economical driving - sajid
yesterday i took my astra 1.4 to manchester airport, I spent £6.50 on fuel,72p a litre and i covered 98 miles till the fuel warning light came on, on the motorway i was travelling an average 62 m.ph, i reckon for a petrol car with four adults my car gave about 45 mpg,question to you guys is it more economical to drive at a constant 60 than go flat out 80-90 mph, saving time, but would i still be driving economically at faster speed?
economical driving - TrevP
Erm, is this a serious question?
economical driving - trancer
Maybe you could try driving the same route at 80-90 flat out and calculate what mileage you get then. Normally I would say you will get less mileage, but recently while driving a borrowed car with an instant readout MPG computer, I couldn't see much difference between driving at 60 and 80mph.

Of course this only applies that particular car in it present state of tune etc etc, but after that experience I would hesistate to speculate on fuel mileage based soley on a given speed.
economical driving - Morris Ox
I'd say the answer to this question lies in your school maths books and the Road Traffic Act. Both give a simple answer.
economical driving - Baskerville
To test out the theory, try this:

1. Ride ten miles on a push bike at ten miles an hour.

2. Next day do the same ride and try to stay at twenty miles an hour.

3. Decide which one was more tiring: that will be the one that used more energy.

Remember that wind resistance at 20mph is more than double what it is at 10mph. The same applies at higher speeds except that the wind resistance curve gets steeper the faster you go.
economical driving - TrevP
As a rough guide, for "average" car, 60mph requires 19bhp at flywheel and car does 48mpg
At 80mph, the figures are 38bhp and 31mpg
And at 100mph, 69bhp and 22mpg
economical driving - mal
It always amuses me when I hear or read of drivers claiming MPG figures based on where the petrol guage needle is at a given distance on their journey or as in the lead post the point at where the fuel warning light came on.
The *only* accurate way to work this out is to brim the tank between fills and use simple mathematics.
Regards Mal.
economical driving - eMBe {P}
Trev - unless we are dealing with trolls, we have to assume it is a serious question.

Sajid: I have no idea what the significance of the amount of fuel, Manchester airport, or the price per litre of fuel has to with your question. As to the answer to your specific question, I will pose you this riddle:
If you think that fuel used at 80mph is the same as used at 60mph, then at 0mph (with your car at a standstill, while in 5th gear, and the engine swithced off), will it still be using the same fuel as at 80mph?
I hope that answers your question.
economical driving - Obsolete
eMBe: So, he'd get the best consumption with the engine turned off then? Or am I missing something?

Maybe this isn't such a silly question. I think the point TrevP and others were making is that 80-90 mph is not legal. I would add that the efficiency of an engine depends partly on the gearing. My 5 speed Ka seems to do best at about 60mph. I suspect that at 80 mph it would drink lots. Maybe having 6 gears would help a bit for higher speeds, though as mentioned wind resistance does increase.
economical driving - Baskerville
>>Maybe having 6 gears would help a bit
for higher speeds


Not necessarily. The handbook of our old 1.4 Fiat Tipo stated that top speed in 4th was higher than in 5th. Presumably this was to do with revs and the available power/torque.

economical driving - jud
sajid
each car has what i call a "sweet spot" that is to say a speed at which the car cruises the best, this varies depending on the power and gearing also the terrain you are covering, for e.g. if the route you are taking has long inclines, to maintain a reasonable speed in a low powered car you need to attain a speed sufficiently high to cover the incline with out slowing down to a crawl. With a 1.4 astra try a cruise of say 75mph this should get you over most inclines with out loosing too much speed but still holding top gear and keeping up with the traffic. The astra 1.3 i had in the 80's was a 4 speed box which was happy at 80 mph at 4k revs.
Obviously the faster you go the more fuel you consume, but with practise you will find the best balance for you car and your pocket.
economical driving - sajid
What about offical mpg figures they are based on a fixed speed say 56 mph, is that the speed where you get the best economy? what if you drive at 56mph, and another car is at 70mph obviously the faster one will get there but at the cost of more fuel consumed, but the time saved, is that any worth??
economical driving - andymc {P}
Sajid, I think what you're asking is whether or not the amount of fuel your engine consumes to do a given distance at a higher speed would balanced out by driving slower, with the consequence that the engine consumes fuel for a greater length of time - is that it?

