Average fuel consumption

[sidepod]

Chaps,

The new van is displaying an odd characteristic. The average fuel consumption is eratic and inconsistant.
I can do the exact same journey on two different days, same traffic, same driving style and yet I get wildly different readings.
I've noted a similar variation on the return journeys.

Temeperature isn't an issue as most of my research has been carried out under similar ambient conditions.

Has anybody noted similar?

Regards,
S.

 

[BMVS-Camper-Hire]

What vehicle are we talking about here? You know there are two computers on them, one lifetime and one is the recent journey?

 

[sidepod]

The vehicle in question is a Caddy 2L 140 but it's got the same engine as pretty much any other VW diesel van.

The read out I'm using is number 1, "current journey" as opposed to number 2 "overall".

s.

 

[choplee]

What you getting then ???

 

[smgcowfoldparts]

Sounds like yo uare getting the real time fuel consumption and not the average which would explain why it varies wildly. Try monitoring the other fuel consumption as that should be the one you are after and stay fairly consistent.

cheers

 

[sidepod]

I'm using the average fuel consumtion readout. Circle with a line through it. Symbol for average.
I'm not using the "instantaneous" consumption read out which is meaningless.

I'm using the readout for the journey just completed, not the overall for all journeys.
I'm pretty confident I'm using the correct readout!

So, day one I drive to work (27 miles). When I get there the readout tells me I averaged 43mpg.
The next day (similar ambient temp),similar traffic conditions, similar driving style, same journey and I manage 52mpg.

This is the same for a number of my journies. Inconsistancy in data.

I don't understand? If it was a small delta than I would accept it but to vary by 9mpg over 27 is nonsense.

S.

 

[smgcowfoldparts]

Difficult to tell without seeing what journeys you are doing etc...Do you have a bluemotion Caddy with start/stop? If you do that could be a factor if it cuts out more on one journey than the other for example

cheers

 

[Californiaman]

Are you also recording/checking your true mpg when you fill up - if you are getting dramatic variations from tank to tank then I'd worry...

 

[TravellinJones]

I'd 2nd that Californiaman. Do a couple of spot checks on what the vehicle is telling you if you're not convinced by the trip computer. 43mpg to 52mpg is not that big a discrepancy. I can get widely disparate numbers on my 25mile commute in my 1.9TDI 4x4 Octavia:

30-40mpg - driving at my comfortable limit
40-50mpg - driving without consciously being 'economic'
50-55mpg - driving 'economically'
55-65mpg - driving carefully, consciously avoiding either pedal, coasting downhill, etc

The Caddy is probably not too disimilar I would guess?

 

[beardie]

I have a golf 1.6 diesel and depending on the traffic flow, condition of roads (wet / dry) have seen a best of 74mpg and a worse of 55mpg. Can't remember the ambient temp etc but can believe a difference of 9 mpg is believable. Exactly the same route to work 22 miles and general traffic conditions. The best one was probably sitting behind a lorry for most of the way, the worse probably more start stop. Neither day would I have exceeded 60 mph (A14 in Cambridgeshire!)

 

[smgcowfoldparts]

Best way to work out fuel consumption is to go to a petrol station and fill tank until pump clicks once. Drive 200 miles and return to the same petrol station and use the same pump and fill back up to one click of the pump and work out how much fuel has been used for 200 miles of driving.

cheers

 

[TravellinJones]

For any fellow geeks/engineers:

27miles + 43mpg = 1.59gallons burned (7.24litres)
27miles + 52mpg = 1.93gallons burned (8.76litres)
delta = 0.33gallons
delta = 1.52litres

So, do a couple of trips and the meter on the fuel pump should be accurate enough to show up the sort of difference in volumes we're talking about...

 

[sidepod]

"but can believe a difference of 9 mpg is believable"

Really? Over a 27mile journey! 33% variation! That can't be right. There's little point in an on board computer with that kind of resolution!

Sure the only true method is to brim it and measure the mileage.

