I've read many times that diesels need a good 20K miles plus to "bed in" and give their best fuel economy.

 

I've owned both a 1.9 TDi and 1.6 CR TDi Roomster from new, but didn't keep either of them much beyond 10K / 1 year.

 

My current Yeti (2WD 2.0 TDI 110 BHP) however has just passed 20K miles and (according to my Fuelly account) the fuel economy seems roughly the same now, as it was when the car was new. In fact the first 5 tanks averaged 44 mpg and the last 45 mpg. A slight improvement granted, but not of the magnitude I'd expected.

 

Over the last 46 fuel ups I've tried a variety of brands of diesel (Shell V-power / Nitro+, Esso standard, Tesco and Sainsbury's), but haven't noticed any appreciable differences between them (in terms of fuel economy). The car averages 45.9 on Esso standard and 45.3 on Tesco's (the main 2 I've used). Of note it returned an average of 44.4 over x3 tanks of Shell V-power / Nitro+.

 

The car hasn't used an appreciable quantity of oil over 20K miles and is subject to servicing every 10K miles (3 services included with finance).

 

The car has in fact just returned from it's second (20K) service and received a clean bill of health. The rear tyres are down to 6mm and the fronts 4mm (Dunlop SP01's).

 

 

Mine was clearly delivered already "run in" and performing optimally!

 

Should I expect an improvement, or is my current MPG as good as it gets?

I don't think my consumption has altered much over the last 60k miles.

Manufacturing tolerances these days are much tighter than they use to be (no running in necessary - although I still built the revs up gradually over the first 1500k, - extended service intervals etc ) so there is little actual bedding in of parts change that will reflect  in improved fuel consumption.

I've read many times that diesels need a good 20K miles plus to "bed in" and give their best fuel economy.

 

The things you can read on t'interweb, eh?  I think this is one of those outdated beliefs that might have had an element of truth to it years ago, and which refuses to die despite the world having moved on.  A bit like the "diesels need to warm up for three or four minutes before you start driving" one that appeared on the bulletproof engine thread recently.

Our 2.0 diesel Yeti has only very gradually improved over its 22000miles to date.  From 43 to 44.1mpg measured on a tank/tank basis.  Our Maxidot averages 10% optimistic.

Done 33.5k in our 110 4x4 and tracked it on fuelly bar the very first fill up and it's been pretty much the same pattern for as long as I can remember.

 

 

TP

........... very nearly at 24k (since new in mid-Oct 12) and "improved" from 43.8mpg to 46.6mpg  Variation from maxidot to real world seems to grow as mpg increases??? :wonder:

........... very nearly at 24k (since new in mid-Oct 12) and "improved" from 43.8mpg to 46.6mpg  Variation from maxidot to real world seems to grow as mpg increases??? :wonder:

 

Noticed something similar in that the maxidot can be giving high 40's to low 50's for one tank full and in the low 50's only for the next, yet the fuel consumption can be higher, lower or the same as the fill to fill reading before :wonder:

 

 

TP

I have kept a spread sheet on the L&K 140 DSG and for our current mileage (approx 5000) it's consistently averaging 42.6 mpg.

Its too along ago to remember exactly, but the consumption wasn't good at first.  My mk1 octavia, bought from new, was still improving approaching 40,000 miles, when the tyres were changed and the dealer put new discs and pads on, "in case" under the free servicing deal.  

The car was only driven the 50 miles to and from work.  Very easy main road commute, what traffic there was, was stuck at 50 - 60 mph.  The A426 was too twisty for any overtaking until the A5 dual carriageway, then it was pointless speeding as it only made a minute or two difference in arrival time at work. 

The car spent 11 years doing this. along with holidays to France and the trips Germany, probably another 8 - 10,000 a year.

What ever your mileage, your driving style will affect the consumption more than anything else.

What ever your mileage, your driving style will affect the consumption more than anything else.

 

Never has a truer statement been made!!

Interesting ... I too have been told "Once it's done 20,000km the rings will have bed-in and the compression ratio will have risen and you'll get more power and better economy." It sounded a little old-school when I heard it, but just figured "what will happen, will happen" and besides, I'm very happy with the mileage compared to the petrol Subie it replaced anyway.

 

But - I have recently noticed increased economy. I don't figure out the mpg or L/100km or anything. Simply how many kms per tank, noting how many litres go in at the bowser.

 

I used to have to think about heading to the petrol station about 650 - 670km of city driving (usually including 3 or 4 x 40min trips to MTB destinations. If it had a 5hr+ freeway trip, it'd be more like 730 - 750km.

 

However the last 2 tanks have had me heading to the petrol station after about 730km of city driving.

 

I am curious to know if it's the petrol (2 tanks ago I went to BP instead of Mobil, and decided to try again after the improvement), or the fact that I have recently crossed the 20,000km point. I'm pretty sure my driving style hasn't changed anyway.

Edited 9 September, 201312 yr by beej

I've always understood that the compression ratio is a product of the volume of the combustion chamber so the piston rings wouldn't affect it?

 

Fred

Correct, Fred.

I've always understood that the compression ratio is a product of the volume of the combustion chamber so the piston rings wouldn't affect it?

The idea would surely be that piston rings that don't fit optimally would allow a finite amount of leakage during the compression stroke and hence a lower compression ratio. Doesn't seem too likely to me, but I can understand the theoretical logic.

Nothing is ever perfect.  The rings and bore aren't perfectly round.  They wear/bed in to match each other-ish.  During this process there is an increased amount of blowby, so effective compression ratio is reduced. 

The theoretical compression ratio is indeed  Vs+Vc/Vc.  But as I said, nothing is ever perfect and the formula does not take into account of leakage in action.  Or volumetric efficienty.

I suspect there was lots of this in the past-my dad had an engineering business which included re-boring cylinder blocks and re-seating valves and he knew his business-but it is a less busy field of engineering and that is because modern production is better as are modern lubrication and engine design. It is nowadays a case of expecting that the tolerances and assembly processes lead to engines which need much less running in and don't use oil.

May have spoken too soon!!

 

Last tank returned a genuine (brim to brim) 56 mpg!! This was achieved after covering just over 500 miles cruising up and down the A1 at 65mph.

 

Pretty pleased with the return nonetheless.