Speedo Accuracy

Been thinking lately how inaccurate our speedo's are, or mine? I know cars have to stick to tolerances when manufactured and most read higher than actual speed. Considering mine has an increased rolling radius of 3.5%, at 70mph on the speedo, my sat nav says 67, so would probably be 65 on original rolling radius. Either that or my sat nav isn't that accurate? Has anyone else spotted this?

I have never noticed it but then again ive never run the twat nav while driving  .... i think the police have or meant to allow 0- to + 10%  for inaccurate speedo calibration .

Yeah I was thinking it was about 7% either way. I know my sat nav shows 69 when my transit is doing 72. My sat nav is 8 years old though  

would be nice to see mine work at 75+ it starts flapping about, then sometimes reads 0 mph then it might start working again  

Do you give it the magical tap here and there 

ha ha yeah normally works, but sometimes not, so i tap harder  

I did have a spare set  that i gave to stu ...not sure if he put them in the white r before he got shot of it .....?

my pulsars speedo is absolutely spot on with the satnav amazingly enough.
heres the thing though my gtr is way out (whilst abroad lol) at 186mph on the clock ((which is a digi display) the satnav shows 169-170 which means the clocks way out.
ive actually seen 218mph on the clock but in reality it will be less than 200 and that's with correct size oem wheels

That's a fair bit of difference Bob. I'm thinking of running a slightly bigger tyre to adjust mine slightly.

yes not very accurate at all really

you could just hold the needle on the speedo with plastic cover off and move it back a tad to see if that sorts it but personally I wouldn't worry about it.
its better that your going slower than speedo reads as you wont get nicked that way, other way round could result in a few tickets lol

This is true...

Mine is so far out the faster I go the slower the needle moves,I know 20mph is really 30 but after that its a lottery.

ha, mine started reading 70mph whilst i was stationary on the drive  the only bit that annoyed me was the mileage started clicking up as i'm on a limited mileage policy  

toaster55 wrote:ha, mine started reading 70mph whilst i was stationary on the drive  the only bit that annoyed me was the mileage started clicking up as i'm on a limited mileage policy  

You were going how fast on your drive   
Never heard of a speedo doing that before?