993 Speedo Reading

[taipan]

Hi there.

I own a 993 C2 which has been fitted with RS bodykit and really nice BBS LeMans 18" wheels. THe tyres are 225/40 front and 265/35 rear. I have always thought that the speedo read a little fast and this has been confirmed. My wife has just bought me a road angel which tells me that the speedo is reading around 10-15 mph fast depending upon speed. This is not good since it is both misleading and adding more miles to my car than it should.

I'm surprised that these wheels have made the speedo fast since a greater rolling circumference should slow it down. I suppose that this might be down to lower profile tyres than standard?

My question is does anyone know how I can get the speedo to be more accurate. Isn't my wheel/tyre combination about the same as the Turbo and if so how does that get an accurate speed (different ratio in the speedo or a gear somewhere in the cable?). Anyone know?

Cheers
Martin.

[AlanPound]

Bigger wheels would make the speedo under-read, but most speedos seem to over-read wrt a GPS, AFAICS...

Alan

>> Edited by AlanPound on Thursday 6th October 18:13

[manek]

I was surprised to find that the speedo would over-read on my 993 by about 5 per cent but, on the Aston I swapped it for, the speedo is pretty much dead on, according to the GPS.

That's British engineering for you!

[paultje]

My C2 overreads by about 5% also running on 17" wheels...at least you shouldn't get nicked in a speed trap!!

[porsche4life]

Seam to recall that manufacturers cannot sell a car that under-reads but can over read by up tp 10%, the odometer though should be more accurate. My BMW reads high against GPS so i adjust my speed to the GPS...

Anyway i sit to high in the porker to see the speedo, i use the "force" to determine mph (officer)

[manek]

los angeles said:

Manek- aa slight deviation from topic: why did you trade the 993 for the AM?

A moment of madness?

More seriously, I found that the time I have available to go out and just enjoy a blast is limited, so the 993 was doing almost all motorway/A road trips. The AM is more that sort of car - and the superb qualities of the 993 are wasted on that sort of driving.

Plus, I wanted a bit more exclusivity...