The most fuel-efficient motorway cruising speed for MINIs

[hyperbluejames]

Hi all!

I was wondering what the most fuel-efficient motorway cruising speed would be for each R 53 MINI (esp Coopers as that's what I've got ). Apparently in the 70s the most efficient average speed for cars at the time was 56mph. Obviously it changes with the size of each engine and what revs it does at what speed - I see this as the greatest varying factor amongst unmodified standard MINIs.

So...any of you mathematicians or people in the know, tell everyone else what it is!

James

[British SD]

I'm no mathmatician but fuel efficient wise, the slowest speed in the highest gear without lugging the engine. about 48-50mph! but that's a dangerous slow speed on motorways because it forces others to make unneccesary lane changes.

[CC07ERS]

i commute to work in my 06 cooper & use the motorway for the travel itsa bout 60 miles i do 55/60 & get about 42 ish on the trip computer

[jez_MCSc]

At 70-75 over 100 miles I have often got over 40 MPG calculated from tank fill up rather than trip computer. Falls rapidly on the B roads though, down to 20 last night!

[Rakey]

surely if you are talking about a fuel effiecent journey then its more to do with finding the sweet spot in the revs and keeping it there instead of trying to keep to a set speed?

As if you are trying to keep to a speed the fuel consumption will increase up hill if you are trying to keep to that speed.

This is also why cruise control is more fuel hungry than regular driving as the car is always adjusting the revs to maintain a set speed, this uses more fuel.

[Fin]

Hmm...

It will be a trade off between two main factors.

1. Drag is proportional to the velocity squared (in general, there are other factors that affect drag). Basically the faster you go the greater the component of drag. The greater the drag the harder your engine has to work to maintain that velocity. Really you would have to plot drag vs velocity for a MINI (taking into account drag coefficent, mass etc).
2. Next, you have to consider the most appropriate gear to travel at (or very near) the speed calculated from point 1. Whichever gear places you well on your engine's torque curve for that speed.

However I am too hungover to consider all that. I would say drive at 70 and whack it in 5th.

[Lawrothegreat]

I once heard someone say the most efficient engine speed is where the power and torque curves cross. I understand that power at a particular engine speed is a function of the engine speed, and the torque at that engine speed, but if this is true I don't know what units you would use - either both imperial or metric? Once you have the most efficient engine speed, you would need to drive in perfect road conditions (e.g. no wind speed) at that engine speed say in 4th, 5th, and 6th gear to see in which gear you get the highest fuel consumption. I wouldn't automatically assume the highest gear (even if the engine is not labouring at that speed) because some sixth gears can be extremely long, e.g. the efficient engine speed may translate to a high road speed, and therefore any efficient gain as fin states above would be overcome by the drag factor. But I'm too hungover to really know either.

[Fin]

The hangover is wearing off a little now. That pesky vodka, will I ever learn?

Power is always a function of engine speed, and torque. They are all related by the equation:

Power = (Torque x Engine RPM)/5252

You don't actually measure the power of an engine. When you dyno/RR test a car you actually measure the torque generated by the engine. The power is then calculated using the above equation. With regard to units, if you are measuring Power in horsepower, torque is measured in pound-feet. If you want to use Newton Meters (Nm) for torque, your Power Units will be in kilowatts (kW)

Horsepower comes from James Watt, the dude who did all the work with steam engines. In order to advertise how good his steam engine(s) were he had to come up with a way of comparing them to something that was already used. Down the mines they used to use horses to lug the coal up to the surface. So Watt went to study some horses and discovered that the average horse could lift a 550lb weight 1 foot in 1 second. That works out to be 33,000 pound-feet per minute for a rough 8 hour shift. He then stated that 33,000 pound-feet per minute of work was 1 horsepower. He could then advertise his steam engines as being 'x' amount of horsepower to show mine owners how much better his steam engines could be for getting coal to the surface.

Since then horsepower has been used to advertise the rate of work of a particular engine. Note that brake horsepower just highlights that the measurements were taken with a brake applied rather than a weight for the measurements. Nowadays as with most things, we tend to state power in horsepower and torque in Newton Meters. Which is incorrect. But hey, nobody cares.

When your engine is sat on the spot of the torque curve where maximum torque is being generated, it is generally not near the maximum power. Therefore your engine is doing maximum work, at a low power (power is rate of work done). This is good for your fuel consumption as to work at a higher rate you have to burn more petrol. This spot is good for if you wanted to accelerate rapidly. Even as your torque curve begins to drop slightly as you increase the RPM, you can see from the above equation that the horsepower will increase as a function of RPM. Therefore, although your engine is generating slightly less than maximum torque, it is generating that torque at a much higher rate.

You will find that 5th/6th gear will most likely have chosen gear ratios to postition the engine speed at an RPM that is near maximum torque at around 60-80, so you can cruise most efficiently. Well, I would have thought.

Right, back to my ham I'm roasting...

[Lawrothegreat]

Great stuff - I've always wanted to know what the difference was between 'hp' and 'bhp'.

[hyperbluejames]

Some very good stuff! Very 'physic-al', but very good. I'm starting to get a good picture of how all the intricacies work!

James

PS any other details welcome!

[Richard Crofts]

In the S, I can set the cruise for 10mp higher than i should and the car will do 75 miles on a quarter tank marker with ease. In theory this would mean 300 miles from the tank which is 27 and a bit miles to a gallon. Not bad in an S IMHO

[Jamesr022]

Quote: Originally Posted by Richard Crofts (original)

In the S, I can set the cruise for 10mp higher than i should and the car will do 75 miles on a quarter tank marker with ease. In theory this would mean 300 miles from the tank which is 27 and a bit miles to a gallon. Not bad in an S IMHO

SO how many miles you actually get from a full tank then?

[Curry23]

On a recent trip to London a set my cruise control at 70 and i was in 6th gear and my OBC was giving out a reading of 44mpg and thats in my MCS

[stuvoi]

I have had computer say I can do 405 miles on full tank (in R56 MCS), normally manage about 250 miles on the tank though, worst has been 150 miles <erk>

[Jamesr022]

the trip computer has said that my S will do 420 miles on a full tank, but the closest i have got is about 320 the lowest i have had is 250! best motorway cruising speed i found for my S is between 70 and 80 and the best 'A' road speed is about 50.