I have a question with supercharge engine. I compare a 2005 XJR and 2005 Quattroporte, I found that they output similar power, 395bhp vs 400bhp. But their fuel consumption reported from Fuel Economy has significant differences, 18 miles/gallon vs 12 miles/gallon.


What is the cause?

 

The jag has an all aluminum body which is over 400lbs lighter than the masser.
The jag has a final drive ratio that is more suitable for cruising. The lighter body and higher engine torque and gearing give the jag the best of both worlds. Performance and economy.
See the comparison here:
Maserati Quattroporte vs Audi A8 vs. Jaguar Super V8 - Luxury Sedan Comparison - Motor Trend

Notice the difference in weight and final drive gear ratios.

On a side note I recently made a trip on my 04 xjr from Dallas to New Orleans, a 520 mile drive. I averaged 27mpg while cruising at 70mph during the cold holidays. Gotta love that.

 

On an 800 mile drive from MD to ME I averaged just under 27mpg in my 2005 XJR. On the way back only 24 mpg I believe because I used 93 Octane from Sunoco for 1/2 the trip. I notice that 93 octane gas seems to differ quite a bit from station to station. From my local grocery store I only average 14 mpg locally but using Shell V Power it jumps to 22 mpg local driving. So now I am very careful where I buy my "petrol".

 

good comparison; looking at the stats, the engine speed at 60mph.

Jag 2,000rpm's
Maserati 2,850 rpm's

being NA that lovely ferrari engine needs to rev; and at $100k price, designed more to feel the power all the time than worry about gas mileage.

 

Ok, so I have a 2006 XJ8L, and as much as a nice Super V8 enticed me, I went for a NA car partly for more reasonable mileage and 2nd because all the amenities in the Super V8 (well most) were designed for the back seat passenger. I think in my X308 I was in the back seat for exactly 1 trip.

But having seen all the anecdotal reports of 30 MPG on X350's my first trip, a nice 350 miles, the best I was able to do, running on cruise @74 MPH was 26.6 - also my around town driving seems to end up around 19.x and while its difficult to quantify, especially the "City" or "Combined" stuff, my X308 got 19.x for combined and just about 25 Highway, but I had to really work for that, almost hyper-mile style.

Only been in the car less than a month, but found a horribly black Throttle Body for starters. I cleaned it up a lot with 3M Throttle Body Cleaner, that convinced me the fuel system ain't all that clean. Also, cleaned the MAF Sensor with electronic cleaner designed specifically for it. Put in a new Air Filter, though I may up it to a K&N drop in filter, no CAI's for my Jag, thank you!

Next up is oil change to Mobile 1 Extended- This stuff is only $2 more per 5 qt container. It gives me flexibility and less guilt if I miss my intended change by a few weeks. I am still trying to decide what intervals I will follow- certainly less than the recommended.

Lastly I have new NGK Iridium plugs all set to put in next weekend. My question is this: Is there anything else I can do to increase my mileage; am I missing something, or is there anything I should be investigating to determine if something is reducing my mileage? Oh, and I have checked tire pressures etc. Any suggestions to help me bump it up a few mpg?

Not sure I believe that Shell V Power can up your mileage by 8 mpg, but I always run Shell 93 unless its unavailable, in which case I get BP Premium.

 

93 Octane is the highest I can find in my area. Shell V Power is 93 Octane here but also claims to have proprietary cleaning agents not Ethanol based. I did about a year of research before buying my XJR a few months ago and part of the research was viewing as many CARFAX reports as possible. One common theme I noticed was the fact that many cars had fuel system flushes during their lives. My guess is that owners were buying cheap fuel. Just my guess. Also my guess is that Ethanol plays a big role in increasing fuel consumption. My owners manual says "do not use fuel system additives/cleaners". Which is why I go for Shell V power when I can instead of using an Ethanol based Walmart product every once in a while.
As part of the buying transaction I insisted that the dealer do an oil and filter and air filter change and replace all brake pads as well as some other things. I am sure he did not use synthetic oil so will replace that myself very soon, Since I am just over 100,000 I will replace my spark plugs but my mpg figure is good so not expecting them to be in bad shape. I make sure that my tire pressure is what it should be. When driving I am not afraid to use the XJR's power when necessary and usually travel at about 80mph on highways. Oh by the way the dealers mechanic over tightened the wheel lug nuts. Spent 1/2 a day in a garage where the mechanic there had to actually chisel one of the lugs off. The dealer paid for that. I replaced all Jaguar lug nuts with Toyota one piece lug nuts. Took a while to find the correct specs.

 

Can you recall what model Toyota the Lug nuts were for? This is something I have been wanting to do since purchasing my XJ8L

 

Almost any Toyota, except GT86.
You need a M12x1.5 nuts.

