Classic Audi Community General Chat » C3 Speedo - opinions please

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Old 19-01-20, 05:57 PM   #1
Ringmaster
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Default C3 Speedo - opinions please

Folks,

The speed showing on my sat nav matches exactly the speedo reading on my daily driver (E46), but the speedo on my beloved C3 always shows I am doing exactly 5 mph more than that same sat nav is telling me.

Assuming the sat nav and my C3 speedo are not faulty, could that exact 5mph difference be down to the speedo being set up for 15 inch wheels? The car has 14 inch wheels and always has as far a I know, or am I being thick here?

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Old 19-01-20, 06:46 PM   #2
tractor Dave
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Most older speedo's do over read by about 5-10%. This is a built in safety by manufactures who don't want to get sued if you get a speeding ticket when your speedo is showing 30 at a true 35.
I just found some nice Golf wheels for my boy's polo. They are 14" instead of the standard 13" that were fitted. We did some checks after fitting (using the sat nav) and now the speedo is 100% accurate. It previously over read by about 5 mph.
Newer speedos are more accurate.
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Old 20-01-20, 09:21 AM   #3
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If the incorrect reading was down to different wheel sizes, it would be a percentage difference (so it would always be, say, 7% out) rather than a constant MPH difference.

Maybe the needle has been replaced, and put at a different position?
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Old 20-01-20, 10:14 PM   #4
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Quote:
Originally Posted by Ringmaster View Post
Folks,

The speed showing on my sat nav matches exactly the speedo reading on my daily driver (E46), but the speedo on my beloved C3 always shows I am doing exactly 5 mph more than that same sat nav is telling me.

Assuming the sat nav and my C3 speedo are not faulty, could that exact 5mph difference be down to the speedo being set up for 15 inch wheels? The car has 14 inch wheels and always has as far a I know, or am I being thick here?

Now, I had this same thing happen on my 200, only it was the other way around. When I changed from the standard 15" wheels to the 16" ones I now have on I noticed on the nav I was using that 70mph on the motor way had gone down from approx 68-69mph to 65-66mph. I was in the habit of checking the speed against the nav so it was something I noticed pretty much straight away. I can see no other reason why the speedo would change the mph other than the change in wheel size so you're not being thick here.
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Old 21-01-20, 08:20 AM   #5
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I used this site for my boys Polo. It tells you the % change for the wheels size.
https://www.wheel-size.com
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Old 21-01-20, 04:09 PM   #6
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Thanks Chaps.....
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Old 22-01-20, 11:25 AM   #7
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Thanks to the fact magnets deteriorate with age my Cq now under reads by 4mph at 70mph (satnav says 74mph) My A5 is spot on accurate as I discovered now, until recently assumed it was under reading as cars used to do so always set my cruise control at 32mph in a 30
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