Hey Guys,

So I was driving along today (high 80's here loving the top down), and I had the trip computer set to Instant Mileage. While cruising down a hill with my foot off the gas I was surprised to see the mileage reading jumping around. Initially it jumped up to 70mpg, but then while the car was coasting downhill at close to 80 the mileage starting jumping around between 14 and 21 mpg.

So my question is... How intelligent is the trip computer ? Is there something wrong with an injector and the car is really consuming fuel while coasting or is this just a novelty feature in the car and it's prone to errattic (sp?) behavior ?

Thanks for the thoughts.

Dave B

 

Yeah Dave B, I get the same low reading with foot of the gas. Strange, would have though I'd be getting at least 100 mpg ! Perhaps it's doing 1000 mpg and computer can't cope !
XK8 4.2 2003
George

 

Mine jumps around too but in the end, when I fill up, I find that the computer is pretty much spot on. The gallons used is usually correct to the 10th of a gallon so the MPG is correct too.

 

I have previously posted also about my impression that the mpg function of the computer in my XKR is absolutely the LEAST accurate of any cars I have owned that have this function. First, I noticed that the average mpg I calculate manually (I fill the car up, note the gallons, note the mileage since my last fill, and then do the division) is always significantly less than the computer is telling me (e.g., I calculate 14.5 and the computer indicates 15.7). In any other car I have owned the difference has always been maybe one or two tenths. One explanation possible would be that the use of the larger (20") wheels has not been properly calibrated into the onboard computer but that does not seem likely as the speed readout on the speedometer seems to be in the right ballpark.

Second, in my previous posting I had also noticed the strange behavior of the mpg readout when coasting. This happens on a long downhill stretch of freeway near me (405 North from Los Angeles into the San Fernando Valley) that is at least a few miles long. You can remove your foot from the gas pedal and still easily maintain/coast at 60+ mph. When I do so in my Corvette, the instantaneous mpg read-out goes up to the max 99 mpg. However, when I am coasting down in the Jaguar, the instant readout keeps bouncing back and forth between as low as 14 mpg to up to maybe 50 mpg. If I gently push the pedal enough to just open the throttle blade, the instant readout keeps steady at between 30-40 mpg. It is definitely weird that the mpg readout would drop to 14 mpg while coasting with NO foot on the pedal.

Doug

 

[quote=o2manyfish;140563]

So I was driving along today (high 80's here loving the top down), and I had the trip computer set to Instant Mileage. While cruising down a hill with my foot off the gas I was surprised to see the mileage reading jumping around. Initially it jumped up to 70mpg, but then while the car was coasting downhill at close to 80 the mileage starting jumping around between 14 and 21 mpg.

From the original post above as I understand things,

1 cruising with foot off gas in gear will use LESS petrol than coasting foot off gas out of gear. Because the ECU etc will shut the fuel down to the absolute minimum when there is no input from the driver. Even though the tacho still reads 2000 rpm, there is hardly any fuel going in. It "knows" it is still in gear, and that you don't want it to "do" anything and are forcibly putting it on a diet..

2 However coasting in neutral will use MORE fuel because the engine will be running at Normal idle speed or slightly above just as if your car was parked. It "knows" it is not in gear and does not "expect" anything from you, and so goes into "self preservation" mode and feeds itself how much it feels like..

So the jolly old Jag computer will give a much higher reading for 1 and a much lower reading for 2.

Although I do not trust my computer at all. I have a feeling that it is linked through the fuel gauge sender in the tank and calculates backwards. from the point of, I know what I began with, I know what I have left, I know what distance was covered since I last did my calculation, thus that is what I am using right now.

ie. I start with 10gal, I drive 10miles, I end with 9 gal, thus I get 10 miles to the gal. If I rock the car or take a hill or a curve the fuel sloshes around, moves the intank sender and "tricks" the computer in to thinking that there is more, or less, fuel in the tank than is actually present.

I do not know how often the Jag computer makes this calculation.

Any Tech's or specialists want to rip me to shreds feel free. (I'm also here to learn).

Cheers,

 

Hi,

The MPG calculation is all done by the engine ECU. The amount of fuel injected at each stroke is minutely calculated by the ECU. The amount injected, together with the distance travelled, is used to calculate the MPG figure.

The fuel sender (and therefore the fuel tank contents) is only used in conjunction with any 'distance to empty' or 'Range' calculation.

You will expect to get a high mpg reading with your 'foot off the gas' going downhill, as virtually no fuel is being used by the engine in this instance.

As SesmicGuy says, fitting different sized wheels will affect the calculation, how much depends on the actual rolling circumference if the tyre, rather than the wheel size. Big wheels (20") usually have a much smaller side wall than smaller ones (ie 17"), but the rolling circumference of the whole wheel & tyre assembly remains roughly the same.