My wife was consistently able to average 33 mpg in our 2005 S-Type 3.0 on her solo trips to and from Tampa to visit her elderly parents. That's using cruise control set to between 72 and 74 mph with a fully-loaded trunk and the HVAC system running to keep her comfortable at 70 degrees winter or summer. I keep the S-Type tires at 33 to 34 psi all around....

Since acquiring her 2006 XK8 in February, it is always her vehicle of choice. Same cruise control settings, a fully-loaded trunk/pretend back seat/passenger seat/passenger floorboard, same HVAC settings, and tires at 31 psi all around (if I pump them up above 31 psi, I wind up with excessive treadwear down the middle). She averages more than 28 mpg on the Tampa trips, a bit better than I expected from that car....

I keep trying to convince her to redeploy our S-Type for her Tampa trips (that's primarily why I purchased it for her way back in December 2008), but she just laughs at me and heads straight for her XK8 now....

 

Mine is a 2004 v8 4.2 non-R and I get 25mpg daily and 34mpg on a motorway cruise.

Daily driving is country and A-roads at 90kph

Motorway cruise at 130kph.

 

I just had a tuneup- and am now getting 33-34 at 65mph and about 24-26 easy driving around town. Keep burning the rear tires if I'm not careful with the sport mode engaged. 150K now and still love this Jag. Also had the transmission serviced - new filter & fluid change. Now it's time for rear struts or bushings. Better than a Camery car payment

 

No, but our six-speed trannys help a lot.

 

True

 

Hi All,
Well considering that the US Gallon is 20% smaller than the Imperial Gallon, some of you are doing very well on your fuel returns.
Here in the old country my S with a V6 2.5Ltr., with 5 speed manual gearbox will do about 33mpg on long distance runs and averages about 27/28 on local use.
The manual versions are lower geared than the autos, in that at 70mph the manual revs are approx 2750rpm, against 2250 engine revs for the auto. So the auto can achieve 35/38mpg on long runs for the 2.5 and 3.0 Ltr in this country. However the autos are not so good in town conditions, when fuel return figures are generally in the lower 20s or worse.
Regards,
Inver.

 

Very impressive numbers! Thanks.

 

I'm confused. Are those great fuel economy figures for imperial gallons, or US gallons? And is your 2.5L a diesel, or petrol?

For the techs... The final drive is determined by the differential, correct? Hmmm, I wonder what would happen if a diesel manual got swapped in a auto differential? A billion MPG?

(Yes, I am joking.)

 

2.5 is petrol Diesel is 2.7

 

I'm sure he's speaking in real gallons, not 20% off discount US mini-gallons.

Final drive ratio is a combination of transmission gearing- all modern transmissions are the overdrive type AFAIK and not 1:1 direct drive like in the old days, multiplied by the differential ratio. Auto transmission applications also have to consider torque converter slippage which effectively raises the equivalent numerical ratio. Most cars have a locking type torque converter which allows for better economy when engaged.

Lower (ratio) is not necessarily better. Any engine has a sweet spot where maximum efficiency is achieved. Going higher or lower than that can be counterproductive.

 

I find I get the best mileage running about 75-79 mph and at that speed I get around 28mpg. I always thought driving at 55 mph is supposed to get you the best mileage but I don't think that is true with the Jags. In the city driving it gets the worse at 15-16 mpg US.

 

My 2004 STR is getting between 23.6 to 24.5 mpg for normal highway cruising.
Around town, I get somewhere between 11 and 16 mpg.

My best MPG average so far has been around 26 mpg, when I was able to drive at a significantly higher speed than the posted speed limit for a long period of time. As someone else posted, I agree that there appears to be a sweet spot at higher speeds where MPG increases.

I always use 92 Octane of higher in the car.

 

Muhuhaha! I have a great excuse to drive faster now.
"Um, determining an optimum distance traveled versus fuel consumed ratio as a function of vehicular velocity, officer."
Nope, that won't get me out of a speeding ticket.

 

Well you're in town numbers match with mine it's just that where I live it's such an urban pseudo combat zone that no car save a hybrid does all that well.

As for that sweet spot well there'a problem with at in the NYC tri-state area as well. Light traffic on highways is pretty much a mythological occurrence and that "sweet spot" coincides with law enforcement's "sweet spot" too. With 65 mph limits 75 is generally safe and even 80 is tolerated at times (with the herd) but in light traffic you're prey.

I'd have to be on along downhill to ever see 26 mpg as an average for any appreciable amount of time. Certainly never between fill ups no matter what the road conditions are and how stingily I drove.

These variations remain a mystery to me.

 

The North American S-types have been optimized to run on 91. No advantage or benefit to using anything higher.

 

The Sunoco station I buy gas at sells both 91 and 93 octane. Since there's only $.02 a gallon difference I get the 93 octane.

 

Has anyone tried 87 octane in their tank?
I know the knock sensors can compensate timing somewhat, but the compression ratios have me somewhat stumped.

My motorcycles have compression ratios of 11 to 1 and 11.6 to 1, and with manufacturer specified 87 octane gas, they never knocked. Admittedly, they rev much higher (12,000+ rpm) were not torque monsters either.

So when I see that the V6 compression ratio is 10.5 to 1, I think, why am I putting premium in? Is it because of the physically larger combustion chamber, that the likelihood of hot spots is greater, and therefore pinging (knock) more probable?

Correct me if I am wrong, but doesn't the lower octane fuel have more readily available energy? And would not that imply greater fuel economy? (less volume of fuel required for desired energy output)

I am full of questions today.

 

Our S-Type 3.0 has been fed a steady diet of either 87 or 89 octane since we purchased it four years ago in December 2008 at 18,000 miles. Now approaching 73,500 miles and never any pinging or power loss. I do add a 20-ounce bottle of Chevron Techron to the fuel tank at every oil & filter change (6,000-mile intervals) to help keep the fuel injectors cleaner....

 

I drive my V8 cat predominantly on the highway and I've come up from 18 to a current 22 mpg since I bought it. I cruise at a steady 70-80, with an occasional 100+ mph burst. I'm sure it would drop off if I drove it in stop and go traffic. I specifically wanted the V8 model when I bought it, so I wasn't expecting stellar mpg numbers. These numbers are about what I got from my '01 Cobra when I drove it gently and not far from what I get in the TL.

 

See bolded below. This question gets thrashed on a regular basis and I am the proverbial moth to a flame.


I see Jon89 uses 87 also.

Compression ratio is just one of many factors that determine what level of octane is required to avoid detonation. In general, the higher the compression, the higher the octane required as a rule of thumb, but there's so many exceptions to the rule it's is a weak thumb at best. No direct correlation IOW. Cylinder size has no direct correlation to octane rating either other than being another factor to consider. Cylinder design might be a better criteria.

Pinging (detonation) is not caused by hots spots, that's 'pre-ignition' which is a different phenomena entirely. Many people erroneously use the terms detonation and pre-ignition interchangeably

No, false. No connection whatsoever. All gasoline has the same amount of energy (within production tolerances) but there is no correlation between octane rating and energy level, burn speed, burn temp etc etc. These are all myths. The octane level simply designates a fuel's resistance to detonation. Nothing else.

Yes you are.