Has anyone noticed a difference between the mpg reading on the computer screen under the Tach compared with the reading on PCM? Mine seems to be off by anywhere from .2 to .5. Maybe just a software bug. When I fill up I reset the entire trip data on the PCM screen. After that I check the screen under the tach and it shows zeros as well. Once I put 20 or so miles on the car I start to see a difference. All of the other options are always correct.

 

I have not noticed that yet but will keep an eye on it. The avg speed is the same.

 

I've noticed the same thing. It doesn't happen all the time but the difference has been about .2 to .3 off. Oddly enough the "miles to empty" doesn't have any variations even when there's a difference in mpg.

 

Have you mentioned it to your dealer. I am stoping by mine this Friday and am planning on asking them about this. I'm hoping it's not a start of problems to come.

 

I think it's just a difference in how each system computes the mpg. On occasion when they have been off after a while they come back into agreement.

I recently went up to J-ville and they were in agreement for the trip up and off by .2 for the return. Again, the miles to empty and the average speed were in agreement. Tis a puzzlement

 

I noted the same thing as far as the discrepancy between the mpg figures...
On a computer type issue--is anyone losing time on their clocks? I seem to be losing about one minute per day--it's minor, but annoying to be continually resetting my clock. My BMW's clock was never off.

 

Yes, my clock is not accurate.

 

I stopped at the dealer yesterday to talk about the MPG issue. I was told that there are 2 different systems in the car. The one in the dash is the most accurate. The one in PCM is "fed by a satelite and is less accurate." Not sure I buy that but that's what they said. I mentioned that some folks are saying they are loosing time on their clocks and was informed that happens all the time and has been happening for years. I guess that means it's a known issue, but not one worth trying to fix??? Needless to say not a worthwhile trip to the dealer.
Only good part was I finally got the wife to test a Cayenne and she was pretty impressed. Here's hoping she still remembers it when it comes time to replace her 325!

 

That's the first time I've ever heard of a satellite link.I find that really hard to believe.

So far as the clock is concerned, mine looses time as did my Boxster before it.

 

PCM only updates it's time from satellite if the navigation module is fitted. This allows it to synchronise with the v accurate Global Positioning System.

 

Thanks for the info on the clock fellas--guess I won't worry about it...the rest of the car is fantastic...I suppose I can live with resetting the clock, as long as I get to listen to the sounds that flat 6 makes every morning!!!

 

Here is another related question. No matter how conservative or hard I drive my MPG shows 18.5 to 18.8 MPG. Never changes beyond that range. I haven't done a manual calculation with the gas receipts. Anyone getting in the 20's as advertised for highway milage?

 

On two recent road trips I got 23-24 mpg based on the PCM readout. My speed was on the high side, around 85-90 so I would expect better mileage if I slowed down - but why bother My "S" had less than 1000 miles so i would expect some improvement as the mileage increases.

 

I did a week straight of highway driving for work and got between 23.5 and 24.5 on the highway the whole time. Usually running around 80ish. In town I'm around 18 to 20 depending on how much I sit in traffic. It seems strange that your's is always the same. Is that both highway and city or just city? also do you reset the system after you fill up, if not then it's just your overall average since the last time you reset it. In that case it won't change very much.

 

I reset my trip meter (hold the button) everytime I put in gas. I haven't really played with the PCM controls. I'll look at it that way as well as see if there is a reset on the PCM. Thanks for the suggestions.