Low range double the fuel economy??

Submitted: Monday, Jul 04, 2005 at 00:59
ThreadID: 24406 Views:3557 Replies:6 FollowUps:18
This Thread has been Archived
I use low range alot when on tight bush tracks as I believe it gives better flexability from the gearbox when maximum speeds are 30 -40k and usually alot slower. Apperantly the general rule is twice the tourque and double the fuel usage. But how does that work?? I can see it being true for the same speed in the same gear but whats the difference between 3rd 4th low and 2nd and 3rd . I realize there is somewhat of a gearing differance but would it give half the fuel economy when speeds are around 10 - 20kph??
Back Expand Un-Read 0 Moderator

Reply By: Member - John (Vic) - Monday, Jul 04, 2005 at 01:56

Monday, Jul 04, 2005 at 01:56
Davoe its got nothing to do with speed per say its all about engine revs.

Simplistically speaking.

If you are doing say 2400 RPM in 5th on the Hwy your doing say 100 kph.

If you are doing say 2400 RPM in 2nd or 3rd low your doing say 20 or 30 kph.

Stands to reason that you will get better litres per 100 km traveled if you actually traveling at 100 kph and covering 100 kilometers in one hour as opposed to 30 kilometers in one hour.

VKS737 - Mobile 6352 (Selcall 6352)

Lifetime Member
My Profile  Send Message

AnswerID: 118713

Follow Up By: Member - Davoe (Widgiemooltha) - Monday, Jul 04, 2005 at 02:02

Monday, Jul 04, 2005 at 02:02
yea realize that but as a more exact example if in high range at 15 to 20k I might be in 2nd where as in low range I might be in 4th engine revs will be simular. reason for using low is that I still have 4 gears to change down for rough or scratchbits will in high range there is only 1 gear
FollowupID: 373894

Follow Up By: bundyman - Monday, Jul 04, 2005 at 08:25

Monday, Jul 04, 2005 at 08:25
I tend to agree with you Davoe. It depends on what your doing and how much your giving it to her. A days worth of low range in soft sand will chew heaps more than idling around rocky tracks in low range. But as for what your saying - driving around in 1st gear highrange or 4th gear lowrange over the same track won't make any difference. Engine is doing the same number of revolutions for the same time period = same fuel usage. Except you have more choices if in low range.


FollowupID: 373904

Follow Up By: Member - Davoe (Widgiemooltha) - Monday, Jul 04, 2005 at 10:45

Monday, Jul 04, 2005 at 10:45
I was actually wondering if you might use less coz in high range the motor gets out of its powerrange 1000rpm and lower frquently where as in low it is generally 1-2000 rpm
FollowupID: 373910

Follow Up By: bundyman - Monday, Jul 04, 2005 at 11:01

Monday, Jul 04, 2005 at 11:01
Well I know I once did a trip from brisbane to Landcruiser Mountain park and back in my old HZJ75 (1HZ) and after 2 days of idling around the park with 30 high:70 low range work and bitumen highway there and back my fuel economy (which I record EVERY tank full) was still within an allowable 10%. This suprised me as I was expecting to use a fair bit more. So yes relatively for the length of time the engine was runing during those 2 days and the kays I travelled I used suprising stuff all fuel.
But hey you just can't go past the old 1HZ for torque at idle. Nothing is too much trouble for them. The big girl is still going well after the more powerful and torquey turbo diesels have stalled!!!
FollowupID: 373918

Follow Up By: Mike Harding - Monday, Jul 04, 2005 at 18:16

Monday, Jul 04, 2005 at 18:16
It's about _power_ used.

I should say I'm out of my depth here really - is there a mechanical engineer in the house? (Even a battery operated one would do :)
OK... mechanics according to Harding...

At 100kph you have very significant wind resistance which increases on a squared law basis - so at 100kph you require 4 times the power to overcome it than you would at 50kph - and lots, lots more than you would at 20kph.

