Driving Economically - 56mph Myth??

AdrianHi

loofer wrote: »

There's no science to back this up but here's my theory anyway.

I think most efficienct for 'my car' is in top gear (6th) where the torque band is at it's peak.
For my VW Bora TDI this starts from about 1950rpm I think.

Thankfully for me - 2000rpm is 70mph...so observing that speed limit is most efficient contsant speed - me thinks.

Diesel car's have alot more torque than their petrol equivalants and this peak torque is achieved much lower down the rev range.

Does quite work like that.
The TDi engines are extremely good, and after seeing the following graph for the first time, I see it as one of the best all rounders for fuel economy. The graph is for a Gold GTi TDi, so your's may a different profile - expect consumption to get worse quicker as speed climbs, but you get an idea.

tomstickland

There's no rule that peak torque should correspond to peak efficiency. It's generally more rpm orientated, and somewhere around 2000-2500rpm.
A bit more cam and an engine might make peak torque at 5000rpm, but peak efficiency would still be at lower rpm.

bryanb

Anybody know a way for a layman to determine the most economic (sensible) speed for a given vehicle. Particularly a 1.9 non turbo, non common rail, diesel, without a fuel computer fitted? I realise I could conduct tiresome experiments driving at constant speeds etc but thing don't work like that.

AdrianHi

bryanb wrote: »

Anybody know a way for a layman to determine the most economic (sensible) speed for a given vehicle. Particularly a 1.9 non turbo, non common rail, diesel, without a fuel computer fitted? I realise I could conduct tiresome experiments driving at constant speeds etc but thing don't work like that.

Unless you going to get into varying speed and using momentum down hills etc. to shave the last bit off your fuel use, I think it's best to go with the flow of traffic, or if your own your own, what ever minimum speed you need to do to hang on to top gear. The actual speed may depend on the nature of the road. If you have a road going up and down hills a bit you might need to pick a higher speed to hang onto that gear up hill.
There is an interesting article on What Car which I would post a link to, but I keep finding links I make to What Car web site getting delete by mods, so have a search for "Fuel economy: making every gallon count - The test findings" article 6 June 2008.

tomstickland

I was reading What Car whilst waiting at the dentist. The Aug 08 issue has a report on mpg, including detailed measurements. All of the vehicles tested produced better mpg as speeds were reduced, with the best results at the lowest speed that they tested at, which was 40mph.

tomstickland

bryanb wrote: »

If that's so, what is the point of streamlining? Is it perhaps less turbulent if streamlined?

Having turbulent flow doesn't make streamlining redundant. The profile of the vehicle will still affect the aerodynamic drag forces.

AdrianHi

tomstickland wrote: »

I was reading What Car whilst waiting at the dentist. The Aug 08 issue has a report on mpg, including detailed measurements. All of the vehicles tested produced better mpg as speeds were reduced, with the best results at the lowest speed that they tested at, which was 40mph.

My car has a 6th gear which I cannot use below about 50mph, however all 5 speed manual cars I've had would do 40mph in 5th so I would expect it to be the most economical speed on a reasonably flat road for these.

andor

Always got about 330 miles on a full tank of petrol until we decided not to exceed 60. Now we get somewhere in the region of 370 to 380 miles on a full tank.

PS. Apart from on holidays where we had to drive on about 400 miles of motorway.

arryzed

wow this is a very tricky subject and its not just a random figure...I can't remember all the facts because I was taught this a long time ago.

Basically the magic 56mph is because the energy needed to move a car beyond this point (the dragg of resistance through the air, tyres, gravity, etc) increases at such a steep curve irrspective of what vehicle you have although large heavy vehicles are a lot worse.
The graph ramp (energy/fuel to speed) starts to climb at 40mph so in real terms if all vehicles were geared to have their fuel efficency set to 40mph we would all get even better fuel consumptions but we would have to drive at 40mph....hope this makes sense.

you will notice if you drive your (car Petrol)at 56mph in top gear your revs will be some where between 2,300 and 3,000 (diesel are lower rpm)what ever car you have,
in the past a carburretor's jets were always made to have the leanest fuel usage at approx 2,500ish rpm) which would corrispond with the gear box ratio delivering a speed of 56mph...if you increased your speed to say 60 mph 1. you would have to use more energy because the dragg increases by a large factor as the ramp (graph) is climbing. 2. because the vehicle has been designed to be at its most effiecent at 56mph at approx 2,600 rpm if you increase your rpm beyond the designed leanest (fuel to rpm)you will use more fuel.
beleive it or not your car will use more fuel per rpm at idle than what it will at 2,500 rpm

Many people complain that they dont get the advertised fuel consumption figures.
I get the exact figures on my Mitshibushi Colt CZ1....if I drive at 56pmh on the motoway I can get 63-65mph as advertised.....if i drive combined driving I can get 53mph as advertised ...I would get similiar correct results from my previous car a Clio.. people forget to add in factors such as lights wipers heater etc add to this outside tempurature and their driving habits and anyone of these things will lower your fuel consumption.

My car has a very good fuel computer which I have tested and is extremly accurate

During this winter when most days it was below -5 my trip to work which will give me 50mph in summer was only giving me 35mph it was particaluary bad this year as it was so cold, my heater was blowing full blast heated rear screen on wipers and lights, add to this a very cold ambient air temperature fuel consumption will drop.
When I want to obtain the highest possible fuel figures you do really have to be so so gentle with acceleration and literally touch the throttle when cruising I mean just touch it. failure to drive like this will result in you never achieving a good fuel consumptiopn figure.
other factors include:
window open as much as 5% more fuel
roof rack 10%
underinflated tyres 10%
and most of all badly maintaned engine

Hope all this helps

jase1

Ooh -- old but interesting thread.

I know that our Mitsubishi 1.6 petrol can manage 55mpg when plodding at 50mph (found this particular nugget out on one of those average speed areas) ... this seems to be this particular car's sweet spot. I remember thinking that was mightily impressive for a fairly old-school engine at the time.

56mph is far too arbitrary a number to have any real meaning. Some very small cars have their sweet spot around 40mph in my experience, whereas some very large-engined cars can be more economical at 70 than at 50 (my ex-boss's 3l Toyota Supra twin-turbo for example). Depends on the car.