gnat

I noticed the other day that it was saying that I've spent almost 20 hours in the car for the 500 miles I've done. That seemed way high, but assuming the average speed was correct then it would be right.

Tonight I finally filled up for the first time and while the MFD says 24,6, my calculation says 22.5. Even had my wife check since I suck at math.

So that says something ain't right. Anyone else had such issues? Is this just a Porsche quirk?

user 83838290

bmws, MBs, skodas, vws etc I drove in the past were also way too optimistic, bmw was the worst. same with the speedos, the engineers must be smoking too much pot the way they program the in car computers.
anyways, my p!g always displays something between 7.8 and 12mpg and that seems accurate enough

gnat

I've noticed the lack of understanding about speedos in the 996 forum too. Speedos over reporting isn't an error, its intentional and done to protect the MFG legally.

Your speedo is impacted by your wheel/tire size even to the degree of simple tire wear. As the speedo has no knowledge of that, it can't adjust accordingly.

In some countries (Germany?) the MFG can face some stiff penalties by the state if their speedos are found to be under reporting. In others it is just a liability issue where the owners could sue for getting a ticket. Because of this, they usually have them over report in the 5-10% range.

That fudging, however, is only supposed to effect the displayed value. The odometer is still supposed to be accurate (sans wheel/tire discrepancies) as well as the telemetry data the ecu gets.

Any time we've calculated the mpg and compared it to the 996 or e90, the difference is within a reasonable rounding error. 2 mpg isn't a reasonable rounding error...

steve_Cayenne

Yes, much like gnat states. Speedo error (mostly due to wheel/tire size) can be +/- 5%. UNECE Regulations which are the law in most countries (but not USA I believe) state that speedo can never read lower than the actual speed but can read higher by 10% + 4km/hr. Easy engineering fix, overstate speed by 5% and now your error is within -0% +10%, legally biding but most drivers are then going 5% slower than speedo states.

If the car does not know for sure how fast it is going, distance travelled and fuel efficiency can be no more accurate, and can be thrown off by tire size and inflation level.

My Cayenne does not have PCM/GPS and when I use an external GPS to compare (+/- 1%) my speedo is showing ~5 km/hr higher at highway speeds (no surprise).

Don't know how they calculate/fudge mileage, but my daily trips to work are almost always 10.5-11 l/100km per computer trip 1. Trip 2 which I never reset shows 11.6 (seemingly higher than the sum of the averages). Pump litres vs reported km usually 11.5 (the two times I checked). So in my case the compter reported individual trips report more optimistic, and somehow the long term trip 2 more accurate.

For MPG folks: please take note with l/100km the higher the number - the more fuel burned - less efficient. Which is opposite of MPG, the higher the number - the more miles - more efficient.

user 83838290

I know it's supposed to read high due to EU regulations, but why does a EU version of lexus read +2km/h and a bmw at the same speed +8km/h? porsche reads about +5km/h. with todays technology, I'd accept ±3km/h, but 8km/h is way too much at 100km/h.

anyways, MFDs are much like proclaimed mileage in the EU. you buy a car that's supposed to get 20mpg according to some ECE norm and you're lucky if you get 15mpg combined. you can sue the mfg all you want, it will get you nowhere.
I presume it's a little better in the US as I read somewhere that Hyundai and KIA have/had to pay tons of compensation money as they underestimated gas mileage for their cars.

gnat

8 at 100 is only 8% which is in the allowable +/- 10%. I agree that sucks though.

Really with as cheap as GPS modules are these days (not NAV, just GPS) I'm surprised that they don't use one to keep the speedo and odo calibrated.

steve_Cayenne

Don't know about the Cayenne, but my BMW (no nav) still has a GPS chip since I can go into the iDrive screens and see my location numerically!

gnat

Gave it a half tank in prep for our trip to FL tonight and it came out to 22.1 vs the 24.3 that the MFD reports (since I knew I was going to fill up before the trip I had some fun with this partial tank ). Sadly I won't be able to report stellar numbers from the trip as we're using a cargo box, but I'm still expecting mid to high 20s (actual) which is still pretty damn good I think.

I haven't checked to see how far my speedo is off, but I'm going to guess its probably about the same 7-8%. I suppose it should be expected, but its disappointing all the same (getting really happy about a 35mg trip that obviously wasn't, etc..).

grohgreg

One would think these stated deviations would extend to the miles remaining number, but mine actually seems quite accurate. I've taken it down to <30 miles remaining a few times, and each time the pump says something just shy of 26 gallons. I'd consider that reasonably accurate. That said, I have a GPS app in my iPhone. Checking mph and distance accuracy just went on my ToDo list.

//greg//

gnat

It could get through Georgia, but she'll whimp out and fill it earlier.

What I find interesting is that the average mph reading matches the 65/66 that my gps app says we've been doing when the speedo says 70.

gnat

Filled up after 627 miles and this time the MFD was only about 1 mile off (28 vs 26.9).

At 70mph my speedo app says we are doing 65. At 80 it bounces between 76/77.

So it would seem that the higher the speed the more accurate it is. Not going to test that theory though

Good news is that even with a cargo box on top and a wife that won't keep it at the sweet spot (too fast) it is getting 27mpg!!!