Hi all,

just wanted to ask what could cause this. The tyres, yes,
are an issue. They seem to have less than 40% tread life
left, and are quite noisy. They may cost me a 1/2 mpg.

The air, filter, oil, and fuel filter are all recent. No check
engine light.

I get about 15mpg average without highway, and on a recent
highway journey, at 60, with the cruise control, the little
fuel computer never went over 24mpg. 21mpg at 60mph.
(I reset the trip @ 60, and just ran for a few miles.

On a recent highway trip, I never saw 22mpg. I averaged 73mph
including stops, but I averaged around 19mpg for the trip.

I feel like after I stopped at a certain gas station it got worse,
I seemed to be getting 22-24 before I stopped at Shell in
Castle Rock. The XJ Sport went to the same exact station and
its highway mpg never recovered. (however, I put 235-45-18 on 100
miles before. I can't be sure it was gasoline. I sold the car with those
off-size tyres, so I never knew for sure)

The XJ Sport used to get 27mpg at 60mph, of course, 60
is a rare speed. 75 is more usual.

Do XJRs really cede 4-7mpg to the N/A car?

I get about 20mpg at 70mph. And 18mpg at 80.
in the XJR.

Also, under immediate throttle there is a slamming
sound from the back. I am going to buy some
washers to make sure the spare tyre is secure,
but my mechanic inspected the rear suspension
and drove the car, he said it was probably a trim
item. He didn't say anything about the diff. The
shock bushings back there are perfectly good.

Need your advice! Why such bad gas mileage? Why
the sound when i stab the gas (w/o it kicking down)?

Ian

 

@ gas mileage - get used to it.

noise from the back - is the battery tied down, spare wheel too? anything loose back there?

 

15-16 is about right ..... generally poor ... and any ethanol fuel will be lower.

 

my overall mpg usually stays around 14. If I am using the car in a non-fuel-efficient manner it drops down to 13.5. When I am really trying to "save the planet" I can get 16.5 out of it. On a long highway trip (mostly around 80mph) I have seen 18. 90% of my XJR's activity is short trips on secondary roads

I would agree that your spare could be loose or something

 

I've recently found that running ethanol fuel gives me poorer fuel mileage. But even with Pure gas (no ethanol at all), around town I do well if I get 17.
I guess it is the cost of driving a fun car....
And I don't feel bad at 17- my Suburban pulls about 10 around town!

 

Ian - how much do you weigh?

 

Like I said, my old XJ got better gas mileage
than most people on the forums had reported...
but it was also a low mileage car, and perhaps that
had something to do with it.

If you guys are right (which you are, with your XJR) then that
means that quoted 16/22 is unrealistic. When in my XJ
Sport, 17/24 (all these are orig. EPA est)
was less, I got 17 city, 25-26hwy.

Why would they under-promise on the N/A 4.0,
but over-promise on the S/C 4.0?

@Sean, it seems too immediate to be a trim iterm.
it has almost no delay between
me hitting the gas, and the sound. I don't remeber
if it makes the sound on kickdown. I can wiggle the
spare around with my hand, if I grip it very firmly.
This must be the culprit. Battery looks secure.

 

Are your mpg numbers from the trip computer? Mine shows a bit pessimistic numbers. Less so after a hard reset, but still not correct. My calculated average (over a couple of months) is about 17mpg, while the trip computer says 15-16mpg. But it's gone down a bit since the lower pulley upgrade... probably just because I love the supercharger whine so much.

 

Your XJ sport may have been getting different fuel economy than what the trip computer was calculating due to the non-standard tire size, thereby making it appear that your XJ sport had better fuel economy than normal. If you have standard size tires on the XJR, it may be giving you a more accurate mpg reading. Tire width and aspect affect the tire circumference. For example, 205/45/18 tires would not have the same circumference as 285/45/18. My trip computer says I get worse fuel economy than I really get due to my non standard tire size.

Or if you have been calculating the mpg by hand and not by the trip computer, then ignore what I just said.

 

Well, I was running 245-40-ZR18,

which would mean the engine had to do slightly
more RPM to go the same speed, but then again,
it would affect how many miles it showed. Meaning,
it would show more miles covered.

The current car rides on 255-40-ZR18. And they
are goodyears, therefore noisy. I always used
the trip computer. Btw I do seem to be filling up
with more frequency than I did, so the XJ
Sport definitely used less gas!

 

Well it should, as it doesn't have to keep the supercharger spinning.

 

The EPA numbers at that time are normally the max you will ever get -- the effects of drag are not correctly factored in as the car is not moving through the air while they are being taken.

 

I get on fill up & computer reset, 18/19 MPG at 80 mph. My never ever reached 20+ MPG, even on cruise control set at 65.

I would love to get 23 MPH on highway, want to trade,

All my driving is highway, 143 miles round trip 5 days a week, I rarely do any city driving and when I do, it average around 14/15 MPG.