 

I tried Toyota lug nuts mentioned somewhere else in this forum and they allowed my wheel to "float". Only a bit but dangerous never the less.
The ones I used were from Gorilla and were 73138T. And yes they are Lexus and Toyota M12x1.5 nuts but hex 13/16" with 0.725" width. Also got locking wheel nuts from Gorilla 73631T. The width on the 1st ones was incorrect.

 

I agree with the gas difference. I never believed it until I used chevron and noticed a huge difference! I gotten slightly better results with chevron over shell and way better results over 7-11 Citgo gas. Also using the cruise control makes a difference. I read long time ago when in cruise control the jag goes into an economy mode. One last thing I noticed on my road trips in the xjr is that there is a huge drop off in mileage driving 75 mph over 70mph, but hardly any difference between 65mph and 70mph. So I always cruise at 70 on my road trips to get up to 28mpg.

 

Both octane and ethanol content matter.

Octane because when knock is sensed, the ECM increases fueling to counteract the knock.

Ethanol because it has lower energy content per unit of volume.

Be cautious that ethanol can be used as a cheap way to boost octane. So, high octane with high ethanol does not benefit you in the same way as high octane with no ethanol.

My results on 91 octane Shell V-Power without ethanol:

28mpg at 80mph cruise indicated. With supercharger.

And that's on oversize tires causing the speed and distance to be underestimated by 5 percent.

 

I was a bit surprised that my 2006 XJ8L gets a bit better gas mileage than the 2005 S-Type 3.0 V6 I used to have. The XJ8L gets about 1-2 mpg better in both city and highway modes. City about 20 mpg and highway between 32 and 34 mpg. My guess is that the V8 doesn't work as hard as the V6. Both cars have about the same kerb weight as I recall. The XJL does have quite a bit more low-end grunt which I like.

 

Really? 32-34? Can I ask what driving style and speeds are yielding said mileage? As I said I did 73-74 mph and never topped 26.x - I have since changed my oil, run several tanks of Techron, and have new NGK Iridium Plugs to install this weekend. I will be exchange the Trans fluid soon, as well as topping the differential. Anything else you guys can think of?

I would be ecstatic to break 30 MPG for a single trip at this point, heck I would be happy if it were a portion of a trip.

Note: I cannot find Chevron Gas here, so I generally stick with Shell or BP, but MY SHELL DEFINITELY HAS ETHANOL IN IT - UP TO 10% ACCORDING THE THE BIG BLACK & WHITE STICKERS ON EVERY PUMP. I don't have any Chevron here, so it's always been Shell or occasionally BP for me here. Sucks but what can you do? I wonder why Shell is different in North Carolina than elsewhere.

 

I get the 32-34 by setting the the cruise control to 60 mph in 55 mph zones and 70 mph in 65 mph and higher zones. However, this is in optimal conditions with no traffic jams or accidents. This may improve if I replace the air cleaner and clean out the throttle body. I got the car in July so I'm still getting used to it. I found that using the Sport mode at speeds less than 50 mph and Normal at more than 50 mph works pretty well. I treat the Normal mode as sort of an overdrive.

 

If it was a 1999-2002 S-Type it had a number of things that were improved after that time and the same kinds of improvements would be in the newer XJ. Things like better O2 sensors that allow more accurate fuelling and lower loss transmission.

 

I just completed my last 100 mile highway trip with my 04 X350 for the foreseeable future. Cruise set at 68 it got the usual 34.5. I filled the tank with Chevron Supreme for this trip, it got no better MPG than the Safeway supermarket prem. grade, but I am hoping for a smoother idle from the Chevron gas. Around town still seems to be about 24-25. Brandon

 

I recently changed my spark plugs to Bosch Iridium and it seems that I now get significantly worse mpg figures.
The original plugs were NGK Iridium and were obviously worn looking but with a gap of 38 thou. The new plugs were pre set at supposedly 44 thou but looked more like 42 thou when I checked them. I was expecting at least as good mpg but now after several trips my mpg figure looks to be around 13 around town down from from 22 before.

 

That would scare me as it may well mean excess fuel being dumped in the cats which won't last long.

 

I've taking several trips since acquiring my 07 xj8L back in July (actually driving it back from Daytona beach, FL to Memphis. I get around 30-31mpg on the highway & city, around 21-25 (the gap is wide there bc sometimes around the city, I love gunning her)

I do not use cruise control ever, being it gases it too much going uphill; & actually, I'm always driving 70-80mph (even in the 55 zone)

I just drive with the "Instant fuel gage" on & watch that...

Oh, & I only use BP 93' gas...

 

I changed back to the original spark plugs with a gap of 39 thou and have gone up to 19 mpg around town in cold weather.
This reply is to close my loop.