I have a well run in Golf Mk5 140 (175k on the clock) that will take the new Caddy to the cleaners on both performance and fuel consumption. Very odd considering they're both similar spec motors. If I had to estimate I'd say the Golf was heavier too.
I think VW's latest generation engines are a little strangled to achieve decent performance/economy.

Disappointed.
S.

 

[TravellinJones]

Agreed - you'd want to start with a brimmed tank and a known mileage and then do a few trips/re-fills to calculate the true mpg over a period of time.

I can't comment on the newer VW engines w.r.t. fuel economy. We've only just gotten our Cali (and it would not really be a fair comparison anyway for obvious reasons), and our Octavia is similar vintage and mileage to your Golf... I would say though, per my previous post, that the Octavia fuel consumption is very responsive to 'how' its driven. Given that the comparison of 'how' you're driving on two different days is at least to some extent subjective I think based on the numbers you've quoted it is not certain there is a problem/weakness with your vehicle engine or trip computer.

Sorry for getting too involved in this thread and probably just irritating you - it caught my eye as mpg has been a pet subject in recent months whilst researching which van to buy...

 

[beardie]

For any fellow geeks/engineers:

27miles + 43mpg = 1.59gallons burned (7.24litres)
27miles + 52mpg = 1.93gallons burned (8.76litres)
delta = 0.33gallons
delta = 1.52litres

So, do a couple of trips and the meter on the fuel pump should be accurate enough to show up the sort of difference in volumes we're talking about...

I think there is a typo in your figures, 52 mpg burns less than 43 mpg

 

[TravellinJones]

Doh!

That'll teach me to not check my own work rather than firing it off without thinking twice. Or not, similar errors have not cured this tendencyin the past!

Correct figures I think:

27miles + 43mpg = 0.63gallons burned (2.85litres)
27miles + 52mpg = 0.52gallons burned (2.36litres)
delta = 0.11gallons
delta = 0.49litres

So actually not much of a differnce in terms of volume. Would you notice the difference between 4 and 5 pints?

 

[smgcowfoldparts]

Also the new engines need some miles on them to perform optimally, don't know how many miles yours has done though.

cheers

 

[Californiaman]

When we get to this detailed level of course, we should be doing this by weight as the volume may change given temperature fluctuations!

 

[TravellinJones]

True. While we're at it we should probably sample the diesel each time we fuel also such that we can account for any variations there also... Think it's safe to say that there are a lot of variables to consider here!

 

[James]

"but can believe a difference of 9 mpg is believable"

Really? Over a 27mile journey! 33% variation! That can't be right. There's little point in an on board computer with that kind of resolution!

Sure the only true method is to brim it and measure the mileage.

I have a well run in Golf Mk5 140 (175k on the clock) that will take the new Caddy to the cleaners on both performance and fuel consumption. Very odd considering they're both similar spec motors. If I had to estimate I'd say the Golf was heavier too.
I think VW's latest generation engines are a little strangled to achieve decent performance/economy.

Disappointed.
S.

Really?

I don't think 47 mpg on a run and around 35 - 37 with a lot of town driving is disappointing.

i would never describe the Beach as a champion of fuel economy but it doesn't do much worse than my previous diesel Volvo C30 and in fact, around town, often betters it.

James

 

[CannellWalker]

Have you considered the direction and strength of the prevailing wind?

 

[Californiaman]

So long as the window is open a craic it ain't too bad..

 

[choplee]

Fuel figures aren't the best I'll agree ,but as vw UK say is that they are greener ,but not meaning they are as fuel efficient ..;-)

 

[sidepod]

I feel a little hypocritical now as I've just been looking into ECU upgrades.......more HP, more torques and more MPG. Everyones a winner :lol:

Do you think the main dealer would be able to detect a "software alteration"?

S.

 

[smgcowfoldparts]

Yes. And once it is detected it will be sent online and stored with VW and could have possible affects on future warranty claims.

cheers