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498 Posts
speedometer accuracy

Guillermo said:
Ready for this!
Your OEM 165/65R14 tires are undersized for the speedometer and FCD!
:oops:
Borrow someone's GPS and you will see that even a new 165/65R14 tire has a rolling circumference that is 2% too small. When your speedometer reads 100 km/hr you are only travelling 98 km/hr.
First off, 2% is an awfully small number for speedometer error. It would be very hard to design a system that is within +/- 2% at all times and I'm sure that the Automotive industry doesn't even want to try.

Secondly, Just because GPS uses fancy satellites and stuff, doesn't make it the ultimately accurate device. There are errors there, too and I would not be surprized if they added up to somewhere around 2%.

And finally, the over-stating of the speedometer is by design. Since there are always tolerances involved, car manufacturers design their speedometers to over-state the speed by a few percent. If they didn't, they'd open themselves up to no end of liability lawsuits! Besides, it may be a regulatory requirement, too! (at least in Germany, I know it is.)
 

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Premium Member
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498 Posts
accuracy

G,

yes, it would be nice, but would it be practical or affordable?

All the parameters involved that affect speedometer accuracy would have to be extremely tightly controlled to make sure they never add up to more than 1%. Just look at some of the more obvious ones:
-Tire size (I don't mean the nominal size, but the tolerance from one tire to the next)
-tread wear (just wearing down the tread on the stock tires causes a 1.3% error)
-pressure
-temperature
-weight of the vehicle (load)
-road surface
-curves
-slip

and the list goes on and on. If you ever worked with any type of instrumentation that attempts to measure something this complex, you'd be happy to get 5%!
 
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