I think fuel consumption will often tend to increase in low range because the engine is being asked to produce lots of power - sand and hill climbs come to mind - additionally the manner of driving the vehicle in low range type conditions tends to make for many more abrupt throttle and brake uses than a nice 100kph on the freeway on cruise control. It's like driving in really heavy traffic all the time. We’re constantly slowing and accelerating a load of 3 tons of metal – so there is an inertia factor too.

There is almost no air cooling of the engine so the fan is always running, there is more running gear being turned over (in some vehicles) when 4WD is engaged - all this requires fuel.

Errr... that's it - I'll wait for the people who _really_ understand this area to put us right.

Mike Harding
FollowupID: 373989

Follow Up By: Lone Wolf - Monday, Jul 04, 2005 at 19:24

Monday, Jul 04, 2005 at 19:24
Mike, you are spot on with the wind resistance factor... 100%

On a very recent trip, which covered 6000 km, a third of that was in 3 rd. gear high, couldn't go much faster. We averaged nearly the same consumption as we did when travelling at 100 kmh.

As Mike has pointed out, wind resistance squares with a doubling of speed.

10 kmh could be say a resistance of say..... 100 units.
20 kmh would equate to 1000 units....



FollowupID: 373999

Follow Up By: Member - Davoe (Widgiemooltha) - Tuesday, Jul 05, 2005 at 13:28

Tuesday, Jul 05, 2005 at 13:28
Mike your spot on............... but have missed the point. So I am lead to believe the most economically way to drive a vehicle from a to b (depending on vehicle but generally) is about 80kph because it covers all the factors you speak of. BUT I have heard on more than one occasion that low range doubles the tourque and doubles the fuel usage I was wondering how this works as you usually use 2 gears higher in low range than high. question only applies to low speed travel which by the concepts you speak of can never be as economical as higher speeds
FollowupID: 374098

Reply By: Member - Brian (Gold Coast) - Monday, Jul 04, 2005 at 06:50

Monday, Jul 04, 2005 at 06:50
I agree with you on the low range thing.... As soon as I leave the blacktop, I stick it in 4wd... and low if at ALL possible :-) on tight windy bush tracks, i can drive in low third like an auto......

And because of that I can say that the fuel consumption roughly doubles! Why??? Something to do with the gearing but I kinda think like you do and wonder about it... the only logical thing I can come up with is that if you were driving around in high second.... you'd be pouring a whole lotta fuel into the engine for little k's.... the same probably as low 3rd or 4th.... the fuel economy "stretches" from using tall gears, high 4 and 5.

I think.
AnswerID: 118717

Follow Up By: cokeaddict - Monday, Jul 04, 2005 at 08:19

Monday, Jul 04, 2005 at 08:19
G'day Brian,
How ya doin mate?
you got that beast "unstuck off that damn tree stump yet?
Need any more rain up there? I can arrange some iffin you need it.

Take care,
FollowupID: 373903

Follow Up By: Member - Brian (Gold Coast) - Monday, Jul 04, 2005 at 15:10

Monday, Jul 04, 2005 at 15:10
Hey Angelo.... good to hear from you mate! Hope your rig pic means you still have the mighty GQ!!!
Rain???? Mate... we took a gallon or three from the rainclouds last week and sent on the rest down to you! Hope it's raining on the catchment, and I hope it rains over Sydney like it rained here .... I know you guys down there need it!

the stump???? Guess it's time to change the rig pic... LOL.....

