MSE's 2011 Petrol/Diesel Cost Diet Challenge: Put your money where your pedal is!

JDPower

frugalistic wrote: »

hi can anyone tell me is it worth my while knocking my car into neutral -- though of course it is illegal and i would never do such a thing -- in order to save diesel? i have a bmw 530 and it is kinda costing me a bit these days... someone told me its engine cuts out the useage anyway if ur going downhill ...?

Coasting in neutral actually uses more fuel than coasting in gear. If you want a longer answer:

http://www.popularmechanics.com/cars/alternative-fuel/news/coasting-in-neutral-fuel-economy

http://lmgtfy.com/?q=does+coasting+save+fuel

steve-L

Hmmm....
Obviously MPG is the thing we can report BUT for MSE it shouldn't be the be all and end all. It is after all MONEY we are saving and the planet is a nice byproduct... (Or we would walk/cycle) despite the hardships.

The reasons this might be important is:
1) I can save MPG by doing unpaid overtime.....
If I drive in at 07:30 and home at 20:00 I can get far better MPG...

I don't get overtime but I do have some value attached to spending time with my family ... its also only 15-20 miles so easily walkable.. I used to walk 10 miles a day to school and my father used to walk 15 miles each way every day for work at one point. (Ahem back in the 60's and 70's) of course as a kid I saw little of him!
As it happens my work is often flexible... so I can often compromise and start at 07:30 (not today) and finish at 16:30.... or start at 10:00 and finish at 19:30.
I can also sometimes work from home.....

ALL of these give me the biggest savings on commuting!

If you can change these then you can save more IMHO than 'emptying the boot'.

2) As part of this I get different routes are most economic at different times. Its important to remember that doing 40miles at 55 MPG isn't cheaper than 20 miles at 30MPG (even though it might get me there quicker) AND is devaluing the car.

3) Wear and Tear .....
HOWEVER.... 40 miles on the motorway will cause less damage to your car and require less spent on the car .... especially to be avoided are speed-bumps.... there is simply no MSE friendly way to do them.... you either come to a near stop and waste fuel and incrementally wear your clutch or you incrementally damage the suspension. As a one off this is minor and almost not worth bothering about however do it 200 days/yr for 3 years and it is no longer trivial.
Even the heavy traffic but much shorter route will cause MORE clutch wear.... and clutches are expensive to have replaced, especially on FWD cars.... (the clutch isn't expensive, the labour is)
Same goes with roads likely to give a puncture etc.

4) You might get better MPG from some branded fuel ... or worse MPG from some supermarket fuel (esp. if it has a high bio diesel content) ... how much depends on YOUR engine... both the make/model and to some extent YOUR engines history....
However ... you need to find out if you do a lot of miles....
You might find that the Tesco super unleaded actually gives you more MPG ....
What you need to find out is pennies/mile in YOUR car on YOUR commute.

5) Don't drive your diesel to from the corner shop...... avoid any trips less than a mile by walking.... you get the worse MPG, do the most wear to your car and miss out on FREE exersize.... just make this a lifestyle change if you can!

6) REALLY REALLY learn your car's response to different trips....
If you have a MPG computer check it against brim to brim... you don't need to go empty you just need to calculate how much was put in until the pump won't deliver more....
If you have an instantaneous MPG then many MODERN engines actually have not altogether expected responses to speed/foot on the pedal.....

Basically they are made to deliver power IF needed (mode A), run economically in the meantime (mode and reduce consumption further when possible (mode C).
Depending on YOUR car YMMV (your milage may vary.. to use the internet term in its literal sense)!!!

What I have found is:
Mode A is obvious .... my instantaneous MPG drops to 10 MPG or less in a 280 BHP Diesel BMW with a stage-1 remap and your face is pressed hard against your cheekbones.....and the Turbo is simply putting as much fuel as can be burned into the engine...

Mode B and C is not obvious.....
This is where the cars computer adjusts between WHEN the fuel is injected.
This is easy to see if you have instantaneous MPG in allowing deceleration ... on MY car this means going from say 40 MPG to 90 MPG in the extreme of say allowing the car to decelerate from 70 mph to a stop on a empty motorway exit sliproad....
At the extreme is it dripping in some fuel on the upstroke, just enough to stop it stalling BUT .... between this and Mode B is quite a bit of leeway....

In MY car I can accelerate slowly on the flat and still be getting 55MPG (or quicker downhill) but press the gas a bit more and it drops to 40 MPG... its a noticeable quantum change on the MPG and not noticeable otherwise.....

