Do manufactures purposely manufacture speedos that over read. eg: read 65mph when the car is doing 60mph if so why?

Cheers

 

Yup

Can be as much as 10% on the conservative side

Cheers

 

Cheers Si, any idea why or is it a legal thing(speeding fines etc) if so is it a gentelmans agreement or is it in print somehwere, just need some hard proof for a pub argument hehe..

Cheers

 

well not allowed to underead but can legally go +10% so guess it's bound 2b a bit over 2b on safe side
Si

 

Legally they can read up to 10% over but not under...for this reason they work in a degree of safety by over-reading.

Assuming my tacho is correct then at 3k rpm in 5th gear I'm doing exactly 72mph but it shows about 78mph on the speedo. When in 5th I never look at my speedo but I always know my speed....cunning huh

 

Thanks lads, has any one got any links to hard proof
DOT sites etc, is it written anywhere in type aproval laws?

Cheers

 

Travelling back north on the M40 in the middle lane late yesterday without the knowledge that a police Omega estate was coming up the fast lane. Doing 80mph on the speedo whilst the road angel was reading 74mph. Decided to pull out to overtake a vehicle infront of me. Got to 95mph on the speedo (didn't look at the Road Angel) very briefly and slowed back down to original speed upon completion of the move. The Omega didn't even bat an eyelid. Probably looking for a donut shop.

 

And dont forget that if you increase the rolling diameter of your wheels then your Speedo MAY be more accurate. This was proved with a Fiesta at an RR day not long back.
It showed that a 2" increase = the 10% shortfall in the speedo reading, for a Fiesta that is.
Doc

 

My 02 WRX reads + 6mph, checked using my Snooper.

As mentioned above the law is -0 +10% maximum.