993 MPH speedo reading vs OBC computer reading

[chrisgaia]

Hi people, there is a descrepancy of 1-3 MPH between my needle speedo and OBC speedo. It very slightly worsened when I had my odometer fixed. Am I right in guessing the OBC should be correct as it is a.. err, computer!? Needle is obviously more mechanical and more prone to? Thanks.

[otolith]

My RX-8 consistently did the same - and the speed display was digital. Deliberate fudge-factor because they must not under-read, I think.

[SRT Hellcat]

stick a sat nav on the dash or as I do now use a speedometer app on my phone and then you will know your true actual speed. Typically your Porsche speedo will over read by 10%. Good for speed cameras. I do this with all my cars to get a solid baseline. My truck is almost bang on. The Cat under reads. 189 through a speed beacon against 184 on the speedo

Edited by SRT Hellcat on Thursday 14th September 23:36

[chrisgaia]

SRT Hellcat said:

stick a sat nav on the dash or as I do now use a speedometer app on my phone and then you will know your true actual speed. Typically your speedo will over read by 10%. Good for speed cameras. I do this with all my cars to get a solid baseline.

I was thinking of that but was not sure if it would be more or less accurate. Do sat navs really give the most accurate readings? Probably obvious but I'm not a GPS expert!

[stewart rix]

The OBC uses the same speed connection as the needle so should read exactly the same. Satnav will give you the more accurate level ground speed; my speedo reads just about 8% over at 70mph but very close at 30mph according to the satnav.

[Dammit]

My 996 does this as well- needle speed is 5mph under digital speed. It's annoying, but I don't know how to fix it. Neither do I know which is correct!

[chrisgaia]

Thanks people. I will talk to the cluster repair guy and report back, there seems to be some interest in this.

[AW10]

GPS measured speed is quite accurate when travelling in a straight line at a steady speed in an open area where the GPS aerial has a good "view" of the sky. They're typically not that accurate in urban areas where there's a limited view of the sky and signals are bouncing off buildings (called canyoning) or if your speed is changing.

[SRT Hellcat]

agreed. M25 steady speed. on the older 911's I usually go with the app and go with rpm in 5th gear. Easy to remember then

[Dammit]

chrisgaia said:

Thanks people. I will talk to the cluster repair guy and report back, there seems to be some interest in this.

That would be great, I'd be next in line to get mine fixed if you succeed.

[ras62]

I had my speedometer recalibrated while it was being repaired. Now 28/9 actual reads 30 and it is spot on at 70 mph when read against a sat nav.
highliftproductionshop VDO Electronic Speedometer repair service on ebay.

[bgunn]

Dammit said:

My 996 does this as well- needle speed is 5mph under digital speed. It's annoying, but I don't know how to fix it. Neither do I know which is correct!

Thing is, the needle on a 996 is fecking useless as it's calibrated in a stupid way. I don't use it for at-a-glance checks at all.

[Dammit]

No, I always rely on the digital readout- but that doesn't mean I don't want the analogue read out to work correctly.

[bgunn]

Dammit said:

No, I always rely on the digital readout- but that doesn't mean I don't want the analogue read out to work correctly.

Realistically, how can you tell? The calibration of it is so so awful for reading that you'd be hard pushed to know.

[Dammit]

Probably only when trying to drive at a steady 50mph through roadworks- not an enormous part of my life for sure. But, I can't stand things not working. Largely why my first year maintenance bill for my 996 was £22,000.