Quote:
Originally Posted by CapriRacer
Speedometer calibration is done at the placard pressure. But the RPM (revs per mile) value is given to the OEM
by the tire manufacturer - AND THEN the vehicle manufacturer makes adjustments - and they tend to
have the speedometer read a bit low, because if the speedo reads high, the vehicle manufacturer could
be legally liable for someone getting a speeding ticket!
But on a more practical note: People tend to speed, so judging your speedometer accuracy based on the
traffic around you isn't a good idea.
A better idea is to use the Interstate mile markers. They are at least reasonably accurate. But use several
over a long distance. I like to use 10 miles, although it is sometimes difficult to maintain a steady speed for
10 minutes - even with cruise control!
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Thanks for that explanation.
Although I think the part about the speedo reading high/low is reversed.
You'd want the speedo to read(indicate) high, maybe 1mph/2kmh. That way, if a driver is driving at an indicated 60mph, they are actually doing 59.
In any case, I'm doing things right, for the most part.