That's not very comforting. How do you normally account for this / know what is the true speed you are doing? (Apart from finding a long length of track and getting someone to time you as you test your speedo)
Many major highways still have 5 km markers down both sides. I used to use them to 'calibrate' all my new company car speedos when I was working in the country. This is most accurate way to check that is easily accessible.
GPS is fraught with inaccuracy over short distances, but if you check it over a moderate distance of steady driving on flat road with no buildings or hills it should give a decent reading.
The Australian Design Rules specify acceptable speedo tolerances; last time I checked speedos are allowed to read as much as 20% high but must NOT read low. If your speedo says 100 km/h then you are travelling at 100 km/h or less if you are using the manufacturer supplied standard tyres.
Electromechanical speedos (most common type) also lose accuracy with speed because of the nature of the electromechanical coupling - usually spot on at 50 km/h but lose increasing amounts as speed rises, 80 read ~ 75 actual, 100 read ~ 92 actual, 120 read ~ 108 actual....these are indicative numbers based on my measurements on a host of cars over 20+ years.
Truckies can increase the road speed of their vehicle by putting larger diameter tyres on without affecting speedo logs. This is one way 'speed limited 100' vehicles routinely do 110+, bearing in mind your own speedo errors which makes them seem faster still.
On my cars over the years I have consistently measured a 3-5% speed difference between new and worn tyres - this is in your favour as the worn tyres are slower because of the reduced rolling radius.
In short, mechanical speedos are a very poor way of accurately measuring speed on road vehicles. Best way is vehicle mounted Doppler radar or ultrasound.
If you stick to the correct speedo reading with standard tyres you should be perfectly fine.
almost as quick as a normal 1600cc 4 cyl Datsun/Nissan
Someone in grade 12 had a Nissan 1200 or 1800. I was well jealous.
