Driving Economically - 56mph Myth??

jdm01

I remember reading about a recommendation that driving at 56mph will give the max performance efficiency in terms of driving a motor car.

It was suggested that this applied to all cars no matter what the size of engine.

I am of the opinion that this is an urban myth, can someone prove me wrong? This figure has stuck in my head and I always feel myself trying to get close to the 56mph on motorways (albeit in a 3L diesel engine & I still believe it is a false figure).

If it is a myth, has anyone got any way of calculating the max fuel efficiency from a car.

J

Lorian

jdm01 wrote:

I always feel myself trying to get close to the 56mph on motorways (albeit in a 3L diesel engine & I still believe it is a false figure).

Oh, that's you eh?

I'm suprised a 3 Litre car doesn't have a fuel computer to tell you the mpg.

balsingh

I suppose it is sort of a myth .... i always thought the best economy was acheived when cruising at the lowest possible comfortable speed in the highest possible gear. By comfortable, I mean not so low that it almost demands chaging down a gear but a speed that its well settled at lowish-rpms. For most cars, this is around 50mph so I think 56mph was used to cover the fact that some cars have 3 cylinder 1 litre engines and others have 3 litre diesels with 6 speed boxes.

lellie

56mph is also 90 kmph - which is a common speed limit abroad.

AMO

It's a myth - sort of. It's different in each car depending on the number of gears and rpms (diesel/petrol).

However, I believe that even though most cars might deviate from 56mph, its never going to be too far off, as once you go into the higher speeds you have to calculate adverse resistance from driving into the wind etc.

AMO

jellycat40

one of the questions for my daughters practice theory test was "what is the percentage of fuel saved at 50mph as opposed to 70mph"? the answer was 70% so the DVLA must have some reason for putting this in.

The slower you cruise at the better economy you get providing you are in the highest gear you can be in for the speed.

Louise

philhuff

Similar to all of the above, the very rough ideal speed will be around 35mph in as high a gear as the car manage comforably in.

Beyond that, wind resistance builds and you spend a lot more energy cutting through the air. Resistance increases at a rate square to the speed, so doubling your speed means you'll need four times the energy to get through the air.

The above is very generalised. Please don't pick too many holes in it, or I'll be forced to come back and bore you all to death

Idiophreak

jdm01 wrote:

This figure has stuck in my head and I always feel myself trying to get close to the 56mph on motorways (albeit in a 3L diesel engine & I still believe it is a false figure).

Always wondered who these people are in big cars holding me up on the way to work every day - in a way it's nice to finally meet one.

As a hint, if you're so concerned about economy, just get a smaller engine and GET A MOVE ON!

The_Biff

This (56mph) was the national speed limit in the USA in the 1970's when the petrol crisis was on. This was an average mpg for all US cars.

abarthman

The_Biff wrote:

This (56mph) was the national speed limit in the USA in the 1970's when the petrol crisis was on. This was an average mpg for all US cars.

The US speed limit was 55mph. I don't understand the second sentence.

I think lellie is correct.

ollyk

The_Biff wrote:

This (56mph) was the national speed limit in the USA in the 1970's when the petrol crisis was on. This was an average mpg for all US cars.

A US car that does 56mpg? Pull the other one :rotfl: