Speedometer calibration?

It seems like my speedometer is reading between 3 and 5 MPH on the high side (i.e. showing faster than I'm actually going). This is based on readings from a couple of those radar signs that show your speed. Does anyone know if the dealers can do a good job of calibrating the speedometer? Any other thoughts on calibration? Drive on! Jim

At what speed? Above 80MPH that's probably acceptable, within reasonable tolerance. Indicating 40 when doing 35, no. Did you change out your wheels/tires from original? Tires worn thin or not fully inflated?

From 20 mph and above all 996 speedos run 4-5 mph higher than actual speed all model years , even TTs . There is a TSB for some early '99 cars for speedo recalibration, but that was a software glitch where the digital speedo didn't match the analog pointer. There is no fix for the former problem. Article in Car and Driver 2 moths ago said Porsche and BMW both purposely do this to compensate in case owner puts on bigger tires with larger diameter , since BMW and Porsche owners are more likely to do aftermarket wheel packages.

as the previous poster indicated, they allow margin for different size wheels. Funny thing is they can't adjust for 19 inch wheels. My '99 has always been off by 4MPH with my 19's. Just gotta live with it

I have the 18" factory wheels on the car and I had assumed that they would calibrate for the wheel size but your posts make sense. I guess I'll just have fun thinking I'm at 75 and watching the spoiler pop up! Thanks again for the input. Drive on! Jim

On the C5 Corvette, a microprocessor compares rpm / gear with speedo readout and recalibrates speedo after a hundred miles when you change to aftermarket tires of different diameter so that speed is always spot on. Porsche should do something similar since it's owners change out tire packages a lot also. Speedo calbration running fast from factory on purpose seems so low tech in 2002.

MKW, As soon as you said Corvette, I had my doubts. I don't believe a car can calibrate it's own speedo based on the rpm/gear if you change the diameter of the wheel/tire. Or, another way to put it, the self calibration is based on the fact that for a set rpm/gear, the car will travel a set distance based on the circumference of the stock wheel/tire combo. Therefore, since the self-calibration assumes that there is a correct speed for a set rpm/gear, the calibration would definitely be off if there is a different wheel tire combination that results in a different distance traveled for every tire rotation. The only way to accurately calibrate a speedo is with a measuring instrument independent of the car.

Palting , I know what you are saying - but somehow the C5 does it. Just ask any C5 modder. C5 owners were skeptical initially , too. Even Kevin Brown, the C5's initial roll-out chassis chief confirmed its existance. He did say if you put on a giant truck tire, it would be out of recalibration range.

If Vette recalibrates itself, how come the Hypertech Power programmer advertises that it can be used to calibrate for wheel/tire size changes?

The Hypertech Power Programmer is basically hype, like their name ,as any time spent on a C5 board will tell you. Some of their claims have not been borne out in testing. Only thing it's good for is changing shift kickdown sensitivity on an automatic. If you change tires +/- 1/2 to 1" diameter bigger or smaller it will recalibrate.

I have a 1999 C2 with factory 18" wheels. The digital and analog speedometers match, however they both read 10% high. When it reads 88mph, I'm really doing 80. At 44mph, I'm doing 40. I have verified this by both highway mile markers, GPS and police radar (no, not by receiving a ticket). I have taken it to the dealer (Brumos) and asked if it could be calibrated. It was returned to me in the same condition as I took it in. I don't buy the "done on purpose because Porsche owners have a tendency to put larger tires onthe car". This just doesn't make sense to me. Of course what does make sense is that Porsche would do it on purpose to save on warranty work. If the speedometer reads 10% high, then your odometer will also show 10% more miles on it than actual (also verified with highway mile markers and GPS). Porsche is cheating its customers out of 4,800 miles of warranty by having their speedometer / odometer read 10% high. Since the more expensive warranty usually occurs towards the end of the warranty period, how much money do you think Porsche saves. Now, don't get me wrong, I love my Porsche and think it's a great company, but I also don't like being cheated. BTW, Jaguar does the same thing.

