Hi all,
I have to start this post by admitting that I am poor. Bearing that in mind, I take serious note of the fuel consumption of my '95 XJ6.
Now, as I leave my front door, I am blessed/cursed (whichever way you look at it) by a 5 mile long "track". This being made up of minor roads, dual carriageways and estate roads. So, on a hunch, I reset the trip computer and drove round the track with my wife as passenger, telling her that I needed her to listen for a noise. I was fortunate in that nothing much slowed me down as regards other road users and, when I got back home and my wife had gone inside the house, I checked the MPG .... 25.4
Two days later, I asked her to come with me again, this time as the driver, while I sat in the back and listened to the diff. I reset the trip comp and off we went. The other road users were kind to her and she had an even better run round the track than I did but when we arrived home, the trip computer revealed that she had averaged 18.6.
So either she is a right-foot maniac or these trip computers are not as accurate over a short distance as one would have hoped.
Any ideas which is right?

 

Any chance the engine was already warmed-up and near normal operating temp before your run and stone-cold-start for her's?

 

I doubt that she's a maniac, not on the road at least :-)

I think you were driving to maximize fuel economy and she wasn't, that's all. In my experience fairly subtle changes in driving technique can make quite a difference in fuel consumption.

Cheers
DD

 

5tevie....not scientific enough....lol

You're gonna have to do it again at least another 2-3 times and take an average of both your attempts ( without raising suspicion!! )

TBH though....I suspect your wife's just heavy footed, maybe its hereditary?

 

Neither your wife should be mad nor your trip computer is bad lol
She should let off gas pedal to help your tranny engage higher gear , this is what I do around 3000 RPM

 

A couple of observations from my own experience:

I've found the computer to be slightly, though predictably, optimistic over numerous tanks. What I mean is it under-reports fuel-burned and over-reports avg mpg by <3.5% vs. actual fill-up and odometer mileage.

Furthermore, I have noted a distinct "poorness" in the instantaneous mpg numbers until the car reaches operating temperature. Reliably. Regardless winds/weather etc. My normal daily commute is 50+ mi, so not much impact...
...but that's why I ask above, "Is it cold-start?" because I would think if I did a short route - 5 mi....once cold-start and once pre-warmed.....I'd see big difference in the numbers.

Come to think of it...that helps explain why all my vehicles driven by family members get such poor mileage even though most of the mileage is "Highway"...it is a 5-mile jaunt to either small town near us (for school or shopping) along a divided, 70 mph highway...but normally both legs done on cold-engine or at least one (maybe at oper. temp for return from shopping...from school, not so much)

I think it is to do with the oil...in parting I have just switched the X300 to 5W-40 syn but I have these previous averages:
Avg 10W30 Syn20.9Avg 20W5019.9avg 15w4020.6
enough tanks of fuel in each to cover 5k miles...still LOVE IT whatever the mileage!

Update: I have 2500 miles now since an oil change to Shell Rotella 5W-40 synthetic.
Drive cycle similar (daily commute to work) and currently, the average is 20.5 mpg.

A couple of other points: the true "baseline" is actually the 15W-40 the car had in it upon purchase. Additionally, it is possible that the data from the 15W40 is "better-than-would-have-been" because of the fall time-frame (SEP_OCT 2010) also featuring football games with my children participating to varying degrees around and about N. TX...resulting in proportionately greater highway mileage vs. Stop-n-go on that oil. The 10w30 syn. was Late OCT thru DEC, 20W50 JAN - early APR, 2011 and 5w40 APR - Present. One note, I believe it was about the time of the oil change in early JAN that I realized the impact to fuel econ of my cold-wx habit of taking lunch-n-laptop to shop, backing car out to close door, then repositioning to house and going in for to kiss the wife and get my coffee for the road. (This also afforded an opportunity to rap on the WV with rubber mallet after coolant had begun to warm) In fact, now I'm sure of the timing b/c I bought myself a backpack for laptop for Christmas w/pockets for the thermos, allowing for laptop/thermos on back, lunch in one hand and coffee cup in t'other, so switched routine to kiss wife, then load all my crap and head to shop for car....eliminating probably 5 min. of a 5:30 sec idle from every morning. So that would represent a penalty for the 10w30 and an advantage for the 20w50. My expectation is that the 5w-40 will equal or surpass the the 15w40 by the time I get 5k miles on it. In theory, it should offer an advantage between startup and normal operating temperature, otherwise, offer the same resistance. Cold-oil running is a very small proportion of the run-time I log. I plan to settle on the 5w-40 (though Mobil1 has a 0w40 I'd like to try but doubt it would be cost-effective) but may not be able to stretch the Pirelli P4k's out til the next oil change...so new tires will likely affect the average.....

 

Interesting, it seems that the 15W40 is not much of a penalty. Even the 20W50 is reasonable. Especially as the only synthetic is the baseline 10W30.

Did you know that fuel consumption is a consideration on lubrication recommendations?

Redline, sort of let it out of the bag on their discussion of their ATF D6: "lighter .... due to CAFE requirements". Castrol implies the same thing in their oil specification monographs for each of the weight grades of Castrol GTX. That leads to a belief that while the factory fill might be "adequate" to get through the warranty period, it may not be the "optimal" fill for the type of longevity that is the goal of later owners who hang out here.

BTW, the above has *nothing* to do with the effect of thicker oil on VVT. Those with VVT have other considerations.