I'm told that very broadly speaking, fuel consumption (mileage) is somewhat of an indicator of the engine's general health. Of course tyres, weather, traffic, terrain and whether you had a big breakfast that morning can all affect it, but if you are consistently recording consumption that's too far off the median figures, something's up.

Now, on various FB groups, people have been reporting some crazy numbers -- crazy to me anyway. 30. 32. 35. That's MPG, highway, on regular 4.0 XJ8s.

I've driven my 01 VdP far and wide and have never seen better than 25-26 mpg, whatever external conditions you take into account, including gas quality.

Recently that figure has dropped to 23. Here are some figures from last night's little trip from London to Milton Keynes, including my car's overall mileage.

2001 XJ8 VdP 4.0
Mileage: 163633
Petrol: Shell V Power 97
** trip A **
69 MLS
Ave Fuel 23.0 MPG
Ave Speed 58 MPH
Fuel Used 2.9 Galls

If any of you have any idea how normal/expected that is, it'd be great to know. If on the other hand you think I should be seeing better numbers, suggestions of what to look for would be appreciated.

Thanks

 

If folks are getting anything over 26, I want to know the constant non-stop downhill they're driving on? Not that its not possible at points, but not maintainable.

 

The UK Imperial gallon is 20% larger than the US gallon, so reports of folks getting 32mpg is not unreasonable as that is simply 26mpg in US gallon terms. (Despite noninfinito listing his location as Oakland California he states his trip was from London to Milton Keynes, so I assume there is a UK connection here somewhere).

 

The only way I've been able to get 26mpg U.S. on a full tank is by overinflating the tires (42psi front, 45psi rear). With more sane pressures (32psi front, 35psi rear), 24mpg seems to be the ceiling.

 

I was initally concerned about the MPG but this was quickly overshadowed by waves of sympathy for anyone spending a Sunday night in Milton Keynes. How bad can Oakland be if is this is a holiday.

The overall fuel consumption for my 2001 XJ8L Sovereign 4.0 litre was 20.07 MPG in Imperial gallons.

Graham

 

The Oakland fuel economy is right where it should be. Note the difference between US gallons and imperial as CK pointed out.

Now, if he is in England, maybe not so good.

As bad as Milton Keynes is (I found it sort of charming; lots nicer nearby, tho), it beat the s... out of Oakland! That said, even the homeless in Oakland spell tires, not tyres. Old tires on the street make wonderful toilets for all the homeless and you can toss your old syringes in the side wall, which keeps people without shoes from stubbing their toes on HIV infected needles.

 

Thank you all so much for the commiseration. Indeed, Milton Keynes is no Pizza Express in Woking, but compared to Oakland, well...

Anyway, back to the issue at hand: the XJ I reported fuel consumption of is a US (California, albeit not necessarily Oakland) car that I brought with me to the UK. So, whatever it reports, it is in US units of measure.

I think the conversion answers the question which has been the cause of my anxiety. I can now go back to the various fora without the overpowering sense of inferiority about my gas mileage.

 

(I think the "Pizza Express in Woking" alibi could backfire once the media find customers who were definitely there on the night in question.)

Your US specification XJ has a different ECM programming to the equivalent European specification XJ so you will not necessarily obtain like-for-like consumption even after correcting for the US/Imperial gallon difference. The consumption figure I gave in post #5 was actual as calculated from purchases against distance travelled over one year and not taken from the vehicle computer.

Graham

 

For those of us in the Colonies: Peking Express in Woking?

 

Lots of variables in fuel mileage and it certainly could be an indicator of engine health but usually I find if just the mileage has dropped (you're not using any other fluids) the drop can be related to a sensor that's not working as well as it had been. I usually use a 10% drop in my average mileage as a sign that I need to change out my O2 sensors. Either way anything above 20 mpg is pretty good for one of these big cats. I'm apparently without the proper restraint to get better than 23 mpg out of mine but my wife, who is infinitely more mature seems to squeeze a little more out of the tank...I'm not sure that really makes her a better person however.

 

Don't forget these are all steel body automobiles. If we had a carbon fiber hood, roof, boot and doors...we possibly could be pushing 28?

 

I sustained 30.3 MPG New Bern NC-ATL Ga. on my most recent trip. 28 on the previous trip.

 

I think if you drop below 16mpg (US) (20mpg Imperial) as an average you are pretty lead footed, or only running very short trips to the grocery store just down the road every day. (or there is some mechanical/electrical/technical/fuel problem)

On a long distance trip with plenty of freeway driving you should comfortably average around 26-27mpg (US) (33-34mpg Imperial), and more if you are careful, and use the "non-sport mode" transmission select.

Over a long period of time with a variety of driving an average of 17-19mpg (US) (21-24mpg Imperial) should be expected.

If you normally have the car in "sport mode" (as I do, and I suspect most owners do) I think it makes about 1mpg (US) (1.25mpg Imperial) difference.

 

Another thing to take into consideration....the XJ8 and XJR have different transmissions and depending on if your running in Sport mode or not. I'm almost always in Sport, it holds your shift point longer to draw more into the rpm power band...and more into the not so economical mpg band. I've gotten 26 out of mine coming out of Ft Lauderdale, not using Sport, and being rather surprised the SC was an asset and wasn't that hungry.