I have both an XF and an earlier S type both with twin turbo diesel engines and 6 speed gearboxes. In a recent test, both cars were driven in convoy on motorways, dual carriage ways with no stop start or town driving. The S type did 49.1 mpg and the XF 38.2 mpg with an average of 64 mph for both cars. The Jag dealer says that is "normal" for XFs. Jaguar themselves say that the S Type should do about 50 mpg at 75 mph (which it does) and that the XF should do 51mpg which it clearly does not! As the engine and gearbox are identical for both cars, I cannot get any explanation of the XFs relatively low mpg. Are there any other XF users out there who could give me an idea of their fuel use when cruising in 6th gear at about 2000 rpm?

 

did you try posting on the sister UK forum? i dont think anyone here has the diesel since its not avail in the US

neither number is anything to worry about though lol!

also, the trip computers might be calibrated differently

 

XF weighs the same and has a much better Cx of drag. Frontal area is not a lot different.

Therefore, either the measurement of fuel consumption was inaccurate (and accuracy for one trip is notoriously difficult) or there was an unusual mechanical condition affecting the XF.

Was the engine fully run in on the XF? Were the tires at proper inflation pressure? Is the DSC working correctly, is the park brake functioning correctly or are the rear brakes dragging?

My petrol (gasoline) XF returns around 30 to 32 mpg under similar conditions (per Imperial gallon which is the measure I assume you are using and the one we last used in Canada eons ago, and now we use the much more logical liters per 100 km travelled which is a fuel consumption figure, mpg is a mileage figure) . There is no way that the 2.7 diesel could be getting such poor consumption figures unless summat is oop!

You say you were in convoy. If the XF was in front and you had a substantial headwind that would significantly increase the fuel consumption of the leading car.

 

In reply to various posters. We travelled in convoy with say 300 yds apart, so no drag effect. The XF has done 6k miles so run in. Both S Type , and similar computers and XF have similar weights, and . have the makers tyre pressures
The 75 mph fuel figures are 51mpg (imp gallons) and the S Type is 49.8 mpg.
The S type reaches very closely the fuel figures and the Xf does not. The 140 mile "in convoy test" followed a 380 mile round trip again in convoy and very similar results were obtained, but included stop start and town driving, so the latter test was done when reaching dual carriageway sections and which were typically 60-80mph travelling with no stops starts, but simply cruising in cruise control with it set @73mph , which according to the sat nav was a true 70 mph. Both cars have automatic parking brakes which work and are not dragging, as the car can be pushed (by my wife) with some effort (o n her part!)

Ideally I am hoping that i can talk to another diesel owner, and repeat the test of doing a motorway run having reset everything. Frankly the round town for 6000 odd miles are about average and not far off the S type. However we recently did a mixed passage of stop start and motorway driving. The car was actually using less fuel when in stop start town driving than on the motorway at 70 in 6th gear and 1950 rpm.I have never owned a car which uses less fuel in town driving as compared to cruising on a motorway.

Finally on a practical note, we are doing up my sons house which means a commute of about 120 miles round trip of mixing driving. The SType will do the trip 3 times,before fill up, but the Xf only 2+ times

 

Are you actually calculating fuel consumption or relying upon the trip computer read out?

 

Yes I am relying on the two computer read out on the two cars. However I am also relying on the fact that I can drive to and from from my sons house on 3 journeys per tankful on the S type and only 2+ on the XF. In other words which ever way you cut it the XF is using about 20% more fuel on the same routes and similar speeds