I'm a little puzzled by my latest Fuelly "fuel up" - see link: http://www.fuelly.com/driver/pinkpanther/yeti

 

The car has settled down to mid to high 40's mpg average over the last 30K miles.

 

I recently set a personal best at 56.6 mpg (long / slow drive), but was somewhat alarmed to find the last tank equated to just MPG 35.7!! 

 

My driving style / journeys undertaken haven't changed to any great degree.

 

The car was serviced (3rd fixed / yearly service) just before the last refuel and I also changed back to the summer (pressures OK).

 

The car seems to run fine and I'm not aware of any leaks/ binding brakes etc.

 

I've doubled checked my records and it isn't a typo!!

 

Cause for concern or "see how it goes"?

You sure you've entered the information into fuelly correctly?

You sure you've entered the information into fuelly correctly?

 doubled checked it

Is it possible your PB didn't fill the tank fully thus giving you a high mpg?

 

Andy

Is it possible your PB didn't fill the tank fully thus giving you a high mpg?

 

Andy

Sorry Andy - not sure on PB?

Sorry, PB = personal best. I was just thinking that if the pump clicked off early, then you hadn't filled the tank.

 

Andy

Andy C makes a good point - was the personal best an unusual spike in otherwise generally consistent readings? I don't use Fuelly but just another thought, could you have incorrectly entered the mileage in one of your recent entries in Fuelly? It's very easy to put in, for example, 28653 when you meant to enter 28563 (I've done it in my own mileage spreadsheet) and the elation of a personal best might have distracted you from the mistake. (If it had gone down most people would immediately check). This sort of mistake would also give you an unusual high mpg figure followed by an unusual low one, or vice versa. As for "Cause for concern or see how it goes"? I would say wait until the next fill (and don't be tempted to fill up early). Unless you think or know you have a problem with the car things will probably (hopefully) return to normal. 

Did you buy a different brand of fuel or fill up at a different garage? There is quite a lot of regional variances in fuel which can affect economy fairly significantly.

Also if you drove into a headwind or followed a vehicle creating a turbulent air pattern you could get a very different result

I would not be overly concerned unless the economy does not improve over the next few tanks.

If i get a 'pleasant surprise' on a fill, I just know it's not proven 'till the next fill.

...I also changed back to the summer

 

Are your summer tyres the same size as your winters?  My winters are slightly smaller, so the mileage reported on the Maxidot is higher with the winters on than the summers for the same journey (more wheel revolutions to cover the same distance).  Hence the calculated MPG ends up higher.

 

I'd also second the suggestion that your fill at the time you saw the personal best might have been slightly short of properly full.  In my experience the pump cut-outs vary quite a lot in sensitivity so you can't be sure that you fill to the same point every time - there's always scope for a bit of variation.

Edited 30 March, 201412 yr by ejstubbs

Thanks for all the suggestions thus far.

 

I'm pretty confident it doesn't represent a "typo" on my part.

 

I think the most likely answer is, as suggested above, that the pump  knocked off early on the second to last fill up. Averaging the last 2 fill ups gives an average suspiciously close to my running average.

 

I ran 205/55/16 winter tyres (on 16" steelies) between Nov-Mar and am now back on the summer 225/50/17.

 

I'll update after the next fill up.

Did you buy a different brand of fuel or fill up at a different garage? There is quite a lot of regional variances in fuel which can affect economy fairly significantly.

Also if you drove into a headwind or followed a vehicle creating a turbulent air pattern you could get a very different result

I would not be overly concerned unless the economy does not improve over the next few tanks.

 

Interestingly I kept meticulous records over the last few thousand miles and have concluded the supermarket fuels are no better (or worse) than "premium brands". At least in terms of MPG.

 

My other car is a petrol mapped to run on SUL, so Tesco's Momentum99 has virtually become my default choice.

Sorry, PB = personal best. I was just thinking that if the pump clicked off early, then you hadn't filled the tank.

 

Andy

 

Sorry Andy - I wracked my brains, but for the life of me couldn't work out PB!!

 

I think however you have probably hit the nail on the head!!

I ran 205/55/16 winter tyres (on 16" steelies) between Nov-Mar and am now back on the summer 225/50/17.

 

That's a 4% variation in favour of the winter tyres right there.  That means that the odometer will read 4% higher with the winter tyres on than the summer tyres - ie making it look as if the same journey is 4% longer - and thus the MPG will always appear to be 4% better with the winter tyres on.  Obviously it's not nearly enough on its own to explain the drop from 56.6 to 35.7 but it has to be a contributory factor.

 

I use the same sizes of winter & summer tyres as you do and my Yeti consistently returns about an average of ~2MPG better running on its winter tyres compared to the summers.

Mine seems to go up and down like a dog's whatsit (MPG that is  ) without rhyme nor reason so I wouldn't worry about a one-off ...............

Did you fill up your reserve tank on one fill then not on he other?

Yeti's don't have a reserve tank!!

Not very clued up on yeti's but was trying to help. What I meant was an expansion tank pressure release switch (the little lever just inside the filler hole).

This enables you to put extra fuel in on my fabia. Thought they would be on all diesels across all modern skoda's.

Sorry if I am wrong.

Not fitted to a Yeti.

No one has mentioned the obvious - The mpg in fuelley is set to UK & not USA as  it happened to me once before where i  put in the numbers & the results came back with a drastic drop in mpg, then noticed it was US mpg. Switched it back to UK & hey presto figures matched.

Nope, still set to UK.

Would be interesting to see what mpg subsequent refuellings give. Perhaps the initial fill up was not to capacity. Please keep us informed.

Hmmmm now that is puzzling ?

The first thing I would check regarding less mpg in a diesel engine with DPF is the engine oil level to see whether the level increases or not.

As well as being used in normal operation of the engine, extra diesel is also injected into the cylinders to find its way to the DPF during regeneration to burn off the soot/ash deposits.

If the DPF regeneration system (either injecting too much or too often) is faulty, the extra diesel is drawn into the crankcase and mixes with the engine oil

Thanks for all the suggestions thus far.

 

I'm pretty confident it doesn't represent a "typo" on my part.

 

I think the most likely answer is, as suggested above, that the pump  knocked off early on the second to last fill up. Averaging the last 2 fill ups gives an average suspiciously close to my running average.

 

I ran 205/55/16 winter tyres (on 16" steelies) between Nov-Mar and am now back on the summer 225/50/17.

 

I'll update after the next fill up.

Todays fill up (54.53 litres / 573 miles / 47.8 mpg) shows everything back to normal - as above it must have been the pump knocking off a little early.

 

My Fuelly account tells me I've now "refuelled" 66 times and have spent a staggering £4340.90 in diesel over the last 30963 miles

 

More here: http://www.fuelly.com/driver/pinkpanther/yeti