mpg has dropped since service!

Lum

27col wrote: »

I think that you will find that it is not only illegal to coast downhill, but also quite dangerous. My car tells me that it is doing 99.9mpg when in cruise control and running down a slight incline. But it is always in gear, while in cruise mode.

You will use less fuel leaving the car in gear and letting it roll downhill than you will by putting the clutch in and/or putting it in neutral.

The car needs to keep the engine turning over, and while you are rolling downhill, if you leave it in gear then this can be enough to keep the engine turning so no fuel will be added, however if you put it in neutral then there is nothing turning the engine over for you, so it has to burn a bit of fuel to keep it turning.

No idea if this is what the OP is doing or not since "coast" is an ambiguous term, but it's worth mentioning.

In addition to using less fuel, it's safer and also not illegal, which is a bonus!

gilbert_and_sullivan

I doubt the OP is coasting as such, he's probably on a trailing throttle most of his downhill journey.

myhooose

Lum wrote: »

You will use less fuel leaving the car in gear and letting it roll downhill than you will by putting the clutch in and/or putting it in neutral.

The car needs to keep the engine turning over, and while you are rolling downhill, if you leave it in gear then this can be enough to keep the engine turning so no fuel will be added, however if you put it in neutral then there is nothing turning the engine over for you, so it has to burn a bit of fuel to keep it turning.

This really depends what you are doing after the coasting. If you are coming to full stop then leaving it in gear will be best.

However if you are trying to maintain momentum to go up the next hill, it may be better (from a mpg not legal point of view) to coast in neutral as the braking effect of the engine will be removed. This will leave you with a faster speed after coasting. This faster speed may or may not be offset by the fuel used in tickover. It probably depends on the type of car used. Serious Hymermilers turn off the ignition and bump start at the bottom of the hill. I certainly don't advocate this.

thefishdude

been an amusing read this thread with loads of people having a go at the op over his simpole question. bascially people seem to be missing the obvious.

if mpg correct before service and now wrong after service = garage did something
if mpg was wrong before service and now correct after service = garage did something
if mpg correct before service and now correct but lower = garage did something
if mpg wrong before service and now still wrong but lower = garage did something

basically no matter if its accurate or not so people can stophaving a go at the op, the fact is the display has changed therefore the garage did something on the service. now what is the biggest question, judging by the change being so large i would say maybe a software update i would imagine it would be normal routine to plug a car into diagnostics in a service hoping to find erros that need fixing

DUTR

thefishdude wrote: »

been an amusing read this thread with loads of people having a go at the op over his simpole question. bascially people seem to be missing the obvious.

if mpg correct before service and now wrong after service = garage did something
if mpg was wrong before service and now correct after service = garage did something
if mpg correct before service and now correct but lower = garage did something
if mpg wrong before service and now still wrong but lower = garage did something

basically no matter if its accurate or not so people can stophaving a go at the op, the fact is the display has changed therefore the garage did something on the service. now what is the biggest question, judging by the change being so large i would say maybe a software update i would imagine it would be normal routine to plug a car into diagnostics in a service hoping to find erros that need fixing

I wouldn't say they are 'having a go' at the OP, perhaps the OP had a blinkered view on what the display is telling him, as others have mentioned a car coasting the display will show astronomical MPG figures , was the OP getting consistently >840 miles per tank of fuel? Highly unlikely. But indeed the garage did something, that's what they are supposed to do when the customer pays for a service.



this is a realistic return on a 2 litre petrol turbo , the A3 1.4 diesel was rather optimistic with the display compared with the milage/fuel recharge figures.

roddydogs

Surely the oil will be thicker as it ages with all the gunge etc, and anyway its not going to account for 20 MPG is it.

Lum

myhooose wrote: »

This really depends what you are doing after the coasting. If you are coming to full stop then leaving it in gear will be best.

However if you are trying to maintain momentum to go up the next hill <snip>

The OP stated that their morning commute was downhill all the way.

roddydogs wrote: »

Surely the oil will be thicker as it ages with all the gunge etc, and anyway its not going to account for 20 MPG is it.

Maybe the original oil was running in oil that has now been replaced with the regular stuff?

anotherbaldrick

roddydogs wrote: »

Surely the oil will be thicker as it ages with all the gunge etc, and anyway its not going to account for 20 MPG is it.

If you have got to the point where the oil is sludging up you have missed about four oil changes :eek:

ianbar0

i have pretty much stopped responding due to the sense that people have had a go, and really have missed what i was trying to ask about. motoring issues seem to get people more revved up than most things and i should know as another post i put ended in a right free for all! i should try and work out my mpg from my tank though, i do have the numbers so will have a go at that. the point was that one day i was getting x amount then next day it had dropped. maybe the figures are unrealistic but its nothing to do with running in etc or cold mornings as the weather was pretty much the same. than you to the people with constructive help, i am going to mention it more in passing to the garage. i will be filling up next weaken too so time will tell.

Lum

That's probably the real way to tell, you should have an idea how many days you get between tanks of diesel. If this doesn't change then you can safely ignore the read out.

Assuming the software update didn't also affect the fuel level reading too!