Welcome to the club, so to speak. I took delivery of a 97 Boxster, first in the DC area and immediately noticed the problem. Did a whole thread on Pete's Boxster Board. Checked via marked distances, time and with the radar information carts the locals set out. 10% over is just what it read. Complained for 2 years. Then there supposedly was a software mod. Got that done, NaDa. I hoped that when I got my 996 that this problem had been solved. Seems like Porsche doesn't give a ??? about this issue. My 87 Bronco II is spot on speedo display. Seems like if Ford can do it to a vehicle that is just as likely to have wheel mods, Porsche could on a car this expensive. So now when I am going down the road it is all that new math I have to do, let's see actual times 10% equals... <img src="graemlins/soapbox.gif" border="0" alt="[soapbox]" />

10% above 40 MPH is about what I am experiencing on my 01. Not acceptable. Confirmed with GPS. Plan on talking to my dealer and Porsche. I don't get it. Why not fix a known problem? This type of error is not excusable. At some point somebody is going to get involved (government agency) and make Porsche look very bad and potentially cost them some $$. Regards Jim

Posted my results of a AAA dyno job on my car a couple of times at funcarsonline. Short answer is I have an 8% error factor on the high side across the board. Interestingly enough the test showed the odometer exactly correct, even if just a one mile test. Don't know if odometer is really off 8% also. It it were true you'd think some enterprising lawyer would have already have a class action going against Porsche. I was told by Porsche service that anything up to 10% was acceptable. Maybe to them, but not to their owners.

My car seems to be a difference of +2 mph on the digital readout and it's a '99. Seems like 10 percent is a bit too high. I have 18 inch rims. Regards. Mike

Reviving this old thread. I bumped it up to first page so someone who is looking for it can see it.

My 2003 Targa has 285/30R18 on rear, 225/40R18 front. It has upgraded stock 18” Porsche rims. My build sheet shows the 18s. Is this the correct tire size or could I go a bit bigger? Incidentally, I am off reading low by 7 Kmh at 100 Kph. Is there a programmer I can use to address this, speed app on my phone and Waze both verify, digital speed signs also.

I did confirm, for the 18" rim upgrade my car has stock, these are the correct size tires front and back. Still, 7 Kph off seems a lot, programmer?

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I don't know about newer Corvettes, but on my '96 C4, I had to use the HyperTech PPIII to change adjust the speedo for the gearing differences when I swapped the D33 for a D44 with 3.73 gearing. Maybe newer cars with built-in GPS can auto-calibrate, but my older Corvette definitely needed to be tweaked. The digital speedo now perfectly matches GPS-indicated speed... As far as the 996 - my 996 has ALWAYS indicated a few MPH faster than actual speed using the digital and analog gauges. I thought this was a known issue/fact about the 996 - there are countless threads on the Interwebs about it. As far as I know, there is no way to correct for this. An old thread in this very forum from 2013 ( https://rennlist.com/forums/996-foru...naccuracy.html ) suggests " European regulations mandate that speedos not show less than actual. So european cars have built in errors ( - 3mph) to comply."

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That dude posted that 20yrs ago.

<edit> don't pay attention to the village idiot. Also, I'm the village idiot...too dumb to know I got necrothreaded. I'l be in the corner counting my thumbs if anyone needs me. </edit> 2022 - 2013 is 9 the last time I checked... Regardless, the internet is awash with similar posts. https://www.theglobeandmail.com/glob...ticle11487709/ https://www.northamericanmotoring.co...-accuracy.html https://www.caranddriver.com/feature...meter-scandal/ ...

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I get you, I'm just saying the quote you replied to was posted in 2002... I don't think that person is going to see that.

Ah, f@ck. What you meant to say is "Schnell, you're a moron."

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Not even close.

Your 7kph difference is within the 3-5MPH range of most 996 owners , but is your speedometer reporting 7kph higher than you are actually traveling, or is your car traveling 7kph faster than what the speedometer is showing? If the latter, then I would think that�s a bit odd. The wheel and tire sizes on my car, I believe, are stock. Wheels & Tires Sizes: Front 225/40ZR18 Rear 265/35ZR18 Tire Pressures: Front: 36psi Rear: 44psi Ran a calculator comparing what you have to mine and looks like the total diameter is different:

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I went to 285/35/18 Michelin Pilot. Theoretically this would case speedometer error, cause traction control problems etc. The increase in diameter from 265/35/18 (Achilles dealer fit) has only reduced Sperdometer error, compared with GPS not eliminated it. The speedomter remains optimistic.