As I promised a little while ago, here is the report on the fuel cons. of my 2006 S-type 2.7D. (120,000miles covered)

Fuel.......BP ultimate diesel.
Price......£1:20/litre. (£5:46/IMP. gallon/$8:19approx. or $7:16US.gallon)
Cons......Fulltank/fulltank method..........27mpg. (23.6US)
Cons......Car's own computer................27.6mpg.
Jag. claim.........................................26.4 mpg.
Usage....Mainly urban, conservative driving.
Check distance...112miles.

It was noticed during the test that fuel cons. is much heavier when engine is cold, which is often in this winter. I shall be looking for improvement to see if this BP ultimate diesel achieves what is claimed for it. I shall be mindful of comparing like-for-like, no change in driving habits etc. But I shall expect better cons. when weather gets warmer.
As a comparison, my old Mercedes 300D 1987 did 28mpg. Similar weight, similar driving, same autobox.
Leedsman.

 

Merc 1987 - same autobox? Is the ZF 6HP26 that old? (I don't think so.)

 

My deepest sympathies on what you guys across the pond have to pay for fuel, be it petrol or diesel. Every time I shake my head when filling up at the pump, I try to think to myself, "Jon, just be glad you don't live in the UK...."

 

I take it that BP is somehow advertising this fuel as being able to provide
better fuel consumption than other fuels.

Keep in mind these claims are usually based on the old marketing spin which assumes that a given car is in poor state of tune (ie clogged injectors) and that the blend of magic additives in the new fuel will cure this.

If your car is in good state of tune there will be no difference.

 

Same autobox as opposed to manual, which makes a difference (acc. to Jag, figures). Figs. are better than Jag. claim which will be for a new car, so let's see the results over time, without prejudice.
Leedsman.

 

I thought the diesel would get much better mpg than that.

 

Hi Leedsman. When I was doing my short commute of 5 miles, early starts in cold to very cold weather and my car parked outside, I was getting the same as you and 26-27 mpg. Now I have a 13 mile commute, mainly on motorway and fast A roads, not always having frozen early starts and I get 30-31mpg.

The best I have got was a long A road run and hit 39.4mpg at my destination.

I use whatever is the cheapest fuel in my area, which is mainly Shell and their standard diesel.

 

The mpg question tends to get 'hyped' a little for various reasons, esp. in the not too distant past by car's mpg. computers which used to be regarded as a joke by most serious investigators. Having said that, the Jag's modern computer seems to be accurate within the limits of "experimental error" as scientists like to say.
One has to remember the weight of Jags -- the S-type is near 2 tons at abt. 1.7 tonnes metric. This is by far and away the BIG factor for mpg. The next is the driver and his/her style in stop-start traffic. Economy techniques here can make a considerable difference. The next is the type of gearbox, the worst was the old Borg-Warner and its primitive torque converter, a better hydraulic g/b being the lock-up in the converter, the next your average manual + it's driver, the best being the electric-shift clutchless g/b. This last has been shown in UK., where at one time the Citroen C4 diesel EGS box passed the band 'B' emissions test inside the 120g/Km CO2 limit, where the manual did not quite, and qualified for next more expensive band C. I think this situation has been since rectified by Citroen.
Currently, my interest is in the effect slight injector clogging can have on mpg. Back in the 1970s, I had a diesel London taxi. Even then I logged all mileage and consumption assiduously, and one time there was a noticeable increase in consumption. No other effects were noticed, no black smoke, no loss of power. But removing the simple injectors and having them serviced (before my very eyes as it happened) brought the mpg. back to normal. The man showed me the dribbling from a bad injector, and the "puff of smoke" from a good one with his little machine.
The heavy consumption from a cold engine MIGHT be partially cured by very thin engine oil (say 0W30?) or by additives to cut friction (very dubious on this one!) such as molybdenum disulphide. Anyone have opinions on this one?
Leedsman.

 

Is that one of the additives which was found to poison catalysts and/or O2 sensors?
(with disastrous consequences)

 

Research indicates that an oil additive called molybdenum disulphide does indeed reduce engine friction, but has a nasty side-effect in that it can clog the oilfilter. There is also the sulphur problem with nikasil coated bores, where the coating can be damaged by sulphur. This means XJ8s up to about year 2000.
I would not use this additive for these reasons. Other additives such as PTFE based ones work, but only for so long. Therefore I'm looking into engine oil itself now. Past experience has shown that full synthetic oil is no doubt long lasting and good for high temp. turbo bearings, but its use does increase fuel consumption as shown by my records for other diesel cars.
Leedsman.

 

Is there any additive you would recommend for the diesel?

 

Occasional injector/engine cleaner. Very fine nozzles, need every bit of cleaning on offer. And anything to reduce carbon/varnish build-up anywhere from fuel in to exhaust out must be good. No need to go mad with treatments as all fuel has some additives.

 

Each commute I do I add another 0.1 to my mpg!

 

Finally achieved 40mpg, 40.5 to be precise on a motorway run for a couple of hours at an average of 60mph exactly. A week of country roads averaged 35.1mpg with some spirited driving.

Overall average is now bang on 30mpg.

 

I noted this week that petrol just hit a staggering 126 pence p/Ltr (Average) but no mention of Diesel - What is that running at and how much of an improvement in economy does that yield for you?

 

Latest fuel consumption figure for my S-type 2006 diesel is 31.5mpg(UK) using fulltank/fulltank method in urban driving. So something is reducing the fuel consumption over time...
Leedsman.

 

Diesel tends to be the same or a penny or so more. 126ppl is thankfully at the top of the market and it is possible to get cheaper. So a diesel at roughly 5 - 10mpg more than a petrol makes sense.