The 56mph Fuel Economy Benchmark.

[st4]

Is yours a manual?

I wonder how much efficiency is lost in the autos as they don't fully lock-up until quite high speeds?

Nah an auto, easier to kick down to get up to speed

Come on Steve, when did you ever drive at 50mph long enough to check your mpg?

When slowing down after a tug from a lady in uniform.

I'll get my coat
http://www.MBClub.co.uk/forums/members/***-albums-my-e-class-picture885-how-beat-combined-cycle.jpg

 

[arryzed]

There is a reason for 56mph

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 at 56mph in top gear your revs will be some where between 2,500 and 3,000 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.

Hope all this helps

 

[Deleted member 11979]

you will notice if you drive your car at 56mph in top gear your revs will be some where between 2,500 and 3,000 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

Our Volvo XC70 does around 1400 rpm at 56 mph, but our C30 is more like 1800 rpm at the same speed, and my Land Rover Series III must be doing about 4000. Have I missed something?

 

[finisterre]

The implicit 'as a broad generalisation it is often the case that...'.

 

[Deleted member 11979]

The implicit 'as a broad generalisation it is often the case that...'.

whatever car you drive at 56 mph the revs will be somewhere between 2500 and 3000?

It's just not true, but I do understand some of the other points he makes.

I think cars are geared to maximise engine torque curves, which is why, generally, turbo diesels rev lower than petrols at the same speed.

 

[arryzed]

sorry i did omit to say that the 2500-3000 rpm is only the case for petrol engines ...diesels are some where near 1700..to 2100

 

[arryzed]

there are some vehicles that do not complie to this mainly older off road vehicles am sure there will be others...i do remeber the early VW golf 1100's were nearly flat out at 70mph they had great economy around town but were terrible on a run..but in general you will find that what ever car you have the optimum fuel consumption will be achieved at 56mph because the car engine managemnet and gearing will be designed to do so...I have owned in excess of 40 cars (all petrol) and not one of them has ever achieved better fuel consumption at 65mph than at 56 , from what i was taught its just not possible

 

[Sp!ke]

My SL is doing about 1700 rpm in top at 56mph. My rather low geared 190E is a little closer but I'd say its about 2k rpm at 56mph. Both Petrol engined cars.

I understand your overall point but I think your generalisation regarding gear ratio's is well off.

 

[arryzed]

Thanks Spike
I get what you say it is always hard when making generlisations...lets just say a many cars

 

[john925]

It is all about engine revs as far as my memory serves me. 2500 rpm is supposed to be the most fuel efficient speed for an engine. So it depends on the individual vehicle to some extent. I had a truck driver mate driving a 16 speed splitter geabox and his rev counter never varied from the economy band other than stopping or starting He would change gear on the slightest incline to keep the revs spot on. But he got maximum fuel economy. Boring driving though LOL

 

[Dieselman]

56mph is 90kmh so that was the figure used for A road travel.

So we had figures for;

Urban
A roads
Motorways.

 

[DITTRICH]

Dieselman is correct. 90Kph / 1.606Km = 56.03Mph.
Older mercedes manuals from the 90's had inserts where consumption at a steady 90Kph and 120Kph were shown.
It's like quoting 0-62mph = 0-100kph acceleration times.
Also the same Dieselman answered this question some while back in another thread along the lines of "go as slowly as you can in top gear with just enough revs to stay in the rpm band where torque is high". Or words to that effect. I don't think 56Mph is the most efficient speed but it is not that far off the mark for alot of cars. The c230k is in top gear from around 35-40mph. I obtained up to 42mpg in eastern europe going through the baltic countries at very sedate speeds without aircon. Also in the e300td I managed 45mph to wales once at just over around 50mph on the motorway. That is more than the manual quoted figure of around 42mpg at a steady 90kph. So I think that 56mph isn't the same figure for all cars nor is likely to be the exact figure for any car.
My 2c
Les

 

[Mactech]

56mph is 90kmh so that was the figure used for A road travel.

