Driving Economically - 56mph Myth??

LandyAndy

movilogo wrote: »

The most optimum driving speed depends on each engine's power torque characteristics. There can never be a single optimum speed for all cars!

An engine is at its most efficient (ie burning least fuel for the work it is being asked to do) at its torque peak.

This is typically somewhere between 1750 rpm and 2500 rpm for a diesel and 2500 rpm and 4500 rpm for a petrol.

Combine that with typical gearing in top gear 25 - 30 mph per 1000 rpm and you get an idea of what is likely to be the most efficient speed to drive at.

However take my Mazda RX-8 PZ. Peak torque is at 5500 rpm. Gearing in 6th is 20 mph per 1000 rpm. So most efficient speed in 6th gear is 110mph:D

I was just driving efficiently , officer.:j

boyse7en

>So most efficient speed in 6th gear is 110mph

No, the most effiecient use of fuel in a your engine when static (i.e. if it was on a test bench) would be @ 5500RPM - that is it will give the best ratio of power to the amount of fuel required. This does not mean that it is the speed at which it would use the least amount of fuel.

I know it was a tongue-in-cheek comment, but don't get everyone to confuse efficiency with economy.

Plus, there is no consideration of aerodynamic nor rolling resistances, but there has been enough mention of those already...

movilogo

An engine is at its most efficient (ie burning least fuel for the work it is being asked to do) at its torque peak.

I beg to differ. Torque peak is the point where it has highest hauling power, however, you don't always need the highest hauling power!

Why?

Because when a car is run, it has to overcome these resistances = friction, gradient (zero on level ground) and air drag.

The engine produces torque and which manifests as power. Power = Torque x Revolution x [some constant]

Now, as soon as the power produced by engine is more than the power required to overcome friction + air drag, the car will run easily. Usually, this happens at an RPM much less than torque peak RPM (because of the gear) - which is usually quite high [unless it is a diesel or petrol engine with Variable Valve Timing].

If you shift up to higher gears at low speed, the engine will often jerk [since torque is less at low RPM], however, bigger engine you have, sooner you can shift up [because of high torque].

In most engines, lowest fuel consumption tends to occur at 2000-3000 RPM. But this can vary depending on several factors.

The gearbox simply acts as torque multiplier - nothing else, that's why we never need to wait till the redline to change gears. In electric motors, torque is constant at any RPM. So, electric cars don't have gears!

movilogo

And remember, if your tyre pressure is not correct, you'll burn 10-20% more fuel irrespective of how good you drive or not.

LandyAndy

pigeons, pigeons, cat, pigeons, pigeons.

MickKnipfler

thescouselander wrote: »

Obviously you dont frequent any BMW message boards then. BMWs are anything but reliable, just ask someone who has one of the recent diesel powered models whos plastic inlet manifold has disintegrated and destroyed the engine. This has happened on cars that are well under the 100,000 mile mark!

www.bmwlemon.com\dontbmwcare.htm

harveybobbles

Think it depends on the car's engine/gearbox combination.

I had an Audi A4 1.8T which was more economical at 65 than it was at 55...
I replaced it with a BMW 528Auto which was at its best doing 84mph.

It's all to do with gear ratios etc...

MickKnipfler

I drive a motorhome and actually find it'd most economical arounf 70mph as it get's up more hills without changing down

shown73

I'd be interested if someone with one of those whizz-bang fuel computer thingies could run a test on slipstreaming. I used to have a mk3 Zodiac, manual, (remember them?), and travelled about 30 miles on the then new M40 to Oxford, to work. Well, this thing did 18mpg, whether driven briskly or sedately, and I spent ages behind lorries trying to improve this, but it didn't make a blind bit of difference. It's bothered me ever since, because in theory it should have. Maybe we just weren't travelling fast enough to make a difference, I don't know, but with all the modern widgets, if anyone is seriously bored one day, they could have a go for a few miles, just to see if it works. I wouldn't want to spend a long journey like that, but just as a matter of interest......

harveybobbles

shown73 wrote: »

I wouldn't want to spend a long journey like that, but just as a matter of interest......

No, but you may spend the next day touching up all the stone chips on your bonnet thru being behind a lorry for so long lol... :rotfl: