For no good reason I was driving around with the display showing the range for the fuel in the tank and of course it varied as I drove. In my wife's Japmobile it only does increments of 25 miles but the X300 seems to give the nearest mile.

But does anyone know how it calculates what to display? It clearly isn't using the overall average mpg because it changes mile by mile quite dramatically depending on how fast you go and how much acceleration which clearly affects the recent mpg.

So, does anyone know how it calculates the range for the fuel it thinks is in the tank?

Also again my wife's Japmobile recalculates the average mpg for a trip resetting it after a two hour session of non driving. So the average mpg will cover three sessions of driving with half an hour between each one but starts afresh if you leave it for more than 2 hours. I assume the Jag does no such thing.

Just idle curiosity.

Mike

 

Hey Mike,
I'm not entirely sure but I believe it uses "instant fuel" to calculate, which is the last setting on the trip computer. This would explain why it wavers so much.

 

Yeah, I think it is some form of that, but there are buffers. When you are at a stoplight, inst. fuel goes to 0, which would drive range to 0 as well if it were a straight calculation. I believe it is some form of running average of instantaneous fuel over a set of time.... but I have nothing to base that on.

 

The way I saw it is complicated.

As you drive off I suspect it uses the previous trip average consumption
Then after X seconds or minutes or something it uses the distance travelled and fuel used for the new trip
After I filled up with fuel I could see that my range wasn't as high as I thought it should be from the previous journey once I had driven a short distance.
It was quite an interesting mental challenge to try and work out why the range fluctuated between 65 and 85 miles over say 4 miles travelling prior to adding fuel.
I tried teasing it by pootling along and that did quickly boost the range and then booting it which reduced the range but not as dramatically as I thought it would.
I'm sort of hoping someone actually *knows* the answer to this

 

This got me thinking.

It probably combines the instant fuel reading with the average reading.

I'm wondering, though, how accurate the fuel reading is. The "total fuel" seems to creep down slower than the gauge does.

 

I suspect (based on a heuristic analysis over many miles of boredom while driving), is that it is using an average mpg that is different from the one that is operator re settable on the display. I think I came to that conclusion watching it during playful attempts of modifying my driving to cause excursions in both directions on the mpg gauge.

I still do that with my XFR from time to time.

 

This all made me curious, so I consulted the "Electrical Manual" on my X300 CD.
The Range Display is computed from the speed signal, the fuel used signal, and the fuel level.
(Fuel used is computed from a pulsed signal from the engine management system)
Both Average and Instantaneous Fuel Consumption are computed from the speed signal and the fuel-used signal.
Distance and Avg Speed are both computed from the "Speed signal from the ABS Control Module"
From the context, that is the "Speed signal" mentioned in the range calc.

 

So fuel used is the real time amount monitored as being consumed for that trip I suppose?

Given the technology of the fuel tank sender using the amount of fuel in the tank would seem to negate much of the value of this calculation.

My wife will happily drive her car while the range is showing 25 miles when the next step is zero as it works in steps of 25 miles. We have only ever had one crisis when we couldn't find a filling station in an unfamiliar city and had to run on a range of 25 miles for at least 15 miles. She has a touching faith in this display.

 

I've found both the X300 and the Excursion range displays are skewed to the idiot-proof side...having driven both for a number of miles whilst they claim, "0 miles to empty."
But in all those cases, I knew the mileage when I topped up previously, had a high degree of confidence that no fuel had been pilfered from them, and had a keen appreciation of the max and min miles I could expect from a full tank. The most recent such "nervous" run was in my X300 was from work in Hurst to the Beach & Western Ctr. Kroger location, about 10 mi. of stop-n-go city traffic with range showing 0. I put 19.74 gal in to top-up vs. a total capacity of 21.3.
I recall one top-up in the Excursion that was 42.5 gal against the 44 gal capacity. (I'm much more comfortable with 1.5 gal in the X300 than 1.5 in the Ex!!!)

Haven't logged enough time/miles in the S-Types to get to know them that well...but I have a sense they are also of the alarmist bent....however, in their case, I guess some propensity for the pump to overheat and fail if you routinely run them low?

 

In defence of the X300, I think the fuel used part of the equation is considerably more sophisticated than using the tank contents gauge. As I understand it, it counts the number of injector firings, and applies a standard quantity assumption to each. Thereafter an algorithm is used to calculate the consumption and range, based on instant and recent speed and consumption.
Hope this helps.

 

But to calculate your range it needs to know how much is in the tank and that's basically down to that ping pong ball on a stick in the tank. It stuns me that such a crude device is still used but it does seem to work.

 

I was only talking about the fuel used part of the equation, which seemed to be the question raised earlier. I agree that tank contents are an element of the range calculation, and rely on the tank contents gauge. I am not sure what alternative you have in mind. There are certainly different types of senders, such as those used in aircraft, but as well as being much more expensive, these can also be inaccurate, especially if there is water in the fuel tank. The inaccuracies with the existing sensor type are mitigated to a degree by the fact the calculations are made frequently. That said, there would seem to be a built in understatement of range, due to the contents of the filler pipe not being recognised by the contents gauge, and the fact that the contents gauge is designed to show empty when there is still fuel in the tank. On balance, I think I prefer a built in understatement of range, as it does allow a margin for error. My own current record for 0 range travel is 30 miles, but I would have to say anxiety levels were pretty high towards the end, and I chickened out at that point rather than pushing on to the next filling station 3 miles up the road!

 

Once I cut it fine on a long trip in the XJR

It said range about 10 miles from memory
It took 67 liters to fill, so I must have had about 3 litres left
I usually fill up when it drops to 1/4 tank, as I read here once one can cook one's fuel pumps in the tank if enough juice is not sloshing around them..