OK, so two more questions here...

After I fill-up, a RANGE value is displayed. I assume the car knows how much fuel it has. But, what mpg value is it using to calculate the RANGE? I reset both Trips after fueling, so the AVE FUEL reads 0.0.

Also, how is AVE FUEL calculated? Reason I'm asking is after spending several days at the dealer, my AVE FUEL dropped from about 16.5mpg (normal) to 14.8mpg. My dealer is kind of lame, but I really don't think they were siphoning fuel or resetting the odometer. The car was only driven 4 miles while they had it, which doesn't seem enough to explain the large drop in AVE FUEL. Now that I'm driving the car again, the AVE FUEL has increased to 15.0mpg.

I'm going to fill-up tomorrow, so it will be interesting to see what happens to these numbers.

 

Hi Kevin

the range is calculated using the previous average stored in the cars trip computer - even if you clear trip a and b it still holds that info for that reason.

if your car was being worked on and they were running the engine without driving it then your mpg would get worse as you are using fuel but the car is not going anywhere so when you drive the car again and and it is recording distance your mpg would improve as you add distance.

 

Fuel used is actually calculated by the system by using fuel injector "on time". I "knows" how much fuel an injector flows for a given 'on time' and fuel pressure. I tallys that and multiplies it by the number of injectors.
It basically uses interpolation to calculate the total fuel usage, that's why someone who really diligently monitors MPG the old fashioned way notices some variation in the calculated MPG/Average Fuel/Range figures.

You'd think there would be a better way, huh?

Cheers,

 

In reality, it's not that bad. The important thing is for it to be consistent while not getting you stranded somewhere without fuel. All you have to do is check your actual mileage a couple times and compare city and highway driving against the computer. Don't be surprised if it's less accurate during spirited driving.

The computer is accurate enough to teach someone how to drive the X for best MPG. I went from an average of 16.5 to 19.0 on the same daily commute. Another thing is I can get almost the same mileage in Sport mode as normal but it takes a lot more concentration, gaging lights and using hills to my advantage etc.

 

Very well said, C5pilot! I agree 110%.

My comment was really just mocking some of the folks who think the onboard trip computer should be "dead on" when they try to "check it's accuracy", which is what they usually say when they complain it doesn't match their calculations. If one just uses it and forgets the calculator, you can train yourself to squeeze a nice bit of extra MPG! [or figure out you'll be needing the gas station a day or two sooner than you planned]

Nice to see someone else likes to describe fun driving as "spirited". Nice English term! I use that term all the time when I try to coax folks to get the most fun out of their Jag.

Cheers,