Question about mpg...

murphyebass

Hi guys,

Really quick question for ye in the know.

I bought a 2.2ltr diesel mazda 6 a few weeks back from a dealer, all serviced, new NCT etc..

Like most cars these days I'm sure it has a display you can show remaining Km's in the tank.

I've filled it about 5 times now from the same station, coolmine Topaz Dublin 15.

Other than yesterday when I filled it the reading post fill up showed roughly 960km's left in the tank so full up I could do around that with proper longs drives etc.. I know its only an estimation and town driving brings it down considerably etc..

But yesterday I filled it again and this time for whatever reason it showed up as 1150 ish... I cant remember the exact number but well over 960km's.
I can understand some variation but 200km seems a bit much to me although I'm absolutely no expert in this type of thing.

Anyway just wondering why that'd be. I'm happy obviously that it potentially could go the longer distance but it just seems odd to me.

I've done a few long distances in it recently if that makes any differences to filters etc maybe but other than that I cant think of a reason why it'd suddenly show a higher reading. Fair enough if I was using a different petrol station maybe but nope same one.

I havent even changed the tyre pressures so I'm baffled. Any ideas?

Cheers,

dar83

The longer journeys will have made the ECU recalculate the MPG figure or more specifically the 'distance to empty' figure. My own car does the same and it only takes one long (100km motorway) drive for this to happen. Takes maybe one to three fills for it to settle back to what it realistically is.

murphyebass

dar83 wrote: »

The longer journeys will have made the ECU recalculate the MPG figure or more specifically the 'distance to empty' figure. My own car does the same and it only takes one long (100km motorway) drive for this to happen. Takes maybe one to three fills for it to settle back to what it realistically is.

Ahh ok that makes sense. I had to drive to Cavan and Laois recently so thats whats happened there then so.

I dont do any long distances Monday to Friday normally, all short hop 15-20km each way type stuff, but on the weekends I'm planning to do more hour plus motorway journeys so will be interesting to monitor it.

To be fair its strange going from the 2ltr petrol Mazda 6 I had which was doing on average about 450/500km to a full tank. Some serious difference. Please god I'm not hit with any large diesel type issue. Lots of servicing and blowing the backside out of it hopefully keep it right.

bazz26

I would not take the accuracy of the onboard computer as gospel to be honest. They can be hit n miss and only an indication.

Jesus.

Its all meaningless OP. If you took off hard from the petrol station in first gear and ragged it a bit after filling up, it might calculate you have about 300 m left in the tank. If you moved off easy it could give you a reading of up to 1,000. Its constantly calibrating against the type of driving you're doing.

When you filled up the second time, your driving prior to pulling in was probably easier than the last time, that's all. If you reset the dash Com before pulling off, the figure it gives you is completely meaningless because in a couple of minutes it'll be telling you something totally different depending on how hard you're going.

wonski

The long trips you've made recently caused this sudden jump in a range.
To increase accuracy try not to reset the display at all - the more miles it takes into account, the more accurate it is.

Jesus.

I'm presuming the OP resets his Com regularly (upon each fill up)?

If not then what the others have said is more applicable

HandsomeDan

It's a variation of 20%.

Fuel consumption can vary by 100% between min and max consumption, depending on how you drive...

murphyebass

Jesus. wrote: »

Its all meaningless OP. If you took off hard from the petrol station in first gear and ragged it a bit after filling up, it might calculate you have about 300 m left in the tank. If you moved off easy it could give you a reading of up to 1,000. Its constantly calibrating against the type of driving you're doing.

When you filled up the second time, your driving prior to pulling in was probably easier than the last time, that's all. If you reset the dash Com before pulling off, the figure it gives you is completely meaningless because in a couple of minutes it'll be telling you something totally different depending on how hard you're going.

Yeah I see what your saying but it doesnt just jump back to say 900 it goes down gradually.

wonski wrote: »

The long trips you've made recently caused this sudden jump in a range.
To increase accuracy try not to reset the display at all - the more miles it takes into account, the more accurate it is.

This makes sense. I always reset after a fill up just a habit more so than anything to see what milage I'd done in a week or again more just the habit of it.

Jesus. wrote: »

I'm presuming the OP resets his Com regularly (upon each fill up)?

If not then what the others have said is more applicable

yup as above

HandsomeDan wrote: »

It's a variation of 20%.

Fuel consumption can vary by 100% between min and max consumption, depending on how you drive...

Ah yeah I get that alright. I know its only an indicator, etc.. but after filling my car (bearing in mind I had a near identical Mazda 6, same model etc, only petrol as my last car) I'd never seen it as a different amount in the km's remaining section. It was always the same. On the Petrol one I cant remember the exact figure, lets say 500km remaining. Thats what it was each time no matter if I'd done a trip to the moon and back the previous day or how I set off from the petrol station.

The new 6 is a bit different it seems in this way. Its taking something else in to consideration. Having read all your posts the most logical one to me seems to be that its taking into account the last couple of drives. At the same time I still find it a bit odd. Anyway it was more of an out of curiosity question than anything else.

Thanks for the replies!

R.O.R

Different cars calculate the DTE in different ways.

Some are very sensitive to how it's been driven in the last few hundred meters, some take the average over a lot larger distance.

Sounds like your last 6 took averages over a greater distance (hence why it didn't fluctuate a lot), but the new one takes it over a shorter distance so gave a larger DTE based on the better consumption from the longer, recent, drive.

Jesus.

As an aside, I was on top of the hill of howth yesterday and set the dash Com to zero and drove down it. From top to bottom I averaged 0.1 l/100km which works out at 2,825 MPG.

Beat that Mad Lad!