It really does not affect your post in any way, as you used 91 in both directions. 87 octane fuel contains as much energy as 91 and, if the car is driven very gently on fairly level roads, the mileage could well be the same.

However, Jag recommends a minimum of 91 octane to avoid problems with detonation. If an increased load (acceleration or hill-climbing) is placed on the engine, the computer advances the timing to avoid or minimize detonation. It's much more likely to do that on 87 octane but, in any event, the result is that the fuel is burned less efficiently.

All I meant to say was that an increase of 4 mpg between E10 and plain gas seems just as likely as getting 30 mpg in a 4.2L Jag running on 87 octane. Hey, you may both be right!

 

It retards the timing, not advances it.

 

The percentage would depend on which was being used as the base.

At 128 and 160 fluid ounces, but easier to think of using quarts since the numbers
are then 32 and 40 ounces respectively,

the 32 ounce difference between a US gallon and an Imperial gallon is 25 percent
of the US gallon

therefore, the Imperial gallon is larger by 25 percent of the US gallon
or, the US gallon is smaller by 20 percent of the Imperial gallon

It's all in how the sentence is stated or parsed.

 

And either way is irrelevant since NO ONE is using Imperial gallons as measure.

 

Yike! You are correct, advancing could severely damage the engine.

 

The 30+ mpg (US) claim seems to be a stretch unless it was down some country lane
at low speed going downhill driving like a granny.

fuel economy: gas mileage, fuel efficiency & fuel mileage calculator

30 mpg US = 36 mpg Imperial = 7.84 liters/100 km

those conversions are before the "greater than" consideration

 

semantics

"retards less" or "advances more"

yields the same result as "absolute instantaneous ignition advance"

of course, one can datalog with odb-ii and observe two absolutes for
two tanks of different fuel:

LTFT
ignition advance

additional data that would be useful would be:

speed
air mass

Jaguar does not reveal its advance strategy in detail, but it is known that
many ecu implementations have large limits on advance and advance
is controlled by, and adapted by, knock sensor activity amongst
other factors.

We do know however that JF member XJREngineer has stated that in his
personal knowledge Jaguar adopted less than optimal advance parameters
on the AJ6. His products are intended to reverse that choice.

Given that the considerations regarding tolerance drift and warranty claims
would not disappear with the V8, it is reasonable to believe that those
considerations were also applied to the V8 development program.

 

Really?

How did you derive the 30+ mpg number?

Did you do a manual conversion from L/100km as displayed on the onboard computer?

Or, did you flip the onboard computer to a mpg reading?

If the second, then it's time for an experiment.

Obtain both the L/100km and mpg readings from the onboard computer display
and post them here.

Why?

Because your S-Type is a Canadian market vehicle and imperial gallons was
the standard of measure in Canada prior to the introduction of metric measure.

Jaguar may very well have programmed the mpg display to use imperial gallons
as that would be the expected gallon measure in Canada.

 

Maybe not the best or most reliable explanation, but a quick search yielded:

THE TRUTH ABOUT WINTER GAS. BY JOHN HUNKINS

Bonus ... the writer is based in NJ.

As for "coming up with something", your tests need to be repeated for you to trust your
own results. Long trips on the same route would be good.

I have done this and posted in the past about it.

It is pretty much the same result in that higher octane and ethanol free fuel are both
contributing factors. I have done the tests for each factor alone and combined.

The uphill/downhill factor is isolated by considering the outbound and return trips to
be separate tests only comparable with other matching outbound/inbound legs.

Each leg is a 5 hour leg at 80+ mph cruise without cruise control. The trip computer
is reset at the beginning of each leg just as the vehicle hits cruising speed and is
left alone after that even for rest stops.

The consistent real world result is that greatest mpg is using premium
ethanol free fuel.

Given that the greater $/gallon is offset by the greater mpg, I now only use
premium ethanol free fuel.

BTW, fuel science predicts that your outcome is the expected outcome.

 

No. That's what I used to think but it is wrong.

US and UK fluid ounces are not the same (I didn't know this). Then there are different numbers of each per gallon as well (I knew this part). It surprised me.

 

Good grief! It's no wonder that the French decided to define the litre. Even then, it's a liter in the US.

 

Gallons, imperial gallons, litres, liters, octane irrelevancies, retard, advance!

I wish I could drink that alcohol that didn't end up in my gas tank right about now ...

 

Many people forget the principle of Occam's Razor.

 

So what is the simplest answer and to which question are you referring?

 

I got 7.7L/100km or 36.6mpg (UK) out of my XJR, so it's not unheard of...

 

If you have unbelievable data, the data is unbelievable.

 

Or misunderstood.

BTW you used unbelievable I used surprised.

So you think the car reported erroneously?

I'd characterize you judgements in this thread as dismissive whereas I'd like to figure this out.

I agree only that I'm surprised by the huge difference. Calling it unbelievable doesn't help to explain anything.

 

Yes, that's the message I've been trying to convey. The calculators on the car are more of an on-board entertainment device rather than something that's intended to give accurate, credible results.

Claims that a switch between two fuels returning a 15% difference are instantly suspect- even if the person doesn't know the science behind it. If this were a typical result of using E10 vs. pure gas- why has their not been huge public outcry?

 

I checked the mileage info early on in the car's ownership and I never notice any issues. That was on a trip from Atlanta where I bought the car to NJ over a two day run. I did have to change out the the gas levels senders in the beginning though.

But I can't see how changing the type of gas would make it erroneous so if it is then it's always reading wrong which is pretty depressing because most of the time the mileage numbers suck which would mean it's even worse!

OK I don't know when I'll have a chance but I'll check over 1/4 tank of gas on a local drive then on a long highway drive and see if the calculated average equals what the pump and odometer report.

Or should I ignore the odometer too?

 

Keep this in mind when doing additional testing.: