[Drivingforfun]

I mentioned this on another forum in a response to someone but thought I’d put the question here…

Do others notice the economy of “economical” cars scales much more strongly with driving style; i.e., how economically you drive has more impact on the real mpg in a car considered to be an economical car??

On a sedate ~60mph run my mum can get 75mpg in her 1.0L Toyota Aygo, she recently rushed my brother to the airport in it, not driving like a lunatic, just 75-80mph, and the economy went to ~45 ... so 40% less

On the motorway I tend to potter at 65mph in my 2.0L Mini, I get about 43mpg. I found myself having to get to hospital recently, again not rally driving, just a smooth 75-80 on the motorway, and the economy for the journey was 35 so I lost less than 20%

I have lots of examples but that’s just an idea… maybe I’m just asking the obvious. The Top Gear episode comes to mind where the Stig drives a Prius as fast as he can and Clarkson has to just keep up in a BMW M3; and the BMW gets better economy as it’s not working flat out

[olduser]

The simple answer is drag (the force needed to push that particular vehicle through the air) is proportional to speed squared - double the speed and drag increases by a factor of four.

Added to that effect is the efficiency of the engine over its RPM range.
We get a petrol engine to run slower by reducing it's efficiency, by reduce the amount of air entering the engine by closing the throttle, and reducing the amount of fuel to maintain the correct fuel air ratio.
This has the effect of reducing the effective compression ratio* (the cylinders don't fill with air), and the engine, when throttled has to work harder sucking air past the throttle, both of these effects reduce the engines ability to turn fuel into useful power output.
Opening the throttle increases the engines efficiency but as the RPM increase we run into problems with getting the air in in the time available, and completing combustion in the time available, causing the efficiency, and power to fall again.

* Compression ratio, is the volume of the cylinder at the bottom of a stroke compared to the volume when at the top of the stroke (the combustion space or combustion chamber).
Effective compression ratio, is what actually happens because the cylinder doe's not fill with air on the suction stroke, (the air in the cylinder is below atmospheric pressure) therefore on the compression stroke the pressure is not at the maximum.
The higher the effective compression ratio the greater the efficiency.
Of course there are limits to the compression ratio, as it gets higher it becomes increasingly difficult to prevent combustion turning into an explosion.

[olduser]

Economical driving, is primarily about conservation of momentum in an effort to reduce the effects of inertia.

To do this acceleration and deacceleration becomes long and slow, changes of speed are to be avoided, and running in the highest gear possible, all of which involves thinking ahead.
In short, drive very very smoothly.

Not carrying unnecessary weight reduces inertia, and keeping windows closed, and removing empty roof racks reduce drag.

Running the tyres at a higher pressure will reduce rolling resistance but it is unwise to go above the maximum pressure (= vehicle fully loaded) in the vehicle's handbook.
NB. Doing this will reduce grip, you are reducing the tyres footprint, beyond the recommended maximum the centre of the tread will be doing the greater majority of the work and therefore wears faster than the rest.

[Rolebama]

A friend used to work for a manufacturer as a Tech & Public Assist Manager. One of the duties he took upon himself was to meet up with people who were not happy with results from dealers. On one occasion he met up with a guy who had taken his car to the dealer six times complaining of poor consumption. So he met the guy at the dealers, and asked him to take him for a twenty mile drive on mixed roads. At the end, it was calculated the car had done around 25mpg. He also timed the journey. On returning to the dealers, my friend took over and drove the same route and achieved 41mpg, and it took him 2 minutes longer. He then followed the guy to the nearest petrol station and paid for the petrol to fill the tank. Then he told the guy he never wanted to hear from him again, and maybe he should learn to drive.

[Drivingforfun]

Thanks to both for the info and interesting posts

The main point I was getting at was that certain cars seem to "reward" you more for driving more economically. So, driving the Aygo and the JCW "economically" vs. "enthusiastically", the Aygo "punishes" you more for not being gentle

I suppose that'd be an overly black-and-white conclusion, and perhaps you have to take into account what the car is designed for? The Aygo is supposed to be driven economically, that's why people would buy a small 70bhp car!

Looking at it in reverse, I've actually noticed very little encouragement to driving the Aygo enthusiastically in the first place - a couple of times I put my foot down and there's very little difference in acceleration between a gentle and hard squeeze on the throttle.

Likewise I imagine lots of German saloons are tuned to be economical (relatively speaking) at autobahn speeds, but maybe not in the city

[olduser]

Generally, small cars are designed to be marketed as city cars, not economical cars, city driving is not conducive to saving fuel.

In a city, a car will spend a high proportion of it's time in stop start action, a small engine weighs less (less inertia), uses less fuel when idling, and has lower torque (poor acceleration).
The latter can be compensated for in the final drive ratio but today cars are expected to be able to run on motorways, so the final drive ratio cannot be overdone in favour of good acceleration else the engine will be overrevving at high speed.

The car needs to be compact for easy parking but getting a smooth low drag body, with good all round vision, is difficult with a compact body.
As the car is expected to spend most of it's time at lower speeds, drag is not considered as important.

All these trade-offs' result in a car that will have a big (relative) difference in fuel consumption between low speed and high speed.