Fuel Consumption

Nodding_Donkey

The thing is that with aircon on you probably have the windows shut, turn it off and open the windows and you create more drag so the difference in mpg will be negligible.

PeacefulWaters

PenguinJim wrote: »

That's interesting! Would you mind sharing the make/models/fuels and years of the cars? (I expect you might, so don't worry if you don't want to!)

I wonder if they are new cars..? I seemed to notice a difference in my old '96 Astra a loooong time ago. But I rarely used it, because the windows worked. I've always been operating under the assumption that car A/C eats petrol, and as a "good" MSE, I'd simply not bothered with turning it on (and it's been hitting 40C here by 9am some mornings!). Perhaps some experimentation of my own is in order...

2007 Ford Focus 1.8 petrol engine and 2011 BMW 3 series with a 2 litre diesel engine.

No obvious flaws in my method, although I probably didn't have windows open (can't remember).

But then that would suggest the rule should be "don't open windows" rather than don't use ac.

motorguy

MEM62 wrote: »

You will NEVER reach the MPG figure quoted by the manufacturer. They are obtained under optimum conditions and are allow to 'prepare' the cars for these tests. (They even tape up vents & door seams and remove windscreen wipers to reduce drag, over inflate tyres for less rolling resistance and even remove things like the battery to make the car a light as possible)

Short journeys of the type you describe are no good for decent MPG. They are probably not good for the car in general either. Once every couple of weeks or so try to give the car a decent run out.

My 2012 Golf 1.6TDI 5 Door S has an official MPG of 62.8.

Over the last 22,422 miles i've averaged 65.51 using the brim to brim method on each fill. Best so far has been 69.82 mpg.



I'm probably seeing around 5% inaccuracy on the trip computer above that - so my trip computer would have been reading around 73mpg over the course of that best run.

Thats with predominantly long steady commuting runs and i've turned driving economically into close to an art form.

motorguy

PeacefulWaters wrote: »

The air conditioning thing is interesting.

I buy into the theory that it should reduce mpg.

But I've had two cars with trip computers and tested both. A week of identical commutes with ac on and the same with ac off.

Identical fuel consumption whether on or off.

It will also depend on the time of year. Air con works hardest when it has to cool the cabin down to below an outside temperature and keep it at that, so you'll see the biggest draw on economy at the peak of summer.

Spring and autumn when its merely keeping the cabin around external temperature then its doing very little.

In winter it can use heat from the cars cooling system to warm the cabin.

Also, it will depend on engine size. A larger engined car will have very little overall draw from the air con system to power it as a percentage. With a small car it would be much higher.

I had a 1.25 Fiesta with aircon some years ago and you could actually feel the difference in performance with the aircon on - i think the car only had about 59bhp to play with in the first instance.

PenguinJim

Ah, I'm rocking a 1.3 Micra at the moment. I think I'll just keep opening the windows. Thanks!

LandyAndy

MEM62 wrote: »

You will NEVER reach the MPG figure quoted by the manufacturer. They are obtained under optimum conditions and are allow to 'prepare' the cars for these tests. (They even tape up vents & door seams and remove windscreen wipers to reduce drag, over inflate tyres for less rolling resistance and even remove things like the battery to make the car a light as possible)

Short journeys of the type you describe are no good for decent MPG. They are probably not good for the car in general either. Once every couple of weeks or so try to give the car a decent run out.

Do they? As the tests are supposed to standard and repeatable across all manufacturers I imagined they were done on a rolling road so air resistance would be irrelevant. Of course the lack of air resistance would itself improve the consumption figures.


I'm also surprised the cars aren't tested in showroom condition.

laidbackgjr

PenguinJim wrote: »

Ah, I'm rocking a 1.3 Micra at the moment. I think I'll just keep opening the windows. Thanks!

Opening windows and the extra drag that creates will use far more fuel than AC!

LandyAndy

laidbackgjr wrote: »

Opening windows and the extra drag that creates will use far more fuel than AC!

I believe that depends on the speed.

PenguinJim

"In general, follow this as a rule of thumb: if you are driving in city and at speeds below 70km/h, roll down the windows and let in the natural air. That’s because at slower speeds, the engine develops less power and hence it has to work harder to generate enough power to run the AC and other components that rely on power from the motor. Besides, when you’re driving slowly, the drag resistance is not strong enough to cause any significant dent to fuel efficiency. At higher speeds, as on highways, turning on the AC would be a better option as the engine would anyway produce enough power to meet the requirements of the AC. Also, the amount of drag on the car will be substantially higher."

Most of my journey is at 70. Now I don't know what to do!

It's a tiny car, though - apparently drag is more of an issue with larger cars like Stupid Utility Vehicles.

LandyAndy

And for added complication, aircon does not like being left switched off for extended periods so opening the windows instead may land you with a substantial repair bill