So we had figures for;

Urban
A roads
Motorways.

Quite right! I've no idea why anyone would think that 56mph is the most economical figure for any car!
The biggest factor in steady state running is the fact that the aero drag is proportional the square of the speed The other bid factor is the dynamic drag, which is close (but not exactly) proportional to the speed. So you can see that slower is better.....provided that the car is in the highest gear and the engine working in an efficient band.
So you can see it will be different for each car.....but slower is good!

 

[st4]

Its nearer 45mph in most cars. Most will engage 5th by then without labouring. In mine its nearer 56 to be fair as its in 7th doing 1400rpm so not labouring and not too much drag. Although 50mph in 6th at 1400rpm and it does the same mpg

 

[arryzed]

lol I think I released some new life into an old thred...Mactech your correct 56mph is not most fuel efficient point actually its somewhere just below 40mph, but we dont all want to drive around at 40mph, diesel engines are very differant than petrol engines in performace, fuel usage and power levels. and will consume less fuel the slower you drive them with low revs 100% guaranteed.

Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h) requires about one-third the power to overcome wind resistance, or about one-half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy per unit distance by 1.9 times, and decreases fuel economy.

[FONT=&quot]The graph ramp (energy/fuel to speed) starts to climb at 40mph and rise's a little more to approx 56mph, [/FONT]but [FONT=&quot]travelling at speeds beyond approx 56mph the graph raises dramaticaly the power needed to propel ANY vehicle forwards increase (irrespective of engine or aerodynamics, although aerodynamics helps of corse it does, but you can not get away from the laws of physics) some vehicles mainly diesel's but also some petrol engines can be more econmical at 40mph even 25mph, but on the whole most manufacutures of PETROL engines design fuel efficieny at approx 56mph for a very good reason[/FONT].....I should of mentioned that diesels are differant in that they can have a greatly improved fuel efficienty at slower speeds than petrol engines, petrol engines can become in many case's worse at slower speeds, but ALL vehicles as stated above Diesel or petrol will use more fuel at 65mph compared to 56 mph ....because of the laws of physics

 

[ringway]

lol I think I released some new life into an old thred...Mactech your correct 56mph is not most fuel efficient point actually its somewhere just below 40mph, but we dont all want to drive around at 40mph, diesel engines are very differant than petrol engines in performace, fuel usage and power levels. and will consume less fuel the slower you drive them with low revs 100% guaranteed.

Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h) requires about one-third the power to overcome wind resistance, or about one-half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy per unit distance by 1.9 times, and decreases fuel economy.

[FONT=&quot]The graph ramp (energy/fuel to speed) starts to climb at 40mph and rise's a little more to approx 56mph, [/FONT]but [FONT=&quot]travelling at speeds beyond approx 56mph the graph raises dramaticaly the power needed to propel ANY vehicle forwards increase (irrespective of engine or aerodynamics, although aerodynamics helps of corse it does, but you can not get away from the laws of physics) some vehicles mainly diesel's but also some petrol engines can be more econmical at 40mph even 25mph, but on the whole most manufacutures of PETROL engines design fuel efficieny at approx 56mph for a very good reason[/FONT].....I should of mentioned that diesels are differant in that they can have a greatly improved fuel efficienty at slower speeds than petrol engines, petrol engines can become in many case's worse at slower speeds, but ALL vehicles as stated above Diesel or petrol will use more fuel at 65mph compared to 56 mph ....because of the laws of physics

Thanks Arry.

I'd be interested on your take on this thread. LINK. Although there's a fair amount to read through.


Well, it is thread revival week so..

 

[arryzed]

Thanks Ringway...its not an easy subject but I remeber having all these descusions and arguments back in the 70's I did my traing with Ford and attend many engine tuning courses with Crypton and Sun, the laws of physics are the same now as they were then...unless they have changed....... spooky man!

am now going spend my time reading your interesting link...thanks