Only a layman's answer here (so I welcome any corrections of what I'm about to say), but I think that would only apply if you were driving so slowly as to be below your car's ideal cruising speed, ie the car would be consuming fuel inefficiently because it hadn't yet reached the point where maintaining momentum and countering wind resistance were more or less equal. So in an extreme example you'd get better consumption travelling 40 miles at 80 mph than at 10 mph. In terms of realistic driving, my experience has been that you will tend get the best fuel economy at anywhere between 55 and 65 mph.

My car could do well in excess of 60mpg if I kept it at a steady 56 mph, but I would die of boredom if I drove like this all the time. So I drive in a style that suits me (and generally keeps me within the law) and it still averages mid-fifties anyway, so I'm not too worried. And what was said about the fuel gauge only being an approximate guide is very true - I started my drive home today with a steep climb for the first seven miles, and another 20 miles later my fuel gauge indicated there was more in the tank than when I finished climbing!
andymc
economical driving - eMBe {P}
andymc: I think you are a genius (sincerely said). That is exactly what I thought the initial question was but dismissed it with "I think he is not asking that".
economical driving - sean
I'm no genius and add a few things to help here.

E=1/2 mv squared.

ie energy is proportional to the square of velocity.

The faster you go, the more fuel you use.

There are some truly awful calculations involving aerodynamic efficiency. I will go there if you want.

We can leave it at the slowest possible speed, at the engine's torque peak, in the highest possible gear.

Your call.

This may get heavy and boring now.
economical driving - andymc {P}
eMBe - why thank you sir, you are obviously a shrewd judge of character. ;)

sean - without getting "heavy and boring" (I try to be neither!), would it be correct to say that all other factors being equal (power to weight etc) the more aerodynamically efficient the car, the higher the optimum cruising speed? For example, would the optimum cruising speed of a Passat with the 130 PD engine be higher than the optimum cruising speed of a Touran with the same engine, assuming both vehicles are carrying an equal weight? Just curious.
andymc
economical driving - sean
NO,

power and weight have no influence as substantial as speed.

v squared.

SI units.

Metres per second squared.

So, speed is the biggest factor, then drag.

Weight is halved in the equation, so is not as significant.

To obtain best efficiency, look for ovoid shape ( have not said why) and lowest speed in highest gear.
economical driving - Greg Parker
Related to the question. I have been looking at official fuel economy figures and found there are two official tests. The garages tend to only show 80/1268/eec in their brochures. However, they fail to represent everday driving and the new cycles (93/116/eec) shows a completely diferent picture. For example, with a diesel 2.0 engine, the original figures suggest in town one will get 38.7 mpg whilst the representive figures suggests only 32.5 mpg. This is a big difference.

The question thus is, where can the new figures be found for new and old cars (i.e. 93/116/eec).
economical driving - TrevP
The question thus is, where can the new figures be found for old cars?

They can't. Like asking for NCAP figures for Allegro.
economical driving - sajid
So does that mean that offical mpg figures are misleading, due to not reflecting real life driving, my astra 1.4 is a high torque model, and torque is pulling or turning power, if a car with high torque lets say peak torque is delivered at 2500 rpm, would that vehicle be considered to be more efficent fuel wise than one with less torque?? does that have a bearing in fuel efficency?
economical driving - TrevP
So does that mean that offical mpg figures are misleading

- no

my astra 1.4 is a high torque model

-no (no such animal as "high torque 1.4 Astra")

lets say peak torque is delivered at 2500 rpm

- probably more like 3500rpm

would that vehicle be considered to be more efficent fuel wise than one with less torque??

-no

does that have a bearing in fuel efficency?

-no.

Engine efficiency is obviously a significant factor.
(as in efficient burn of fuel)

However, for vehicles, aerodynamics play a significant role
BUT the major factor is speed.

For any given vehicle, approx twice as much fuel will be used at 80mph as 60mph.
economical driving - TrevP
sorry, - engage brain.

at 80mph, approx 50% more fuel will be used as at 60mph.

 

Value my car