Wingrider, I bought this car from a mechanic who had worked on this car for years for the owner, he purchased it and put in all new plugs, coils,MAP sensor,fuel injectors,air filter,coolant tank, a number of hose's ( not just coolant) , cleaned throttle body and R&Red the supercharger. I've replaced the MAF & both belts,also I flushed & filled tranny with new pan & connector,also I swapped upper O2 sensors to see if that would make a difference ( it did not ) , I've also been all over hose's and connectors and cannot see any problems. When I went for a test drive ( buying the car ) I got on the highway & gave it a little extra gas and it stumbled, went into limp mode, brought it back to his shop put scanner on it ,said it was MAF sensor so he removed it sprayed carb cleaner all over it and reinstalled it and went for a test drive, same thing he thinks it needs new MAF so running motor sprays carb cleaner allover with no change in RPM saying it's gotta be MAF. That's how I bought the car and got the deal I did. When I first got the car home I replaced the MAF and went over everything, finding all four bolts holding top air plenum on supercharger loose and one stripped, so I did a heli coil and tightened them up to specs. I've been chasing these codes and learning alot for the last 9 months, never the less the car runs great and I love it.( By the way the mechanic only had the car for 2 months doing all that work, never titled it in his name so I got a one owner car.)

I went crazy with my 04 XJ8, which was a vacuum leak.
After a new MAF, 4 O2 sensors, PCV valve, air, & fuel filters, plus plugs with coils.
Wound up being, the eight internal, intake manifold gaskets, that nobody knows about when you ask.
Car always ran great, passed tail pipe e check, but kept turning the check engine lite on.
Changed the non leaking valley hose while i was there, not a bad job, but no super charger on mine.

Why do we always have to have a title when we send messages? Anyway I think your right about the gaskets, I just don't feel like stripping the top end apart with it running so good , you know afraid of opening up a can of worms. so I think I'll let it go for a while. Thanks!

Back to this thread, I took my 2005 XJ8L 4.2 NA with 73k miles on one of our typical road trips from Colorado to Utah and back.

I was quite surprised as across Utah and Wyoming traveling at 80+MPH I was getting ~34 mpg, verified by the math between fill-ups as accurate with the trip computer.

All I did was replace all my coolant system components - hoses and t-stat housing complete, intake manifold gaskets and TB gaskets, fresh 5-30 Edge synthetic oil and filter, fix the lower air deflector tray and all of its mountings/bolt locations. I did use a lot of cruise control but I found that even up to over 90 mph for sustained periods I was still getting better than 30-32 mpg.

Admittedly this was my first long road trip with this car, where town mileage has been about 22 mpg, so I was pretty satisfied with a 33-34 mpg reading on the road trip and range of a tank of fuel of + 540 miles for highway.

My XJ6 4.0 would get about 30-32 mpg on that route, and my 2000 / 2001 XJ8's best were 24 mpg on the same route.

Does that jive with others long roadtrips in a 2005 4.2 liter NA?

About what mine gets, twenty two around town, to 34 on the road.

Average highway fuel mileage, over multiple coast to coast trips: ~ 23 MPG
('07 XJR EPA MPG Rating: 15 city; 22 hwy; 18 combined)

X358 3.0 V6. About 10L/100km. Can get down to 9L/100km pure highway driving. Less in city perhaps 8.

10L/100km = 28.25 imperial mpg = 23.52 US mpg.

Now, I'm so light-footed that it's an embarrassment to the car and Jaguars in general. But it's not my fault, why would they put a computer efficiency display on it, it's a Jaguar FFS. I just can't help myself trying for efficiency, whilst all you guys just enjoy yourselves!

I have no problem getting over 30 MPG on a highway trip doing 70 MPH and I have a Supercharged Car, a 2005 Super V8 !

2004 XJR

About 20 mpg in city and 27 - 28 mpg on highway

2003 Super V8
about 25 mpg on average combined use

can be better when avoiding city and driving like a granny

can and will be worse mpg when having fun

Seem to be getting around 21 to 22mpg from my 4.2 SC on normal journeys. Longer journeys I get close to 24 to 25mpg. But never bought it for its economy