I have been enjoying my XJ, but I have found that the MPG calculator is not even close (not that I am expecting it to be spot on). But, I would think that they can get it more accurate than reading 10% high. It is consistently that high. I have done my own "calibration check" of the tripometer and odometer and they are within 0.1 miles for a 40 mile trip (using the milemarkers on the side of the road as my reference). The speedo is even pretty accurate reading 0.6 mph slow at 70 mph. This would leave me to believe that my error in the MPG calculation is in the way the computer calculates the amount of fuel being used. Does anyone know if this parameter is even able to be adjusted to make the MPG reading more accurate? Or is there something else that can be adjusted to make it more accurate? Just curious. I have gotten pretty good at calculating what the real mileage is off of this number by subtracting the 10% in my head. Am I wrong in assuming that the computer calculates the MPG by knowing how far the car has travelled and totalling up the time that the injectors have been open? Kinda like I would have to assume that the instantaneous mileage is a function of wheel speed sensor input divided by the injector percent time open.

Has anyone else noticed that their car gets 1-2 MPG better if they drive their car, get it up to speed (say 60 mph) and then reset the tripometer/MPG settings followed by say a 10+ mile trip where you can maintain your speed for that initial time. From there you can go back to your normal driving. This is something that I have noticed in my car that will happen. This is especially beneficial if you are doing a long trip in one stretch as you can fill up the car, get right on the highway and once at cruising speed, you reset everything. You can net about 30 more miles from the tank driving the same as you would before. I'm just curious if it is a quirk with my car or if this is true of other cars too.

If you can't tell, I do a lot of "playing" with my car when I am going on down the road. I like to make sure that the speedo is accurate when I get a newer car and the MPG thing was something that I was trying to eliminate potentials to get to what the error has to be.

I do like the actual mileage that my car gets. Getting 25.5 MPG (corrected) over a tank of gas is pretty nice and there are times that I can drive and see the MPG (not instantaneous) hover around 30-32 MPG (uncorrected). Really makes people wonder when they see my MPG numbers and then they start thinking about the size of car that they are in. I just laugh knowing that there is that slight error that they do not realize is there.

 

Maybe the computer is not factoring in long term fuel trims?

Anyways, I treat the mileage computer as a relative device.
The only thing I care about is whether the number is higher
or lower than during some other set of conditions.

You'd love the permanent radar setup on some roads.

Drive past and the numbers tell you how fast you are going.

Turns out with oversize tires my car is dead on.

 

plums, oh, I use those radar devices all the time. Remember, I live in Washington DC where if you aren't being taxed, they are figuring out how to tax you and those radar stations also have a camera with them to snap your picture so they can send you a bill in the mail. Those machines are just another one of the many types of "calibration checks" I do.

Like you, I use the MPG as a relative thing. But, I would like to see something a bit more accurate than being way on the side of relative.

 

Not all of those radar devices result in consequences.
Some of them can even be entertaining.

There was one on a particularly fast on ramp designed
to light up a strobe if going too fast ... (in their estimation,
whoever "they" might be)

Great gobs of fun lighting it up at night on the way home.
No good for calibration though ... eyes on the road

 

plums, around where I live, if they installed something that lit up when you went too fast, it would be perpetually on. In the DC area, you get blown off the road normally if you are not doing atleast 10 mph over the speed limit. Hell, I have been passed by cops like I was standing still when doing 15 over (keep in mind that they were simply travelling down the road, lights off).

Oh well, if you can, try out my "trick" with getting the car up to a rolling start and then see if your mileage is improved overall. Curious if this happens with other people.

 

All I remember is the beltway. And we weren't moving
very fast and sometimes not at all. Something to do
with Friday afternoon. Lots of black Chevy Suburbans
carrying various spooks working for lettered agencies
though.

I don't know if this is what you mean, but I zero the 'A'
memory on every long trip as I hit the ramp. This of course
yields the best possible reading. This is not unexpected since
it drops the readings from surface road driving to reach the
highway.

If the traffic is slow but steady on the way out of town,
then the mileage is much improved later on since it
is all part of the average.

 

Something may be wrong. Here, the shown MPH is not allowed to be lower than the actual. Commonly the car (as my STR) knows the true speed (VSS) accurately but then the software adds to it (e.g. 10% + 1).

I expect the MPG is figured from VSS.

However, your VSS sounds to be wrong. You can read it via OBD to check and if it is wrong I'd be wondering why.

I suppose VSS might be right but the cluster wrong such as having the wheel size data wrong.

 

You're saying that you are affecting the actual mileage you get from a tank by resetting the display?

 

James, I cannot explain it, but I have noticed a difference in the readings. Why, I am not sure. But, some of the curiosities that I have noticed. hence why I am asking others to see if their cars are doing the same thing. Maybe I am seeing something that really isn't there. I know these cars are affected by the outside temperature. Amazing how much of a change there is in mileage between a cold day and a hot day even using the same tank of gas.

 

Resetting the display isn't going to affect your gas mileage - but can easily affect the readings you get. As you said, if you start out on a trip and reset it, then drive some distance before you reach an Interstate highway where you can set your cruise control and maintain speed for long distances, the average will be reduced by the lower MPG achieved on the stop-and-go driving before you enter the highway. Waiting until you are actually on the highway and up to speed before resetting would result in a higher average MPG reading on your display, but is not going to change your overall gas mileage or extend your driving range.

Dwayne

 

It seems to me that the display does not affect the mileage, because the mileage can be viewed using external equipment.