Carlo_Carrera

Reviving this old thread.

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

mhoefer

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.

mhoefer

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?

5CHN3LL

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."

TheChunkNorris

That dude posted that 20yrs ago.

5CHN3LL

<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/
...

TheChunkNorris

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.

5CHN3LL

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

TheChunkNorris

Not even close.

Mike Murphy

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:

SealG996

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.