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post #6 of 9 (permalink) Old 27th June 2010
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Cool Re: Speedo readings

Originally Posted by rg1642 View Post
Hi all,
Does anyone know which is likely to be more accurate at 70mph. The car speedo or the Sat Nav?

My Sat Nav regularly reads 4-5mph lower than the speedo between 60 and 70 mph, just wondered which was more likely to be correct/
There is one way to find out--go over the speed limit with plod behind with the calibrated speedo.
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post #7 of 9 (permalink) Old 27th June 2010 Thread Starter
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Smile Re: Speedo readings

Thought as much, thanks
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post #8 of 9 (permalink) Old 27th June 2010
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Re: Speedo readings

Speedo's over-read by up to 10%, in fact they have to by law.

Why? So that when you get done for speeding you cannot blame it on the speedometer giving a dodgy reading.

For example when the speedo reads 70mph you are really doing about 65, which is a nice safe margin for your driving licence. If the speedo told you were doing 70 when you were actually doing 75...

This is why your Sat Nav gives you a lower - more accurate - reading.

The speedo calculates your speed based on how many times a second a wheel rotates (usually the front wheel), and the rolling radious of the tyre.
Change the rolling radius - by tyre wear or fitting different tyres - and the maths produces a different answer. This is why you have to be careful with tyre sizes when upgrading from 15" to 17" wheels for example.
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post #9 of 9 (permalink) Old 27th June 2010
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Re: Speedo readings

Hi i have a 1993 K reg espace and when it says im doing 30 im actually doing 25 i only found out by them signs that read your speed and either say SLOW Down or Thank you lol cos i was holding it at 40 after coming of a 40mph limit road and the sigh flashed 35 slow down. Is there a problem with the speedo or is it normal. ? ( i could do with out any more probs ive spent enough on the bloody thing so far. )
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