the slightest rise or head wind. so i find better lts per kls in forth. this tells me the higher the etg's the more fuel i use.
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Follow Up By: madfisher - Thursday, Apr 03, 2008 at 22:09

Thursday, Apr 03, 2008 at 22:09
interesting piont, In my 3.5 Jack max torque is 3100rpm which is 124kph, to fast for economy in fifth. Fourth is 103kph.
I will have to try and spend more time in fouth when towing to see if fuel economy improves.
Cheers Pete
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Reply By: robak (QLD) - Thursday, Apr 03, 2008 at 11:44

Thursday, Apr 03, 2008 at 11:44
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.
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Follow Up By: Member - Matt (Perth-WA) - Thursday, Apr 03, 2008 at 11:50

Thursday, Apr 03, 2008 at 11:50
That makes sense...the roofrack would not affect drag all that much at 80km/h....its when you move above that speed that the shape of the car (and roofrack) begins to effect the drag on the vehicle.

Matt.
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Follow Up By: KSV. - Thursday, Apr 03, 2008 at 11:59

Thursday, Apr 03, 2008 at 11:59
Matt, I cannot agree with you about roof racks. INHO they suck quite a bit of fuel. Surely there is better construction and not so good one. My experiments with one what I had on my 60 LC indicate that they sip up to 2L/100km @ 110 km/hr when empty! Actually I recon that properly load one can get less. Surely @ 80 number will be lower, but it is still noticeable.

Cheers
Serg
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Follow Up By: Member - Matt (Perth-WA) - Thursday, Apr 03, 2008 at 12:16

Thursday, Apr 03, 2008 at 12:16
You didnt even read the reply Serg...how about you try that before you reply to a post...would save alot of wasted time.

I agreed they use more fuel....did you read that bit.....but its a proven fact (tested in wind tunnels) that the shape of a vehicle contributes LITTLE to economy UNDER 80 KM/H NOT 110!!!!

Oh why do I bother taking the bait to your replies...I should have learnt by now!

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Follow Up By: KSV. - Thursday, Apr 03, 2008 at 12:57

Thursday, Apr 03, 2008 at 12:57
Honestly I do not understand what bait you talking about. You said “the roofrack would not affect drag all that much at 80km/h” and I disagree with that because I recon that it make noticeable influence. Although I agree with your statement that below 80 km/hr shape of the car does not make such difference. Reason for my reckoning that roof rack has very complicated structure versus car’s body.

Cheers
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Reply By: Nifty1 - Thursday, Apr 03, 2008 at 11:53

Thursday, Apr 03, 2008 at 11:53
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....
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Reply By: howie - Thursday, Apr 03, 2008 at 11:54

Thursday, Apr 03, 2008 at 11:54
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.
AnswerID: 296352

Follow Up By: brushmarx - Thursday, Apr 03, 2008 at 12:29

Thursday, Apr 03, 2008 at 12:29
This parasite drag has me intrigued.
Does this mean that vehicles carrying politicians (our biggest parasites, in my opinion) will have worse fuel economy than other vehicles?
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Follow Up By: howie - Thursday, Apr 03, 2008 at 13:04

Thursday, Apr 03, 2008 at 13:04
LOL
another part of parasite drag is skin friction.
the smoother the surface the less friction.
so maybe at the next election, the voting slips should include height, weight and a picture so we could vote for a smooth, less parasitic, economical politicion (if that is possible).

BTW to work the best speed out, wouldn't the best way be to fit a fuel economy gauge (some cars did have them).



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Follow Up By: Ingtar - Thursday, Apr 03, 2008 at 17:37

Thursday, Apr 03, 2008 at 17:37
"the smoother the surface the less friction."

I read an interesting article in a national geographic recently about research into replicating a marine animal's skin. They believe the textured skin decreases turbulence and hence allows the animal to move faster through the water.
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Follow Up By: robak (QLD) - Thursday, Apr 03, 2008 at 18:56

Thursday, Apr 03, 2008 at 18:56
like a golf ball.
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Follow Up By: howie - Thursday, Apr 03, 2008 at 22:17

Thursday, Apr 03, 2008 at 22:17
not a expert on beavers or golf balls (or aviation) but why are the major airlines considering removing their paint jobs to fly naked and shiny to decrease fuel consumption and reduce costs?
that is skin friction.
golf balls- the dimples change the drag from laminar to turbulent (ie causing turbulance). this allows the ball to be held in the air longer and carried further.
marine animals- if that was so, wouldn't the speedo's worn by the swimmers be dimpled or textured, or is beaver technology more advanced than speedo's?
your witness

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Follow Up By: Ingtar - Friday, Apr 04, 2008 at 03:28

Friday, Apr 04, 2008 at 03:28
Only mentioned it for interest (and couldn't remember the animal at the time) but...

I can't seem to find an actual national geographic article (I read it in a magazine) however these are the photos of a shark's dermal denticles and yes speedo have apparently tried to use them to increase swim speeds.

mm fluid dynamics... now there's a crazy subject.
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Reply By: Robin Miller - Thursday, Apr 03, 2008 at 11:54

Thursday, Apr 03, 2008 at 11:54
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)
Robin Miller

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Reply By: Jim from Best Off Road - Thursday, Apr 03, 2008 at 11:57

Thursday, Apr 03, 2008 at 11:57
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.

