We’ve all seen the advice at different times in the past to clear all unnecessary weight from the car before driving it to save fuel
I remember working in Engineering at Rolls Royce and Bentley,
when we used to walk around the prototype cars looking or opportunities to
reduce weight. It’s common now, but back then in the early 90’s, the guy who
wanted to remove the key lock barrel from passenger doors to lose 50g off the
car’s final weight caused quite a debate!
In engineering projects like that of course, lots of little opportunities
to save a little bit of weight add up and give measurable gains in performance and
fuel economy and the same can be said of motorsport, where they go as far as
designing special, thinner profiled nuts and washers made of different materials
to save weight in the search for a couple of tenths of a second in lap time
improvements – a couple of tenths of a second in F1 terms is a big advantage
Aerodynamics are also vitally important to road car
designers and priorities differ between civilian and race applications with
race cars using downforce created by the aerodynamics for grip, whereas road
cars desire good aerodynamics in the quest to reduce fuel consumption and
increase refinement
But once the car is designed and available to buy, being as
we’re only wanting to use it travel to and from work and take the kids to the
park at weekends, is it really worth worrying about these two things on a day
to day basis? Would you notice a difference in your fuel economy if you emptied
your boot or closed your windows in favour of pressing the air con button?
As is the case with many things, this is all about
considering the percentages and adopting a realistic assessment of what a gain
actually looks like
My Ford Focus weighs around 1,400kg when it comes off the production
line, I weigh around 110kg and a full tank of petrol weighs around 45kg, so
when it drives along the road, the combination of a fully fuelled car plus
driver is 1,555kg
When you add another adult and two children (lets say 220kg)
and give each passenger a bag of luggage (80kg total), the fully loaded vehicle
is around 1,855kg
The difference between the lightly loaded single driver and
the fully loaded family going on holiday for the week is around 16%... but do
you ever notice any difference in fuel economy when you drive familiar journeys
on your own in comparison to being having the full family with you? Neither do
I!
Now, those that tow caravans WILL see a difference, and although
they probably won’t be using a Ford Focus for this job, if they did, with an
average caravan weighing around 1,200kg, that’s adding more than further 65% to
the weight of my own fully loaded car, and 85% to the unloaded one - huge
differences!
To put this into context, an F1 car will probably be less
than 700kg without fuel or driver. If you add 10kg to that, you’re increasing
weight by 1.5% and if you did the same thing with my car as it left the
production line, it’d be 0.7%
So yes, weight does make a difference but to see gains at
the fuel pump, you need to be losing a lot of weight – it’s not worth asking
everyone to empty their pockets before getting into the car!
Aerodynamics are also an important part of car design
whether it’s road or race cars you’re looking at
A car’s job, aerodynamically speaking, is to firstly punch a
hole in the air in front of it and then to let air slip over it easily. Both
things mean that the doesn’t have to work so hard to keep speed and momentum.
In F1, this means that the car can move faster and grip the road better (F1
cars rely on aerodynamic downforce for grip) and on the road it means lower
fuel bills, lower wind noise and a quieter driving experience. In spite of some
models having spoilers and things fitted, road cars don’t utilise aerodynamics
to create downforce at all really
Consider holding your arm out of the open window whilst being
driven…
At speeds that are below 30mph, there is very little feeling
of air pushing your arm, at speeds below 20mph, there is almost nothing, but at
speeds in excess of 30mph, there is an increasing effect on the pressure
applied to your hand and at 70mph, it requires noticeable counterforce from
your arm muscles to hold your hand still
You can see from that experiment that air flow over the car
is very important and anything that disrupts the smooth passage of the air over
the car is adding to your fuel consumption by increasing what’s known as
aerodynamic drag
In wind tunnel testing, things like roof racks and roof top
boxes add significantly to fuel consumption and there is consistent advice to
remove them if you don’t need them, particularly if you regularly drive on
faster roads such as motorways
In conclusion, it’s worth thinking about aerodynamics and
removing anything that adds unnecessary disruption to the air flow over the
vehicle. Wind deflectors stop air from being sucked into the passenger cabin
when windows are opened and do a lot to stop buffeting for passengers but I’ve
been using them on my cars since covid and can’t demonstrate any fuel economy
gains with them
With weight, I really wouldn’t bother worrying about it at
all… whether you do or don’t carry that extra 50kg bag in your car, you won’t probably
be able to see any difference in your fuel consumption whatsoever
Eco Safe driving, as it’s referred to in the driving test, is
a complex subject affected by many, many different factors. Many of the
suggestions involve a lot of effort for very little gain that’s often hard to
see in the performance of the car due to the constant variables of weather,
road and traffic conditions as well as the constantly changing price of fuel
The only way to see whether you’re improving the efficiency
of your driving is to fill up until the pump clicks off, and repeat that
clicking process three times to ensure that it’s genuinely full and not
clicking for some other reason. Then zero your trip computer when you get back
into the car and record your mileage when you repeat the process next time you
need to fill up again
When you know how many miles you’ve done and how many litres
it took to get it back to “three clicks full” again, then you can work out the
mileage per gallon by using the following formula and then taking an average over
a few readings to get a more accurate idea of fuel economy
Number of miles divided by number of litres = miles per
litre
Miles per litre multiplied by 4.546 = miles per gallon
Nick Heath Driver Training helps people of all
levels and abilities with their driving. Established in 2005 and based on the
South Cheshire / North Staffordshire border in the UK, I can help you to obtain
a licence, become a driving instructor, regain your driving confidence or
reduce your road risk for work purposes. I am a regular contributor to local
and national radio when driving items come up in the news and am willing to
contribute to the wider driver training conversation
Anyone wanting more information should call 0800 8
20 2038
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