Equally, 6th is not always the most economical gear to be in.... my car can pull hard at very low revs... its a HUGE diesel so I can do 30 in 6th.... BUT it will in most cases use more fuel than 4th or 5th.... and when it starts going uphill the difference is bigger!

This is MY car... you need to learn yours....

7) Because of the above selecting say 65 MPG on cruise control will return lower MPG than me controlling the gas at 70mph..... but allowing me to adjust if the road goes uphill, downhill, turns into or away from a headwind.

8) If you have a longer commute then make sure that you can't save pence/litre on the way to/from work.

frugalistic wrote: »

hi can anyone tell me is it worth my while knocking my car into neutral -- though of course it is illegal and i would never do such a thing -- in order to save diesel? i have a bmw 530 and it is kinda costing me a bit these days... someone told me its engine cuts out the useage anyway if ur going downhill ...?

So its a 530D.... probably better to say...
How old? (Which model/engine)

My 56 330D goes into lean mode which is 'just enough fuel at a different part of the cycle' ... to turn the engine over...

I can chose between 35 MPG and 55 MPG in the E90 on my commute, its all down to how I drive.

mrlevity

I drive an 05 Citroen C3 1.4HDi. Best car I've ever had for fuel economy. I do a 40 mile round trip six days a week and am getting 55mpg. Sometimes more. And only £20 a year to tax it.:j

My wife has an 02 Citroen Xsara 2L HDi Hatchback, does only 20 miles a day 5 days a week and gets aprox 35mpg.

My trip is two 20 mile journeys each day on two major roads, hers is two 8 mile journeys to work and one school run (another mother takes kids to school, we pick up).

When she takes my car, mpg goes down to 50mpg. I take hers and it's up to 45mpg. We once swapped cars for a week and I got 50mpg out of hers.

Both cars have turbos, but it takes hers a whole heap longer to get up to cruising speed, as mine is light and goes like sh*t off a shovel when needed. I feel like I'm driving a tank when in the Xsara !

When we go on holiday we take the Xsara as it's much bigger, and does much better mileage on a long journey. I use the cruise control whenever possible (great for miles of motorway and Derbyshire roads with a 50mph speed limit !) and last year I averaged 60 mpg over a week.

I used to drive a petrol Rover 216s and I admit I drove it like a complete t*sser. I was never going to let the boy racer behind me get past, and if I got 30mpg out of it, I was lucky. I fiiled it up once a week with £25, then petrol prices hit the £1/L mark and I grew up all of a sudden. A blown head gasket (can't think why ) meant giving the Rover up and buying a 1.8L diesel Fiesta LX.

I've never looked back. The Fiesta was doing 50+ mpg and I decided to let the boy racer go past. After that it was the C3, and I would never go back to petrol again. I drive at a steady speed and never go over 75 unless it's completely neccessary (it has a beeper to let me know if I go over).

Righto, off to check me tyre pressures.

MerlinMags

Two people in this thread have commented that Shell gives them a better MPG than supermarket petrol. Someone else said that was codswallop.

Have there any official/serious tests performed to ascertain whether there is a noticable difference, and which types of car it might apply to?

Ideally we would each perform a test ourselves, to see what worked for our own particular vehicle, but this isn't feasible when daily life interferes with the accuracy required for a decent test of sufficient duration.

only_mee

MerlinMags wrote: »

Two people in this thread have commented that Shell gives them a better MPG than supermarket petrol. Someone else said that was codswallop.

Have there any official/serious tests performed to ascertain whether there is a noticable difference, and which types of car it might apply to?

Ideally we would each perform a test ourselves, to see what worked for our own particular vehicle, but this isn't feasible when daily life interferes with the accuracy required for a decent test of sufficient duration.

Any car with a knock sensor can burn higher octane fuel far more fuel efficient my car prefers Tesco momentum 99 regular supermarket fuel is generally 95 ron and doesn't burn as good

steve-L

MerlinMags wrote: »

Two people in this thread have commented that Shell gives them a better MPG than supermarket petrol. Someone else said that was codswallop.

Have there any official/serious tests performed to ascertain whether there is a noticable difference, and which types of car it might apply to?

Ideally we would each perform a test ourselves, to see what worked for our own particular vehicle, but this isn't feasible when daily life interferes with the accuracy required for a decent test of sufficient duration.

Problem is its not economic to do the test......
I usually get 50-51 MPG when I have a clear run on my present commute.....(2x15 miles)
After almost 1/2 tank of VTECH diesel (a somewhat random purchase) I got 54 .. but the problem is the intervening time I got less but traffic was MUCH heavier..... so I wouldn't have expected the same as a 'good run'.

