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I suppose this subject has been done to death over the years,but recently on the way to Billing and back I compared the speedo readings on the Carlton compared to those given by my brothers Sat.Nav system.
He read the Sat-Nav speeds and I concentrated driving at steady speeds.

We found that there was about a 10% over-read on the cars Speedo compared with the Sat-Nav,on speeds up to about 50MPH. Then the disrepancy decreased

70 on the car was 64 on Sat Nav.
76 on the car was 70 on Sat Nav.
85 on the car was 80 on Sat-Nav.
I found that in the road and traffic conditons at the time it was too difficult to compare higher speeds.

I assume the Sat-Nav is accurate,but dont know.

It explains why when I used to go by the cars speedo at 70MPH cars were passing me at about 80MPH. Jerry
 

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Still proud to be a
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i had lower profile tyres on my cav and it only over read by about 5mph usually. Tried gps in my mates ep civic and it was getting on for nearly 10mph out.
 

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Jaybooze said:
I suppose this subject has been done to death over the years,but recently on the way to Billing and back I compared the speedo readings on the Carlton compared to those given by my brothers Sat.Nav system.
He read the Sat-Nav speeds and I concentrated driving at steady speeds.

We found that there was about a 10% over-read on the cars Speedo compared with the Sat-Nav,on speeds up to about 50MPH. Then the disrepancy decreased

70 on the car was 64 on Sat Nav.
76 on the car was 70 on Sat Nav.
85 on the car was 80 on Sat-Nav.
I found that in the road and traffic conditons at the time it was too difficult to compare higher speeds.

I assume the Sat-Nav is accurate,but dont know.

It explains why when I used to go by the cars speedo at 70MPH cars were passing me at about 80MPH. Jerry
Dont forget theres loadsa hills in Wales!
 

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REALITY ASYLUM
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i read about it somewhere, the speed camera are calibrated at speed limit +10% +2mph
because a mechanica speedo can give a reading of +/-10% it became the norm about 20 or so years ago to set the speedo at -10%
So in theory if your speedo says your doing 70mph, is very likely you're doing 63mph, given a gatso is set to trigger at 79mph, at 79mph you'd be doing nearer 71mph so if my calculations are correct (at this time of the night its questionable) you're speedo can read 88mph and you still won't trip a speed camera because your only doing 78mph ish...
**my brain hurts**

additional, sat navs are accurate because you position is triangulated between several satellites.... i think
 

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well all i can say is how on earth could a mk2 1.8 cdi be actually travelling at 135mph indicated.... Speedos imho always joyfully give you faster than the the actual speed (and btw the gearbox didnt like it and went ker - pow at that speed. )
 

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my cav 1990 gsi v6 is about 8mph out it goes off the clock at 160 and continues to rev a bit,the true speed is just shy or on 160
 
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