Cheapest Petrol & Diesel Discussion Area

JDPower

I-LOV-MONEY wrote: »

My tax disc is due to be renewed at the end of the month. With the renewal form they enclosed a leaflet about fuel saving tips. One of these was 'don't coast downhill or towards lights in neutral as this uses more fuel' Is this correct? I would have thought being in gear means you are pushing the engine and therefore using the fuel. Coasting the engine is not engaged and presumably using less fuel.

Can someone please confirm this one way or other.

Thanks.

Apparently running in neutral means fuel has to be used to keep the engine running, whereas if you are in gear the momentum of the wheels means less is required. However it is relative - if you have a half mile long hill that you can coast down in neutral that is surely gonna use less fuel than leaving it in gear which will cause the car to slow down quicker than coasting.

John_Pierpoint

On a modern engine, while the car is pushing the engine rather than vice versa, the car will not be using fuel.
So in theory there is no need to coast, unless you turn the engine off and de-clutch, but that is potentially dangerous.
[cannot accelerate out of trouble , loss of power steering and brakes.]

xposis

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The_Selfish_Gene

All

I can drive to work 28 miles all motorway or 18 miles A roads. Do you think I get a better mpg doing 55 mph motorway driving or using the A roads (more stopping / starting) but 10 miles less.

M

John_Pierpoint

Try it and see - fill the tank full to the same point (ie where you can see the fuel) as you set off and the next day do it again. By the third day you should know if there is a significant difference.
There are so many variables, including the number of stops (ie gear changes) and the type of car you drive.

Personally if there is little in it, I would use the motorway: if I could resist the temptation to speed up, as the risk of accidents is lower and and the pollution imposed on fellow citizens is less.

sabretoothtigger

A constant 55mph is the highest mpg a car engine can return. Any higher and you get air resistance, any lower and you dont cover as many miles for the same wheel resistance/fuel used

That is not an excuse to pull onto the motorway so slow it causes an accident :laugh:

Any stop start is usually sub 10 mpg, quickly brings an average down even if the journey were shorter. A few junctions is ok but any traffic waiting or lights will kill the saving

cpochin

If you really think that the temperature of fuel drops at night, you need a quick physics course. A large volume of liquid kept in an underground tank will take far longer than 12 hours to change temperature significantly, and, as the comment corectly states, even if it did so, the change in density is miniscule.

[Deleted User]

I just pop 30 quid in and drive until the needle is at the bottom again. Life is too short for chasing that last 1 percent of a gallon these days.

steve-L

sabretoothtigger wrote: »

A constant 55mph is the highest mpg a car engine can return. Any higher and you get air resistance, any lower and you dont cover as many miles for the same wheel resistance/fuel used

That is not an excuse to pull onto the motorway so slow it causes an accident :laugh:

Any stop start is usually sub 10 mpg, quickly brings an average down even if the journey were shorter. A few junctions is ok but any traffic waiting or lights will kill the saving

Obviously depends on a few things like the CD of the car....
However the main reason is because that is what is tested for euro-tests .... (I think 56 mph) and US tests (I think mainly 55mph) on a FLAT track.

After a remap my 330D seems to get best MPG* in the high 60's (take foot off almost completely in 6th).

This is quite subjective and a mix of the overall and instantaneous....
Situation: Left Poole for Hampton (90 miles) and Kiddy falls asleep
Warning comes up I have 70 miles fuel left.
Logic, lets get a ways and hopefully not wake kid up .....

So I put my mind to driving as economically as possible....
My instantaneous only shows up to 50mpg..... so I used that and the overall trip mpg and some mental arithmetic on the way.

I noticed that allowing it to go to 80-85 downhill then letting it slow to 65 or even 60 on the uphill required little or no pressure on the gas.

Amazingly I got 68 mpg out of a 3L diesel..... (don't remember wind direction)....I think my Average speed was something like 68 mph....(not sure but I seem to remember the mph was by chance the same as mpg or very close)

Obviously, this is with a remapped car..... I wouldn't be surprised if the factory car did 56mph noticeably better?

John_Pierpoint

The AA did this with a big Honda and a Small Ford.
At 55 mph the Ford was more economical but at 80 MPH they were both about the same.