Speedo reading...

is it picked up from the front or rear wheels?

the gearbox waz

Hi Waz, I don't think so. On models with G50 gearboxes (up to 993) the speed was picked up at the gearbox. On most modern cars (including ours, I think), the ABS wheel sensors are being used. How they use the wheel sensor signals depends on the ABS and DME and is not always the same as the reference speed calculated for the ABS. Don't know how exactly it's done in our cars. I was actually tempted to nip outside, lift the back of my car with a jack and see what the speedo does, but my wife told me to get a life. Cheers, Uwe

I thought it used the wheel sensors too but I simply don't know. I asked the question because I'm thinking about going to a slightly wider/taller tire up front and don't want to throw my speedo off by very much. So if it was picked up at the rear I would be happy as the rears will stay 285/30-18's.

Have you checked your speedo to start with? If you're like most of us, it's already wacked. You can't throw it much farther off.

Actually I went by one of those "your speed is" radar things and it was pretty much spot on.

http://www.abbott-tach.com/era.htm nice little black box that will allow you to set your speedo to be spot-on. nice addition when you change wheel/tire size. i have one. dead on confirmed by gps and (involuntarily) radar...

when i was testingn a GPS program i had created at work, i noticed at higher speeds like 116mph, my speedo actually said 121, but at lower speeds in 80 and below it was dead on

Mine is pretty much %5 high across the board. I think you'll find that is common with 996's

The faster you go the greater the margin of error. If you have a margin of error of -5% which is what most cars speedos are at 50 mph you may be off by 2.5mph worst case. At 120 mph worst case you can be off by 6mph. I don't know the exact numbers but speedos have twice the accuracy they are set at but they shift the margin of error towards over stating your speed then understating. Liability of the car company is less if your going slower than it states.

Guy's, Uk law states that a speedometer may over-read by a margin not exceeding 10% however it MAY NOT under-read. i.e. If you are showing an indicated 50 mph you may be doing 45 but if your speedo shows 50, you cannot be doing 55. What any particular car shows is anyones guess, it all depends on wheels, tyre wear ect. But imagine you are showing 200 mph you may only be doing 180, and i say only in the loosest way Regards, Steve B

Most 996's seem to read 3-5% fast. A taller tire will make it more accurate.

I asked the dealer, 'cause mine definitely reads fast. He said it's set by Porsche to read +5 mph once you're above 30-40 mph. He said it was a conservative margin Porsche programmed in on purpose since so many people put on different size wheels and tires -- They didn't want someone to put on a bigger tire and then have the speedo be too low. Last week I drove on the freeway next to a new Caddy with a really big digital speedo. Confirmed that mine was +5 mph relative to theirs. Couldn't convince them to speed up to 100 mph to see if the difference stayed constant I thought I read somewhere that the dealer can do a software fix. Can anyone confirm this, and tell me how to explain to the dealer? My dealer didn't know about it.