'Petrol efficiency experiment; an increase of 20%' blog discussion

beans

charlieheard wrote: »

Trip computer's usually cannot say if you're using no fuel, as they do miles per gallon - using no gallons means the result is either infinity (or a very large number). What they do (in my experience) is "max-out" at some reading like 99.9mpg. I regularly get that reading when slowing for a junction.

Surely the reason for mileage computers '"max-out" at some reading like 99.9 mpg', is that the car is moving without using fuel. All moden cars have a fuel cut-off valve, that cuts off the fuel when the car's speed overruns the engine. Surely, it stands to reason that miles / gallon or kilometers / litre, if the divisor is zero, the mathematical result of distance / fuel is very large - not very surprising!

The presence of the fuel cut-off valve, means that 'coasting' whether in neutral orby depressing the clutch uses more fuel than merely removing one's foot from the accelerator. Coasting [either type] is also dangerous, since the car isn't under full control.

bookwormem

Hi everyone! :j Never posted on here before.

A friend told me that she was taught that a good way of saving fuel is to only use 1st, 3rd, & 5th gear. I'd never heard of this before, and I think it sounds like a lot of nonsense, I don't know how it would help. However, my friend says it works wonders and the people who taught her this method had tried it out a lot. Any thoughts? Anyone tried this?

scape

I tried this experiment in a 1.25 Fiesta on a 196 mile journey to North Wales from London. On the way there I drove very carefully, and aimed for a constant motorway cruising speed of 69 miles per hour (I heard somewhere this was an optimum speed.) On the way back, I drove hard and fast (within speed limits, and still being safe).

It didn't seem to make any difference to fuel consumption at all! I started both legs with a jam packed full tank and at the end of each leg, the fuelometer read 2 thirds of a tank had been used.

Has anyone had similar results?

Is it because I have an induction kit fitted (fancy air filter, apparently it boost performance, but a lot of people say it just makes noise)?

Interesting.

kwmlondon

If you check fuel efficiency information they test long-distance efficiency at 56mph. You'll find that most cars run most efficiently at this speed. I really noticed it in a hire car, a Hyundai. I took it up to 80mph on the motorway and could see the difference it made in the fuel needle dropping as the miles went by.

At 90mph a Rolls Royce will be more efficient than something like a Smart car as it is barely ticking over.

tinbucket

reducing your speed, will reduce your engine speed which will reduce your fuel bill... Nothing too astonishing there. But, how much by? we're busy people, so much to do and always in a rush. Ask yourself the following questions when you're next on the road:

1. are you in the right gear, is the RPM of your engine as low as it could be? remember always - lower engine speed, lower fuel bill (don't labour your engine, by being in too high a gear as this will cause to you to burn fuel)
2. do you need to be going as fast as you are? will 10 mph off a 20 minute journey really save you all that much time?
3. use the gears to accelerate (set yourself a comfortable max RPM by which to change gear (judge this by the speed your engine is doing in top gear at say 70mph). if you never go above this RPM, you'll save.
4. Do you need to be that close to the car in front? do you keep having to touch your brakes? then you're burning fuel. use a bigger gap between you and the car in front to negate the need to brake so often, or letting off your throttle foot only to put it back again once you've slowed a little. The less that right foot moves up and down, the less fuel you'll burn.
5. junctions: I've taken to trailing in non busy times of coasting with the engine in neutral as i come to junctions, then back into gear as i get closer to it. this has me in tick over anything up to 500 yards from a junction... does wonders for your mpg (I stress - not something I do on busy roads)
6. And lastly - what not change the time of day you journey - leave earlier, or later - miss the traffic if you can and the roads, and thus your car flow better. Less stop start means less fuel.

I've improved by about 25 - 30% on my two litre diesel. which for me is about £630 a year saving.

:j

ChrisWaters_2

Since acquiring a 1.9 Turbo Diesel with a fuel consumption gauge, I have been tuning my driving with the result that my fuel usage has gone from 57 MPG to 67 MPG and still rising. Apart from the significant improvement in fuel economy, and better standard of driving I find I am much more relaxed and arrive feeling much less tired.

In my view, it should be compulsory for all new cars to be fitted with fuel consumption gauges.

P.S. I can confirm on with my cosumption gauge, engine breaking use zero fuel.

savememorecashmartin

harryhound wrote: »

A couple of years ago I used to drive an diesel Vectra Estate (company car). In the handbook it said something like "Do not freewheel down hills, this will give you worse MPG, as the engine management system shuts off the fuel input to the engine automatically"

I have a Mk4 Astra and the handbook says the same. Something like - don't disengage the clutch as when the vehicle is on the overrun and in gear as the fuel system cuts the fuel supply resulting in improved economy.

It's pretty good at 51mpg anyway but I'm sure I could get more out of it. Whoever said 'keep it serviced' - good advice, it makes about 50 miles a tank difference to my van after it's had the oil and filters (air, oil and fuel) changed.

Better than my 2 stroke bike - ridden properly I get about 22mpg out of that one (!) that's why it doesn't get much use!

savememorecashmartin

kwmlondon wrote: »

At 90mph a Rolls Royce will be more efficient than something like a Smart car as it is barely ticking over.

Is it just me or does this seem reasonably unlikely?

Rolls quote combined fuel consumption at 18mpg for their whole range of cars - the extra-urban- figure is 24mpg. I can't imagine a Smart ever getting this bad however hard it is caned. Their published extra-urban fuel consumption is between 68 and 70mpg. Ask Martin, he's got one.

funky1471

savememorecashmartin wrote: »

Is it just me or does this seem reasonably unlikely?

Rolls quote combined fuel consumption at 18mpg for their whole range of cars - the extra-urban- figure is 24mpg. I can't imagine a Smart ever getting this bad however hard it is caned. Their published extra-urban fuel consumption is between 68 and 70mpg. Ask Martin, he's got one.

I think the person you quoted exaggerated quite a bit there, but they do still have a point.

My 1.8 Punto HGT (petrol) should return better MPG at say 90mph than if you push a little 1.2 fiesta up to those speeds. A smaller engine will work harder at higher speeds. If we are talking in the 50's (mph) then the smaller engine should usually return a better mpg figure.

I personally don't trust consumption from the onboard computer but mine returns between 25mpg and 40mpg, depending on how much is on the motorway and how much is town driving. Generally I get 30 odd mpg based off the driving I do (according to the car). I can push the car very hard though and this figure won't go below low 30's on motorway journeys.

The computer may not be totally accurate with the figure it gives, but comparisons should be about right, if you use the same car on the same journey with similar amounts of traffic the difference should be quite accurate.

So if it reads 30mpg and then another time 40mpg there is likely a 10mpg difference, just probably not 30 and 40, maybe 25 and 35 for example. Also some cars only calculate it when you are actually moving and dont calculate time spent sitting still in traffic. Mine counts all of the time the engine is on, but the MK2b punto (2003-2006) aparently they changed it to only count whilst you are moving..

Also remember your cars speedo is likely to be over-reading by approximately 10%, maybe a bit lower, so when you think you are going 70 its closer to 63-65.

jaguar231

Interesting experiment. I try and drive economically, I have a round trip commute of about 135 miles per day and I pay for my own fuel, but other road users just dont get it! I tend to drive at about 50 - 55 mph much to the chagrin of other drivers who try and drive up my boot - presumably to pressurise me into going the speeed they want me to go at - and then when they overtake they look at me as if I'm either totally stupid or just an old ditherer who shouldn't be allowed on the roads.

The result my Jag X-type does 56 mpg and my Clio 67 mpg (both diesel). Not bad though eh.