Thread: Speed vs fuel consumption

Absolutely some roads are designed to for safe traveling at up to 130km/h. The Hume for example.
Sitting on a 110 km/h in a modern vehicle is really a waste of the road and the vehicle's capabilities.
I can't see why they are not raised. It reduces travel time, limiting exposure on the road, a key component of mitigating risk
A lot of Countries have a 'typical' driving speeds of 130km/h on two lane two way divided roads.
None of this clumping rubbish with everyone driving bunched up, 10-20m apart, but too scared to speed.

Clive

We seem to have got a little off topic. The experiment I first posted about was looking at the difference in fuel consumption with speed, and confirmed that fuel consumption per kilometre is directly proportional to speed once you are travelling fast enough that most of the resistance is aerodynamic as theory predicts.

Historically this conclusion has been distorted by vehicles having a limited number of gears and having engines with a relatively sharp peak in efficiency, but the demands for fuel efficiency have ensured that the vast majority of vehicles have a set of gear ratios to allow efficient operation at any reasonable speed - and usually have a gearbox that automatically selects the optional ratio.

Note that this experiment was conducted over a long distance (close to 4,000km) of mostly flat going with little traffic and few bends or intersections for most of the distance. Especially where there are hills, traffic or bends and intersections introduced, driving style becomes much more important, especially efforts to keep a fixed speed (usually the speed limit). This means high acceleration if forced to slow, and braking to avoid going over the limit rather than planning ahead by taking your foot off well before arriving at the corner etc.

A bit off topic, note that the speed limit is just that, a limit, not a target, and in NSW at least, there are classes of drivers and vehicles that are mandated to a maximum speed up to 20kph below the posted speed limit (90vs110), so the authorities certainly do not agree that everyone should drive at the same speed - even on major highways with only one lane each way and few overtaking opportunities. You may not like this, or think this is sensible, but this is the law.

(At least NSW dropped the speed limit for trailers over 750kg of 80kph - used to be a nice little earner from interstate drivers)

Good summary John. Thanks for keeping the numbers and figures from your trip. Lots of good info here for anyone wanting to find out about fuel economy!

I find that the discipline of driving at a defined speed eg 90kmh soon fades
If I have 1000 km to drive today then I will go as fast as is legal safe and within the vehicles capability
Fuel economy becomes irrelevant after a few hours

Interesting to compare your figures John.
Just did a trip the other day in the Isuzu County, down to Melbourne and back.
300km of country road and 150km of moderate to heavy traffic:- 45ltrs of fuel. 10ltrs/100km.
I usually sit on about 90kph to maximise economy. I have compared doing the trip at 100kph on the open road and 90kph.- time difference is probably 10 minutes.
I am always impressed with the economy puttering around in the traffic all day.

Pete

I do very little in the County in traffic, so have little opportunity to see how I go in those circumstances. Driving in traffic - puttering around - the vehicle will rarely reach speeds where aerodynamics are the main source of losses, so driving style will be the principal difference in results, mainly the amount of braking you do (plus things such as tyre pressure, dragging brakes etc).

There are areas where faster speeds would be far safer, reduce road incidents and road rage...

Seriusly????

The ONLY time that extra speed (momentum) becomes a "Safety" factor is when driving on soft sand, claypans or damp black soil country so that you don't become bogged in a remote area.
ANY increase in speed will increase the risk of injury or death and this is a proven Fact.
As far as road rage goes, Well people that are prone to road rage should NOT be allowed behind the wheel in the first place.

Originally Posted by rocket scientist

.....
I usually sit on about 90kph to maximise economy. I have compared doing the trip at 100kph on the open road and 90kph.- time difference is probably 10 minutes.
.....
Pete

That's what I've always found. Trying to go faster to get somewhere quicker usually ends up in frustration and zero nett effect!
I give myself just a few more minutes to get there at the slower pace.