On August 2nd I changed the oil in my 05 XJR to Castrol Syntec full synthetic 5w - 20 and a K&N oil filter. I averaged the last ten fill-ups prior to that and it came to 18.48 MPG (US). The last two fill-ups averaged 20.97. 2 ½ MPG with an oil & filter change is definitely significant IMHO. About 6 ½ quarts in total.

That's not the only purpose of this post. As we know, the on-board display is not always accurate. Here are two photos. One is the Jag disply and I filled the tank right after. The 2nd shows the Fuel Log app and the actual MPG using math... It was much further off than I thought. Numbers don't lie in this instance.

I am quite pleased with the results!!!
The driving conditions only varied by the % of cirt vs highway. Chesapeake, Norfolk, & VA Beach are 20 to 80 feet above sea level and the temperature has been about 80° for the low and about 110-115° for the high, so the Air Cond was well used as of late.









The top one shows the last several fill-ups just as an FYI.

 

My on board display is off by about the same 6-7%, and my speedometer is high by about the same. I think it likely common.

 

Ragman,

That's a very interesting report!

I haven't scrutinized the onboard computer in our '04, but I've done very careful fuel economy tests with our '93 and its computer is as consistently wrong as your '05. Our '93's simply can't be trusted to represent anything other than the trend of the data. It sounds as though the X350 computers are no better, which is surprising, given the significantly greater amount of sensor data that is collected.

On our '93, I suspected that one issue might be that the odometer/speedometer were a little off, but they both agree exactly with my Garmin GPS...

One of the things I discovered while preparing for my fuel economy tests is that internal baffles in the '93's fuel tank prevent it from being filled to more than 90% to allow for fuel expansion and proper operation of the EVAP system (that info is somewhere in the Engine Management System manual, I think). The method I came up with to attempt to fill the tank to the same level every time was to always use the same pump at the same gas station, and to keep filling until the nozzle had clicked off 10 times. By this method I could sometimes get more than a full gallon of additional fuel in the tank.

For my fuel economy tests I mapped out a 60.0 mile route with only six stop signs/lights to minimize the variable of time spent idling. I also did all my tests with the A/C, headlights and radio on, windows all rolled up, and used cruise control on about half the route, which is along the historic Natchez Trace Parkway, a little-used limited-access road. This was the best I could come up with to reduce the variables of stop-and-go driving vs. highway, short trips vs. long, my own natural lead foot, etc.

Cheers,

Don

 

That sort of variation in fuel consumption from tank to tank happens with no changes whatsoever to the vehicle, so attributing it to different oil and filter is a bit of a stretch.

 

Howdy,
Normally, I'd agree. But, after having her since early October and then going over to full syn, which I use Mobil 1 in my other cars, and was told by the dealer it had full syn change just before I got her and then the sudden jump, I'm inclined to think the oil & filter are responsible as I did not change my routes, driving habits, etc at all. A bit of a stretch, I agree that that's a plausible statement. On the same note, the increase over two tanks I feel is just as plausible. I put EVERY dime into the Fuel Log app to get all my costs and to have a record. I'll keep a tally of all of the upcoming fuel-ups and hopefully and she's the same results. The commute to my work and back with this increased fuel mileage allows me a trip there, back home, and there again per tank! Any way I see it, it's real results!
Side note, I use Amsoil in my Harley. Runs like a champ as well.

 

Was there a difference in grade or brand of old and new oil?


How old was the old?

 

I would like to have the answers to those as well. When I got her in October, I was told that she just had an oil change with full synthetic Castrol. When I drained the old oil, it looked like used conventional oil.

One of my seven MOS's in the Army was Motor Sergeant (Chief Mechanic). The first two Jags I had were bought private party & the last three were from dealers. Having owned approx. 30+ vehicles in my time, the first thing I did to them, as I got into my 30's, was change the fluids to full syn. The dealer cars, 99% of the time, when I was told they had fresh fluids, they did.

I can use my 99 Durango as another example. I used Mobil 1 engine oil and K&N filter. I did a COMPLETE, fluid change with Royal Purple immediately following a fill up. Engine, transmission, transfer case (4WD) BOTH differentials, along with all the corresponding filters and the MPG went from 12.75, using the previous ten Fuel ups to 15.15 using the following ten.

Knowing engines, in general, like I do, I have to believe that the HP & Torque improved as well. Needless to say, I'm happy with my outcomes.

R,
Rags