What's up with 996 Odo / Speedo inaccuracy?

Recently bought an 04 GT3 and noticed that the speedo read fast. This weekend i plugged in an aftermarket GPS and did some testing. Odometer reads 5% fast as tested 2X over a 20 mile fwy section. 20 actual GPS miles = 21 indicated miles Speedo reads fast, but not 5% fast???? I'm referring to the digital speedo, not analog. At actual 70 speedo reads 77 At actual 60 speedo reads 67 At actual 25 speedo reads 29 There are several threads with people discussing this, but no answers as to why it occurs other than Porsche designed this as a "safety item" to save us from speeding tickets. That's gotta be BULL ****. All my other cars are accurate within 1%. The car has original size tyres that are at proper inflation pressures. What else in the speed / distance sensing system could alter the accuracy??? Thanks Craig

My speedo shows it is faster then the GPS. Though going though construction speed radars. The radar is dead on with my speedo and not the GPS. Also last ticket I got the cop clocked me at very close to what my speedo read. Some people believe the GPS is correct, but I go off my speedo.

The tire/wheel combo fitted to the car. But I note you state the car has the stock wheel/tire setup. About the only other thing I can think of is there is some fudge, err correction factor that is not set properly. My 02 Boxster's speedo is around 5mph fast at 75mph. The car has the stock 17" wheels/tires. This 5mph error is consistent from 25mph to well, above 75mph. My 08 Cayman S with 18" had no error. My 03 Turbo with 18" wheels/tires has a 2mph error. Other cars have had similar ranges of errors. My 02 VW Golf TDi had around a 2mph error while my 06 GTO was on the money. BTW, all of the speeds have been confirmed using a scan tool to view OBD2 speed and compare this to vehicle speed. I will note that I have also compared OBD2 speed to GPS speed and found them to be in agreement. Also, whenever possible I note road side speed radar readings agree with GPS and OBD2 speed and confirm the car's speedo is optimistic. (One has to be very aware that the radar may not be picking up his car but the one from behind. So in traffic I discount what the speed reading is but I many times pass these road side speed radar setups at times when my car is the only car on the road and then the reading is steady and reliable.) However, all cars have had a very accurate odometer. Well make that an odometer that agreed with a GPS device. For example, early in my Boxster ownership I drove my Boxster 200 miles and the difference between the car's trip odometer and the GPS was on the order of a tenth of a mile.

European regulations mandate that speedos not show less than actual. So european cars have built in errors ( - 3mph) to comply.

Amazing that there are hundreds of posts, (do a search), regarding this issue and no real answer other than "european cars are mandated to be inaccurate". I currently have 3 other "german cars". Their speedo / odo is accurate +/- 1%. None of them are 996s. Sure sounds like this is an individual car, (many cars), or Model or Model Year issue to me. Lottsa people here drinking the kool-aid. have fun, Craig

The speedometer of literally every modern German car I have ever owned has read high: VWs, Porsches, and BMWs. The VWs were way high, as much as 5 mph at 80. I just consider this problem to be part of my speeding ticket evasion tactics.

No experience with BMWs and only one VW but after I confirmed the odometer was accurate in my Boxster I really do not think much about the speedo error. I just allow for it even if it means passing a CHP car driving under the limit in another lane with other cars afraid to pass. If the limit is say 65mph and it is safe to go that fast I'm going that fast and I know that the speedo reading 70mph is actually 65mph so I put the needle on 70mph (actually get the digital speed reading to 70mph) and don't look back.

It baffles me a bit too. Our Cayenne speedometer always reads exactly what the GPS reads as well as road side radar signs. Our Audi was the same way. My 996 is always 3-1/2 to 4 mph high, regardless of speed.

My '02 C4 cab speedo reads ~5mph over actual, my '02 S8 reads ~5mph under actual (w/ 20" wheels - stock are 18") and my H2 is just right. But who cares? It's all good...

Quotes source post: 10638925

If one doesn't buy the explanation German product laws penalize an automaker for having a pessimistic speedo the other explanation is the speedo reads off due to the wheel/tire combination fitted. For some models Porsche offers 2 maybe 3 wheel/tire sets. It could be that say in the case of my Boxster which nominally is fitted with 16" wheels/tires but came with 17" wheels/tires fitted. Thus the speedo reads optimistic or more optimistic than it would have read with the 16" wheels/tires fitted. And 18" wheels/tires might change this reading yet again. The Turbo came with 18" wheels/tires and the speedo error is just 2mph. Maybe with 19" wheels/tires the speedo would be right on the money? But how to explain the odometer appears to be right on the money even though the speedo reads optimistic?... (Is there some adjustment to say the odometer but not to the speedo?) To be sure it is a bit of a puzzle but not one likely to be solved. All I've done is to characterize the speedo error in any car I own/drive and thus know what the car's true speed is regardless of what the speedo may report.

Quotes source post: 10639118

A change in wheel size is irrelevant, as long as the overall diameter of the wheel/tire stays approximately the same. Which they typically do in Porsche offerings... Example taken from some tire spec sheets for OEM 996 size tire/wheel combos Fronts: 996 with 17" wheels running 205/50-17, 25.1" diameter, 827 revolutions per mile. 996 with 18" wheels running 225/40-18 , 25.1" diameter, 828 revolutions per mile Rears: 996 with 17" wheels running 255/40-17, 25.0" diameter, 834 revolutions per mile 996 with 18" wheels running 285/30-18, 24.8" diameter, 840 revolutions per mile. Going from a 17" to an 18" wheel with the corresponding change in tire width and profile results in tires virtually the same size... less than 1% difference. There are bigger variations between brand of tires in the same size than the change resulting from ordering 17" or 18" wheels.

Just checked mine today and I also have a 4-5 miles/hour faster than gps reading. It also seems to be consistent at all speed.

Quotes source post: 10609787

Comrades, I too have an optimistic Speedo, which is weird, but once mentally adjusted for it is what it is. Having read threads on this a year + ago, I recall some saying they would take it to the dealership and they would fix it, and others saying they would take it in and the dealer would advise they couldn't fix it. So there is some type of fix, but it doesn't appear to be global. HOWEVER, the bigger issue here is if the PO is right and the added speedo is clocking additional mileage. I also recall reading a thread worrying about that as a natural progression from the, "if It says I'm going too fast is it also saying I'm driving too many miles" discussion. I never tested mine vs. the GPS, but this may not be a bad idea; although admittedly I'm not sure what exactly we would do with the info if all of our Odo's were off by 5% to the high side besides litigation. I'll test mine in the next few days, today is crazy busy. Good luck, DS