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Follow Up By: brushmarx - Thursday, Apr 03, 2008 at 12:22

Thursday, Apr 03, 2008 at 12:22
Hi Jim
You say "without bloody good reason"
Wouldn't saving fuel and the resulting less greenhouse gasses be a bloody good reason? Think of all the dolphins coughing their little lungs out, and those cute little harp seals getting clubbed to death before they can die from heat stroke.
As for being "a menace and dangerous", I would suggest that the people making dangerous moves to overtake slow drivers would be in that class, but I wholly agree on your frustrating and would add teeth gnashing, blood pressure raising and irresponsible.
Seriously, on busy single lane carriageways, I plan on sticking to the limit if the Jackaroo can do it safely, but on the left side of multi lanes, if I choose NOT to do the maximum legal speed limit, anyone going faster than me can't change lanes, they can get stuffed.
I don't think that most of the travelling to Innamincka and back will have too many vehicles held up by me doing 80 kph.
Don't duck for cover, everyone has a right to their view. (within reason)
Cheers
Ian
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Follow Up By: Andrew from Vivid Adventures - Thursday, Apr 03, 2008 at 13:17

Thursday, Apr 03, 2008 at 13:17
Hey Jim,

I've got to make sure you have time to read the advertising on the back.

Cheers
Andrew rarely at as slow as 80, but more rarely as high as 100 ;-)
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Follow Up By: Gronk - Thursday, Apr 03, 2008 at 13:28

Thursday, Apr 03, 2008 at 13:28
Less time spent on the road means more time at camp sipping on a beer !!!

For the price of a beer ...I think I'll sit on a 100...
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Reply By: Member - Doug T (FNQ) - Thursday, Apr 03, 2008 at 13:35

Thursday, Apr 03, 2008 at 13:35
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.

.
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Reply By: jeepthing - Thursday, Apr 03, 2008 at 15:11

Thursday, Apr 03, 2008 at 15:11
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.
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Reply By: Steve from Top End Explorer Tours - Thursday, Apr 03, 2008 at 15:38

Thursday, Apr 03, 2008 at 15:38
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.
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Follow Up By: KSV. - Thursday, Apr 03, 2008 at 15:56

Thursday, Apr 03, 2008 at 15:56
Something very wrong here. Standard sub tank in 80 50L. With assumption that yours the same it means 25L/217.9=11.5L/100km normal result for such car. But 50/217.9=22.9L/100Km – way too much even for petrol, forget diesel. It is definitely something wrong with your fuel system. As per turbo versus normally aspirated it looks like turboeing does not make any difference on fuel consumption, but greatly increase torque and therefore drivability. As least this is my observation.

Cheers
Serg

NB – fuel solenoid can develop very funny behaviors with time. May it be situation when fuel get sucked from sub-tank, but returned in main?
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Follow Up By: Steve from Top End Explorer Tours - Thursday, Apr 03, 2008 at 17:02

Thursday, Apr 03, 2008 at 17:02
I have never put more than 45 lt in my sub tanks and they are empty at refill, I run all the cars to the light comes on some times longer, If you fill to the brim, you can,t get fuel from the sub to the main, as the main is full.

When I say the TD only uses 1/2 the sub I mean it is probably just over, my comments were meant to be a bit tongue in cheek.

In the same token while on holiday, I had a roof rack and camper trailer, I drove at 90 kph and got 330 km from the sub.

Cheers Steve

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Follow Up By: Member - Shane D (QLD) - Thursday, Apr 03, 2008 at 17:26

Thursday, Apr 03, 2008 at 17:26
Thats not a comparison!
A 1HZ will ALWAYS use more fuel than the newer TD in any circumstance.
Shane
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Reply By: lakedge - Thursday, Apr 03, 2008 at 15:47

Thursday, Apr 03, 2008 at 15:47
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
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Reply By: stefan P (Penrith NSW) - Thursday, Apr 03, 2008 at 17:20

Thursday, Apr 03, 2008 at 17:20
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
AnswerID: 296412

Follow Up By: Steve from Top End Explorer Tours - Thursday, Apr 03, 2008 at 17:44

Thursday, Apr 03, 2008 at 17:44
And if you put 2 under there you wont use any fuel at all.

Who said they don't work. LOL

Cheers Steve
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Follow Up By: KSV. - Thursday, Apr 03, 2008 at 18:30

Thursday, Apr 03, 2008 at 18:30
Hmmmmm..... you got me thinking.... what happens if you put 3 of them? Or even 4? Then fuel can be syphoned from one car to another! Incrtedibly handy in remote areas!

:-))))))
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Reply By: Member - Shane D (QLD) - Thursday, Apr 03, 2008 at 18:13

Thursday, Apr 03, 2008 at 18:13
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.
AnswerID: 296428

Reply By: Richo (SA) - Thursday, Apr 03, 2008 at 19:15

Thursday, Apr 03, 2008 at 19:15
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.

AnswerID: 296438

Follow Up By: Andrew from Vivid Adventures - Thursday, Apr 03, 2008 at 23:55

Thursday, Apr 03, 2008 at 23:55
um that is probably somewhat of a simplification...

Velocity, for one thing is relative velocity... hence the effects of headwinds, et al.

The other important factor is that the power to overcome this drag varies as the CUBE of the velocity! Hence dramatic economy differences for small differences in velocity.
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