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و بالنسبة للعدادت انا قريت فى الويكيبيديا ان عداد العربية بيبقا فيه نسبة من القراءة اللى مش صح و مقصودة من المصنعيين عموما وده اللنك
Speedometer - Wikipedia, the free encyclopedia

Error

Most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Additional sources of error are; tire diameter variations due to temperature, pressure, vehicle load, and nominal tire size.
Modern speedometers are said to be accurate within 10% but as this is legislated accuracy, this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed, although levels of some 3 km/h or 3% are also used in areas of tough enforcement (e.g. Victoria, Australia). This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue[3] is being increasingly blamed for these stricter measures. There are strict United Nations standards in place but it seems not being enforced in some countries, as cars not adhering to United Nations standards, are imported from these, leaving this matter in limbo. Motorists are open to charges of speeding, if harsh methods 3% or 3 KPH are used using a speedometer not complying with accuracy standards.
Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the
percent error = 100x("standard diameter"/"new diameter" - 1). ERRATUM: Percent error = (standard diameter – new diameter) / standard diameter * 100% This is a variation of the standard percent error formula. The one above is incorrect.
Nearly all tires now have their size shown as "T/A_W" on the side of the tire (See: Tire code), and the tire's
diameter in inches = TxA/1270 + W. For example, a standard tire is "185/70R14" with diameter = 185x70/1270 + 14 = 24.20 in. Another is "195/50R15" with 195x50/1270 + 15 = 22.68 in. Replacing the first tire (and wheels) with the second (on 15" wheels), a speedometer reads 24.19/22.68 = 1.0670 times the correct speed or 6.7% too high.
ERRATUM: Using the correct formula for percent error, we conclude that the speedometer will read (24.20 – 22.68) / 24.20 * 100% = 6.28% too high e.g. at an actual speed of 60 MPH, the speedometer will indicate 60 + (60 * 0.0628) = 63.77 MPH (approx).

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