speed that = cheapest driving

indebtbigtime

Hi

I know this is complicated by numerous factors, but in general whats the most economical speed to drive

if I drive at 90mph on a motorway (not that I would ), i get there much quicker than driving at 60 and consume fuel for less time, but I am guessing this is offset by extra engine effort?

I drive a small MPV

thanks

AceCobra

No Doubt every car is different but my MG ZR 1.4's most econominal driving speed is 56mph which is about 2500 rpm. As a rule of thumb I think I beleive quicker = extra cost in the highest gear.

Martin.

Cardew

As you say there are many factors to be considered, but without doubt driving at 60mph will give you far better economy than 90mph; and driving at 40mph will give you better economy than 60mph.

tomstickland

The power needed to ovecome aerodynamic drag rises with speed cubed.
ie: doubling speed requires 8 times as much power.

Because the time spent travelling at that power is inversley proportional to speed, then the mpg is proportional to speed squared.

So doubling speed requires four times as much fuel.

So slow is most efficient.
However, this doesn't work all the way down to 1mph, because the engine has an efficiency band based on rpm and load. Essentially, if you were travelling at 1mph, the engine is burning fuel just to keep running, and most of the output is being wasted.

For most situations, somewhere between 50 and 60 mph is an efficient speed.

indebtbigtime

thanks

thought the answer would be in the 50's

On motorways I couldnt drive that slow but reckon 65ish is doable.

i spend alot of time on country roads / a roads which are 60mph legal limit and dual carraigeways that are the same.....could easily slow down to 50-53 ish without looking too outta place!

every penny counts!

moonrakerz

tomstickland wrote: »

The power needed to ovecome aerodynamic drag rises with speed cubed.
ie: doubling speed requires 8 times as much power.

An Inter City 125 train with both power cars working does 125mph, with only one power car working it will do 100mph.
QED

Cardew

moonrakerz wrote: »

An Inter City 125 train with both power cars working does 125mph, with only one power car working it will do 100mph.
QED

Not certain of your point here moonrakerz:

A one power car will do 100mph, it will take 8 times as much power to get the intercity train to do 200mph.

So even 8 power cars will not achieve 200mph, given the extra weight of the 7 cars.

So doubling speed requires four times as much fuel.

Whilst it is accepted that it needs 8 times the power to double the speed, I have never heard of the above.

Certainly my car does approx 60mpg at 30mph, but at 60mph it gets about 40mpg - not the 15mpg that the above quote would suggest.

tomstickland

Yes, I'm quite intrigued now. Certainly the power requirement scales with speed cubed, and the time spent travelling scales with 1/speed, so you'd expect the total energy used to scale with speed squared.
I've just looked at some typical numbers and it's not as steep as a sqaure law though - more like 1.5. I'm wondering why that is so, and it might well be because at a steady cruise of, say, 30mph, the enging is running at very light throttle and is way off the most efficient operating load. As the steady speed increases then the engine gets used closer to an efficient loading point.

Greensprout

http://forums.moneysavingexpert.com/showthread.html?t=962317
I always thought it was 55mph hence the speed limit for lorries is 55mph (ish) :rolleyes:

Greensprout

Just found this to back up my claim http://ask.yahoo.com/20050826.html

Cardew

Greensprout wrote: »

Just found this to back up my claim http://ask.yahoo.com/20050826.html

That link misquotes their source.

55mph is certainly not the most economical speed for any car.

People are getting mixed up between 'most efficient' and 'most economical'

An engine is normally at its most efficient at peak torque which normally is around 55mph in top gear.

However reducing the speed to 40mph will give you better economy.

There are plenty of charts in car handbooks that show that 30 mph is more economical than 40 mph and 40 more than 50 etc.

You can even show this with the computers on many modern cars - drive at 30mph and compare instant fuel consumption with 40mph etc