See ya
FollowupID: 373959

Follow Up By: Truckster (Vic) - Monday, Jul 04, 2005 at 15:15

Monday, Jul 04, 2005 at 15:15
Ange.. I thought you were selling your rig while back, or have I bolloxed that uP/
FollowupID: 373961

Follow Up By: cokeaddict - Monday, Jul 04, 2005 at 20:24

Monday, Jul 04, 2005 at 20:24
G'day truckster,
Yes mate I was selling it. Had a Genuine buyer too from this forum in S.A. But i was given a second chance with a small inheritance, so I withdrew the sale. Kinda glad i did, she has been cruising so well lately, Done 7 trips to Victoria in past 3 months and she is excellent both on power and fuel economy. Unfortunately though the trips were not pleasant ones as dad passed away 3 weeks ago, just glad i was there when he did.
I tell you what though truckster, its chalk and cheese between NSW and VIC for 4wd's, you guys south of the border really know how to have a good time offroad. Seen me heaps of 4b's and even more of them were dirty, just how i like em. Keep up the good work guys.

Hey Brian, yeh was given another chance mate. And NO.....the pic is fine. Dont change it ...YET !!! :-)
Take care all
FollowupID: 374006

Follow Up By: cokeaddict - Monday, Jul 04, 2005 at 20:27

Monday, Jul 04, 2005 at 20:27
Forgot to mention Brian, have had a couple of sunny days up here, So I am taking dad's tinny out in the bay tomorrow (mum wanted me to take it) for its first bream strike. Hope the old man sends a few my way.
FollowupID: 374007

Reply By: Member - Chrispy (NSW) - Monday, Jul 04, 2005 at 09:06

Monday, Jul 04, 2005 at 09:06
I'd suspect that engine "labouring" would also affect the consumption figures. If you are in steep-ish territory and driving along slowly in high-range - you'd probably use more fuel than if you were tootling along more slowly in low range in a higher gear..... no?
AnswerID: 118727

Reply By: David N. - Monday, Jul 04, 2005 at 09:28

Monday, Jul 04, 2005 at 09:28
If you do the same track at the same speed etc. then it doesn't matter how you achieve the required gearing (say either 1st gear, high range or say 3rd gear low range) it won't change your fuel consumption.
AnswerID: 118732

Follow Up By: Member - Davoe (Widgiemooltha) - Monday, Jul 04, 2005 at 10:49

Monday, Jul 04, 2005 at 10:49
I find I usually end up using higher revs with low range but still around 2500 max briefly here and there where as in high you spend more time with the motor lugging well below its powerband.
FollowupID: 373912

Reply By: Member - Jeff M (WA) - Monday, Jul 04, 2005 at 13:21

Monday, Jul 04, 2005 at 13:21
I found in my little petrol Feroza it use to eat fuel in Low range, but that was probally more to do with the fact that all 4 wheels were off the ground the majority of the time.. In the surf I more than often use low even though I don't need to (because the auto obviously slips at low speed high is good for most stuff if you choose to go that way) but Low gives me torque lock and no torque slipping which means I barley have to touch the brakes (as I can engine brake) but in high I'm constantly riding the bakes so I'm switching my foot from pedal to pedal all day and it wears my leg out and just generally bleep es me off.

I reckon I'd be using almost identical amounts of fuel between high and low as I'm doing higher revs in low but I'm not pushing the pedal as far in as the revs are always up, in high range the auto keeps disengaging and letting the revs drop down to idle, then you have to wind it back up again and it goes on and on like that all day. I find the diesel is bloody good off road anyway with the fuel, low or high range. I spose you can only rev the diesel out to 4 grand anyway, so it doesn't make much diff. A petrol I spose might be revving at 2.5-3k in high and low might be between 4-5k. Diesel's are pretty much always between 2-3k no matter which gear or ratio your in...
AnswerID: 118759

Follow Up By: Member - Davoe (Widgiemooltha) - Monday, Jul 04, 2005 at 13:58

Monday, Jul 04, 2005 at 13:58
did alot of offroad work last job I had and got into an argument with the shwbo who reckoned running it in low range placed stress on things and the other fieldy used high range which I thought was dumb as max speed was 20kph and often 5kph bulldozing trees or slower climbing stumps. I figured I was much better off in low 1 going over tree logs that I knew were going to hit underneath or smashing through thick branches than at high 1 speeds had over 200l of fuel and wasnt paying so dont know consumption figures
FollowupID: 373945

Follow Up By: Member - Jeff M (WA) - Monday, Jul 04, 2005 at 14:20

Monday, Jul 04, 2005 at 14:20
I have heard stories about people damaging their transfer boxs due to overheating, but that's because they were being a bunch of tools and driving flat out along a beach for miles and miles in 5th absolutally ringing out the gears in the transfer box on a 40degree day. There are obviously limits to everyting... Personally when I had a manual I never used the 5th cog in low range as I feel there is way to much torque in low for that little cog and your likley to shear it off! Not to mention the difference between 4th and 5th in low range is bugger all to nothing anyway!
FollowupID: 373950

Follow Up By: Member - Davoe (Widgiemooltha) - Monday, Jul 04, 2005 at 14:45

Monday, Jul 04, 2005 at 14:45
no never 5th even in a work vehicle if speeds were consistantly up to 30 40 kph - high range. I dont know the mechanicals of it but as 5th is always weak on any vehicle if you need 5th low range then it is time for high range.
FollowupID: 373951

Follow Up By: Member - Brian (Gold Coast) - Monday, Jul 04, 2005 at 15:04

Monday, Jul 04, 2005 at 15:04
I got it into 5th low on Fraser last month, but only for a short burst and 'cos I could. But it was easier to stay in Low range for the rougher bits of the track.... I find it easier to stick to 3rd low so I can drive it like an auto, this is only for rough sandy tracks like most of Fraser's inland roads...... But agree that if you get it into 5th low for any period of time, then high range has to be better!
we rented a Range Rover a few years back for a Fraser trip, the rental guy told me of some guys who were tearing up the Eastern beach... 60 k's in Low 4th.... a piston exited the block!!!! ouch!!
FollowupID: 373956

Follow Up By: Member - Davoe (Widgiemooltha) - Monday, Jul 04, 2005 at 17:22

Monday, Jul 04, 2005 at 17:22
landcruisers will easily hit 60 in low 4 and I do regulary on conrod straight up the decline just after stockplile 4 they will also hit 5k rpm doing overspeed in 3rd down as well and have no problems holding 4500rpm in low reverse for hundreds of metres trust me if you can imagine it i have seen it done or done it to all those people that reckon blown motors diffs t/c boxes etc I say dreeeeeming just buy one and see
(and some of them didnt even own cruisers :))
FollowupID: 373978

Reply By: Member - Collyn R (WA) - Tuesday, Jul 05, 2005 at 12:29

Tuesday, Jul 05, 2005 at 12:29
Bye and large, steady state fuel consumption above 80 km/h is mostly a function of weight and speed. From about 40-80 km/h it is mostly a function of weight. Below 40 km/h time becomes a major factor (because a disproportionate amount of energy is lost as heat whilst driving very slowly).

In traffic and off-road, the often major increase is also a result of constantly changing speed, thus necessitating extra energy to be expended in accelerating mass. (For this reason, cruise controls will increase consumption in extensively hilly areas such as the Oz's east coast Dividing Range).

Within reason none of the above is affected much by the gear that one is in.

Good low down torque helps as, in some going, it enables one to maintain a more constant speed.

With diesels at least best economy is usually obtained (at lower speeds) at whatever speed represents the peak of the torque curve (or more correctly at peak bmep (brake mean effective pressure) - but that's often much the same point).

As a matter of interest my OKA hardly varies in fuel consumption at all off or on road below 80 km/h.

The worst ever recorded was a Simpson Rig Road crossing when it increased from a typical 13-15 litres/100 km to a tad over 18 litres/100 km, due very substantially I think to the huge torque (from about 1000 tpm) of the 4cyl turbo Perkins. Not bad for 5.5 tonne.

(I'll just add ,as modestly as possible for a Leo, that I was originally a mechanical engineer, but worked as a vehicle research engineer, until I later started an international electronics magazine).
Collyn Rivers
AnswerID: 118933

Sponsored Links