Member - John (Vic) replied:

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.

Member - Brian (Gold Coast) replied:

Davoe
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.

Member - Chrispy (NSW) replied:

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?

David N. replied:

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.
Cheers

Member - Jeff M (WA) replied:

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 pisses 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...

Member - Collyn R (WA) replied:

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