Economic Speed

Legs

ADC
Book Reviewer
#1
Is 56mph really the most economic speed to drive at in regards to fuel economy.

As I understand it this was how fuel economy was measured when Adam was a lad. Surely with improvements in aerodynamics, engine management computers, tyres etc this must have changed, and surely with the differences in car shapes/engines/gears etc this cannot be the same for all cars.

I can comfortably cruise at 75mph in my Mk3 2L TDCI Galaxy and get 48mpg on a long run - not bad for a 2 ton beast... Would trundling along with the truckers be better for my fuel, if not my sanity?
 

jarrod248

LE
Gallery Guru
#2
In my car the economy is better if I drive no faster than 60mph. It's not a huge difference but now fuel is higher it will be more of an issue. The more miles you do etc.
In the Bedford if I drive at 60mph my fuel use increases by about 25%. At 45-50mph I'll get about 16mpg.
Reckon you are going to have to try it over the same journey at both speeds.
 
#3
The AA's top suggestions are:

1. Check your route on the radio, internet, etc before you leave. Avoid a traffic jam and avoid losing a couple of pence for every minute the car is staionary in a queue.

2. Moderate use of air conditioning. Will the air vents keep you comfortable, particularly early morning and the evening? The impact of using air con can pump up fuel consumption by up to 11% on slower roads.

3. Driving around with an empty roof rack adds 10% to fuel consumption, an open window also disrupts air flow adding a further 3-6%.

4. On motorways, keep to the speed limit. Driving at 85 mph instead of 70 mph increases petrol consumption by 20-25%.

5. Don't tailgate. You have to brake harder to avoid hitting the car in front, lose momentum and have to accelerate harder to regain the original speed. A measured speed keeps fuel consumption more constant.
 
#4
56 mph still seems to be about the best speed on average. Try viewing your trip computer instant consumption figures at 56 and 70. Certainly in my car there's a difference.

Incidentally I had a V5 Passat which I maxed out at 140mph to see if it would do the book speed. It did-at 6mpg!!!
 
#5
Ah a boring old farts thread, i'll be right back just getting my slippers.
At 56mph my Volvo V70 2.0D returns 63mpg on the motorway.

at 80 in 6th it returns 45mpg.

and at 100mph I have no idea as im to busy looking out for plod, motorbikes and people not checking their mirrors before pulling out on me.
 

TheIronDuke

ADC
Book Reviewer
#6
Checking ones MPG is for girls. We were put on this earth to suffer, so get on with it. That said, one of the advantages of a 4.2V8 is that it doesnt matter much if you are doing 80 or 60. Theres only a few hundred revs in it. I could check it on the trip computer but playing with buttons on the dashboard is for girls. The Landy runs on red diesel and paraffin so I couldnt give a toss how many MPG it does.
 
#7
Oh Duke, I'm disappointed. Tripmeters are for ponces who fanny about in Discoveries. Try a Series II, where even door locks were an optional extra. Get your fuel from chip shop waste and a bit of white spirit.
 
#8
I'd put forward the suggestion that you can greatly improve fuel economy by getting off the motorway/dual carriageway. On a motorway it's not only embarrassing to drive a car at 56mph, but you've constantly got your foot on the accelerator - struggling to get up the hills and not having enough speed built up to overcome drag and engine braking going down them. On side roads, nearly half the time, your foot isn't on the accelerator, slowing for corners (without braking) and going down hills, so you're using minimum fuel. The rest of the time, you're maintaining 60mph going up hills (with a bit more momentum than 56mph) or just touching the accelerator to overcome drag.

Obviously, your journey will take longer and you'll cover a greater distance but, if your route is planned to take priority at junctions and avoid really steep hills, the amount of fuel needed to get to your destination will be greatly reduced.
 

jarrod248

LE
Gallery Guru
#9
I'd put forward the suggestion that you can greatly improve fuel economy by getting off the motorway/dual carriageway. On a motorway it's not only embarrassing to drive a car at 56mph, but you've constantly got your foot on the accelerator - struggling to get up the hills and not having enough speed built up to overcome drag and engine braking going down them. On side roads, nearly half the time, your foot isn't on the accelerator, slowing for corners (without braking) and going down hills, so you're using minimum fuel. The rest of the time, you're maintaining 60mph going up hills (with a bit more momentum than 56mph) or just touching the accelerator to overcome drag.