As someone pointed out, yesterday was also warmer....
Also my run home was after the car sitting for 5 hours in a indoor car park vs morning runs where its cooled much more overnight on the drive...

There is a noticeable difference in how long it takes to reach a running temp.... (based on when heater comes on set at 20.5C) hence fuel economy is lower ....

Quite how detailed the test would need to be in terms of different engines is also something I wonder...
i.e. Would the same engine but at 10k,50k,100k and 150k be different? Would how it was treated in the meantime make a difference.... grade of oil etc.????

Would the different tweaks made to engines make a difference.... i.e. small facelift type changes.... where they did a 140bhp model that then gets replaced with say same engine but tweaked to 155bhp...????
How many tanks of clean 100% economy or premium would need to be run.... what happens if you do 1/2 tank of one mixed with the other???

Then how to do all this and keep the ambient temperature the same?

Seems like it would need a rolling road in a climate controlled environment???

Ximian

I was recently researching if it was worth buying premium fuels and found some interesting videos:

5th Gear unleaded fuel test: http://www.youtube.com/watch?v=WTaBngvsPrc

5th Gear Diesel fuel test: http://www.youtube.com/watch?v=ezjknhwcQXU

I recently filled up on Shell V Power Diesel and I can notice a difference, the idle is slightly smoother and quieter and acceleration is slightly better.
It appears that premium fuels do make a difference, but ...... do they make a big enough difference to justify the higher cost. I'm still testing, still have almost a full tank to go but I'd like to see the end result mpg.

johncolescarr

frugalistic wrote: »

hi can anyone tell me is it worth my while knocking my car into neutral -- though of course it is illegal and i would never do such a thing -- in order to save diesel? i have a bmw 530 and it is kinda costing me a bit these days... someone told me its engine cuts out the useage anyway if ur going downhill ...?

Definitely do NOT do this, it is dangerous but also gives you worse fuel economy. Basically when your car is in neutral (idling) you need a small amount of fuel to keep it idling, but when in gear and off the throttle, almost all modern cars use NO fuel (ie the fuel is shut off) as there is no need to maintain the idle.

Your 530d is a 6 cylinder high performance diesel engine, so its not going to be great on fuel (although lovely to drive). These cars do well on a run generally but crap around town.

anotheruser

frugalistic wrote: »

hi can anyone tell me is it worth my while knocking my car into neutral -- though of course it is illegal and i would never do such a thing

I do this in my car.
It's not illegal but depends how you do it. Keeping your foot depressed on the clutch for a longer-than-usual amount of time will ruin it in the end, this is commonly known as "coasting".
But I don't have any feet on any pedals when I do it, especially downhill.

People suggest it is dangerous as you "do not have full control of the car". You have the power to stop, just not accelerate away from danger.

The only danger I can see is that you wouldn't be able to get away from an incident like an out of control car behind you. However when travelling along, the gear I would have selected would not give enough pull to get away from anything.

The only downside I see is a little more wear on the brakes as the engine isn't helping the car slow down like it would if you cycled through the gears properly.

JDPower wrote: »

Coasting in neutral actually uses more fuel than coasting in gear. If you want a longer answer:

http://www.popularmechanics.com/cars/alternative-fuel/news/coasting-in-neutral-fuel-economy

http://lmgtfy.com/?q=does+coasting+save+fuel

Not yet read the links (will do soon), but if a car is at much lower revs then it's using less petrol.

I slide my car in neutral when I am expecting to come to a stop or have slowed enough that excessive strain is not put on the engine when putting it back in gear.

johncolescarr wrote: »

Basically when your car is in neutral (idling) you need a small amount of fuel to keep it idling, but when in gear and off the throttle, almost all modern cars use NO fuel (ie the fuel is shut off) as there is no need to maintain the idle.

I understand this but I would have thought the same if not more petrol is being pumped into the engine when when you're not actually accelerating.
More research for me I think.

Richard53

anotheruser wrote: »

I understand this but I would have thought the same if not more petrol is being pumped into the engine when when you're not actually accelerating.
More research for me I think.

No, johncolescarr is correct. If you coast down a hill in neutral, the engine has to use a tiny bit of fuel to keep the engine idling. In a high gear, on the over-run, the fuel is shut off completely and it uses none at all.

If you add in the danger of not being in full control of the car, coasting really isn't worth it. It's not just being out of gear and having no ability to accelerate; it's the fact that when rolling out of gear there is nothing to take up all the slack in the suspension and transmission, and the car will feel very 'loose'. If you had to take evasive action, you might find that the car was much less stable than you thought.