Re: Spedometer/Computer Speed Discrepancy

["S. Joseph Mann" <joem@pswtech.com>]

Khan,

I recently took a 200 mile trip to Dallas from Austin.  Over the course of the
trip, using the chronograph on my watch, my Newton and mile markers. I
consistently came up with, on average, a 3 mph discrepancy at various speeds.
I'm certain that the "speedometer" is consistently fast and incorrect.

Joe


Khan M. Klatt wrote:

> OK, check this out.
>
> My friend drives a Corrado VR6, and he and I (driving my Jetta GLX) both
> get on the freeway and peg our cars at 60mph w/ cruise control.
>
> We both checked our computers, and reset the computer's "Average mph"
> guage, and both cars showed 57mph.
>
> I have stock wheels and tires on my car, and he bought VW wheels to replace
> his BBS wheels, but is using the same stock tires (or the same type of
> tires as stock).
>
> It seems this speed discrepancy exists at all speeds-- approximately 3mph
> difference between speedometer and computer.
>
> So, is there anyone out there that has facts or figures that would indicate
> which one is accurate and which one is not, and for what reason?
>
> I would tend to think the computer is accurate, but I want some hard facts.
> Any mechanics or engineers out there that know why this discrepancy exists?
> How about having it fixed?
>
> If the computer is wrong, does that affect the miles per gallon and time of
> trip calculations, etc.?
>
> Has anyone tested the accuracy of the temperature guage?
>
> -Khan