Obviously, your journey will take longer and you'll cover a greater distance but, if your route is planned to take priority at junctions and avoid really steep hills, the amount of fuel needed to get to your destination will be greatly reduced.
Cruise control.
 

jarrod248

LE
Gallery Guru
#12
Any evidence to prove automatics consume more fuel. Or is that another urban myth
Yes they do check out manufacturers figures or a car magazine. They are much better than they used to be when most were only three speed, some now are 7 speed.
Then mine won't let me over-rev in each gear it changes up.
 
#13
A quick look in your owners manual will tell you at what revs peak torque is produced by your engine. When in top gear, just drive at that revs and you should get max fuel efficiency....

Automatics of old used some of the engine power to change gear, modern Autos use more electrics to do the same, so should be better on fuel.
 
T

Tremaine

Guest
#14
Yes they do check out manufacturers figures or a car magazine. They are much better than they used to be when most were only three speed, some now are 7 speed.
Then mine won't let me over-rev in each gear it changes up.
Thanks. So I looked it up and found something from the AA. I wonder whether we should also be looking at fuel efficient cars, other fuels, etc. It's a given, that fuel efficient driving techniques work when done properly. And how many of us can afford a new motor anyway. I just feel for anyone who has to drive long distances every day on their own costs, these days.

[ "Manual or Automatic?
For a smooth drive around town, it's tempting to opt for an automatic or a car with continuously variable transmission (CVT). These cars are easier to drive as they have no clutch, and they change gear for you.

The flip side is that they use more fuel. Traditional Automatics may use around 10% more than manuals whilst cars with CVT use around 5% more.

There is however now a growing number of "automated manual" transmissions around. Essentially clever hydraulic and electronic systems take car of the clutch and gear change. These can offer a fuel consumption saving compared with 'normal' autos and manuals.

At motorway speeds fuel consumption evens out and there's not much difference between manuals and automatics. Modern semi-automatic features such as button-operated gear change and automatic clutch control help you use less fuel".] Car Buyers Guide - Advice : greener choices when buying a new car - The AA

Trem
 

jarrod248

LE
Gallery Guru
#15
Then official figures can be misleading but how many people do you know who always drive manuals in the wrong gear? A modern automatic tends to be in the right gear.
 
#16
I'd put forward the suggestion that you can greatly improve fuel economy by getting off the motorway/dual carriageway. .
My experience is exactly the opposite - I've done a regular journey of about 70 miles on both dual carriageway and old "A" road - and I've found much better mpg on the dual carriageway. That's cos I stick to a steady speed the whole way, not keep slowing and stopping for junctions, and then burning fuel accelerating the mass of the car back up to 60 again. Of course, I'm happy to stick with the trucks in the inside lane at 60 and ignore everyone bombing past at 70 - 85 mph in the outside lane. It's a habit I got from driving a Series III Landy, for which 60 was flat out, with a following wind.
 
#17
Automatics do use more fuel, you've got far more transmission losses within the torque converter, particularly when you're accelerating and using the most fuel anyway. Locking clutches improve things once you're up to speed and cruising, but as soon as the conditions change (more torque, change of speed) then you're back the swirling oil about again. The exceptions are the new DSG type boxes where you've got a dual clutch manual gearbox, with the changes being controlled and actuated by electrons....

I've found that to get the best MPG you have to resign yourself to sitting in the nearside lane spending your whole journey reading the back of a lorry! Result: 65-70 mpg from a 1.9 tdi Skoda Fabia.
 
#18
I think the answer to the OP is - it's complicated.

There are so many factors that it is pretty much impossible to say "this is the most economical speed" for everyone.
 
#20
Just diverting the thread slightly, is running a petrol car any cheaper than a diesel when you take in to account the higher cost of diesel and higher mpg over cheaper petrol with a lower mpg?

Discuss

STILTS
 
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