Car has new plugs, coils, fuel filter, mobile 1 synthetic oil only. 100k miles and it runs excellent. I work at a car dealership. Those are accurate mpg figures for my car. I've had the car for over a year and its been consistent. Fuel trims are normal.

 

The str was actually 1mpg worse than my 6sp gto so I always figured my mpg was normal

 

Hang on. Lots of experience among the forum members says some of those are not a good idea. Don't change plugs. Leave TB alone. In fact don't mess till you've got a decent OBD tool and taken readings. Much cheaper than randomly throwing money around and gives you facts rather than guesswork.

These are computers. Not lumps of metal.

 

Is It possible that the 05+ computer calculates mpg differently? I know when I had the elm hooked up it showed I was getting a lot better than my average on the car displayed.

 

The things I speak of are maintenance items!

 

No. Some are and some aren't. And some are but aren't due for a long time. But it's your car so do as you wish.

 

Well I just got home with the STR! I averaged 21.4. Guess that's better than I expected being how I was doing 80mph most of the way. And of course held her to the floor on a few occasions as well!

 

Not so! Of course we had considered the issues you raise. Other owner is senior Inspecting Engineer with NRMA (equiv of your AA) and we work on our cars together in my workshops. Whilst we all had a good laugh at your mention of plugs, gaps, coils etc and then your belated admission that they aren't exactly standard on diesels, you do raise some questions about variances on two otherwise identical cars - identical except for wheel sizes and tyres. You are entirely correct about the adaptive ZF 6HP26 - and the fact that we had overlooked this. Is this likely to have major impact under steady motorway conditions? We'll see. For technical reasons flowing from the two cars' shared maintenance, we had already ruled out any substantial impact from your other objections.

Actually, your claim that this test is impossible, even if changing wheels on the one car as you propose, is also wrong - but we credit your arguments for a solution that should have been obvious to us from the start - repeat the test over the same stretch of motorway, using the same "no traffic" time of day (actually, night into day), same cars, with the same drivers, same fuel etc etc etc but with each car now fitted with the wheels and tyres of its twin.

We have exchanged wheels already in preparation. We will drive off early tomorrow morning - once again, both vehicles will be in convoy but never slip-streaming, and I will post our results when we return.

We do not expect that, despite your objections, the 10% variance in fuel consumption is substantially due to anything other than the increased width and rolling resistance of wider tyres. Remember, we are not rubbishing fat tyres - and you would hardly call 245/40R18s "skinny" - nor are we chasing fuel economy per se - we simply seek to know what is the impact of "wider than the widest factory option".

Perhaps it will amount to nowt (whatever that means LOL). Tomorrow should tell all.
Cheers.

 

You might get the results you expect and you might ineed find the fuel usage difference is due to tire width, but I have seen identical vehicles, purchased the same time, serviced the same, carrying identical kit and have seen more than a 10% difference in fuel usage between vehicles, we used to use 3 identical range rovers to patrol a stretch of the m6 motorway and every driver noticed a difference in fuel usage between them but 1 in particular was much worse than the others. Due to the kit we carried and the light bar on the roof we never got anywhere near the official figures and land rover just dismissed it saying all vehicles, even identical ones, use different amounts of fuel.

It wasn't just the range rovers, most of us noticed a difference in fuel usage between the same vehicles of the same type and age.

 

Some sort of variation in manufacturing, I suppose - but I'm surprised it's so big.

 

another thing we also noticed with driving same age, make and model vehicles was some feel noticeably quicker than others, for example the range rover that used the most fuel seemed to be 2nd quickest, to the other 2.

 

any results cat_as_trophy ?

 

Yes ... Sorry for my delay in posting results ... my locum work intervened unexpectedly. I hope that the attached results translate successfully, because, while I have included some details of test methods, I have not had time to detail a comprehensive analysis. None are intended to convey typical or reproducible fuel consumptions - simply, that the comparative relationship is as valid as possible for our two VIN N8xxxx cars traveling at steady 110Km/h (70mph) on a major open motorway.

As can be seen in the results, the 9.8% variance from the first test was reversed to an opposite 6.9% in the second test, along with the transposition of wheels and tyres. The difference in increased fuel consumption was reversed - but by a significantly reduced margin.

As was expected, the greater adhesive factor and friction co-efficient arising from the road contact of the wider tyres is a significant factor in fuel consumption. However, even though wider wheels & tyres accounted for about 8% change in fuel consumption - as was predicted by at least one correspondent (police666), other factors accounted for about 2% change in fuel consumption.

These results have not been construed to achieve any predetermined goal. A range of contributors have pointed to sources from Newton, the continually evolving Laws of Physics, to the field of bicycle racing using very thin, hard tyres to reduce rolling effort and wind resistance.

Make of the results what you will - we do not see any Jaguar owners (particularly STRs) racing off to buy skinny tyres - a dangerous and foolhardy move indeed - but our findings do put some previously untested numbers against fitting wheels significantly wider than those which Jaguar provided as the vehicle manufacturer.

We look forward to your feedback and are happy to try to answer any questions.
Cheers.

 

Thanks for posting the results, I'm surprised that wider tires increase the fuel consumption that much but I live and learn.

Thanks for your time and effort researching this.

 

Well my 2.7 diesel has gone from doing 22.5 mpg totally urban ,to doing 25mpg ,I have owned the car for 4 weeks and the mpg is slowly getting better .Now a friend of mine is saying that the engine ecu is learning my style of driving compared to the previous owner .Is this right ?

 

More likely the gearbox is re learning