I just got back from 1200+ mile run up to NC and back. The outside temp was 95 degrees + on the run up and a max of 103 degrees on the return trip. The car ran flawlessly and delivered 28.4 MPG on both legs, accordingly to the trip computer.

I did not know the car was capabile of this kind of performance. The speed limit varied between 55 and 75 MPH the whole way and I usually set my cruse control at 5 MPH over the posted limit.

I took a similar run at Christmas and got 27 and change MPG. The only difference back then was Cold weather and I have sense switched to Mobile 1 synthetic in the engine and transmission.

What do you think?

 

I get about 24-26 on the highway with my 04 XJR. I drive about 75 and do have a heavy foot from time to time. City is between 17-18. I have found the computer to be off by as much as 1 MPG.

 

Reletive to computer milage data, I have found the same optimistic error in every car so equiped, regardless of make. It's a small chore to do the calculation by pen and paper, so I just go along with the game. Still-- 28 MPG and change even allowing for the computer fib is empressive.

The thing that really got my attention was the full 1 mile per gallon in hot weather over cold weather. I wonder if anyone has compareable data with synthetic fluids vs mineral fluids?

 

Every car I have drove always gets worse GPM is cold weather. I have always wondered why? Didn't know if it's denser air, the car chokes longer because of warm up time or the fact I let my car runs for 1-3 minutes before I drive.

 

In my naturally aspired XJ8, I regularly get 29-31 MPG on the freeway.

 

The diference is "summer and winter grade" gas. One is more volatile then the other. I believe the summer is more than the winter grade.

 

In Australia the speed limit is 100K/HR ie 62 mph and if I reset the computer when I am actually on the freeway and drive on cruise set to the speed limit, I have recorded 41MPG!!! over about a 30mile run.Incredible.

 

WOW! is that downhill? haha

 

Yeah - I am pleasnatly surprised that I get 26 mpg driving 75 - 80 mph (with an occaisional burst to 100). And yes I get 18 mpg city.

 

Another thing about my 1300 mile road trip. I drove the hell out of my 05 XJR with 51,000 miles on the meter and only used 1/2 qt. of oil. This in 95 to 103 degree heat with Mobile 1 synthetic oil. You can't complain about that.

Damn, I love these cars.

 

I have noticed they do burn some oil. I don't know if the engine was just designed that way or other. Mine burns about 1/2 qt. per oil change and the last one had about 4,200 miles on it.

 

My oil has always remained constant between oil changes. I'm using Castrol Syntec.