Speedo WAY off....

[andyb]

Not really very much, no.  If you've got a lack of grease on the speedo drive cable, you'll get a bouncy needle at low speeds (at least, that's what mine did).  Turns out that aftermarket speedo cables are not lubed the same way that OEM ones are, I just bathed the guts in antisieze goop (that nasty silver permatex stuff that you find all over everything after you use it once), and that fixed the bounciness.  Accuracy won't be altered unless it's somehow skipping or slipping someplace, and it'll read low rather than high in any case.

GPS are usually considered to be the most accurate way to measure short of a radar gun, but timing lights at a high-speed racing venue (Maxton, Bonneville, etc) will also be accurate, if not exactly pratical.

[rktmanfj]

GPS are usually considered to be the most accurate way to measure short of a radar gun <snip>

Most of the time, anyway...     

[Sideburns]

Not really very much, no.  If you've got a lack of grease on the speedo drive cable, you'll get a bouncy needle at low speeds (at least, that's what mine did).  Turns out that aftermarket speedo cables are not lubed the same way that OEM ones are, I just bathed the guts in antisieze goop (that nasty silver permatex stuff that you find all over everything after you use it once), and that fixed the bounciness.  Accuracy won't be altered unless it's somehow skipping or slipping someplace, and it'll read low rather than high in any case.

GPS are usually considered to be the most accurate way to measure short of a radar gun, but timing lights at a high-speed racing venue (Maxton, Bonneville, etc) will also be accurate, if not exactly pratical.

Don't cops in the U.S. offer kind of the same service?
They take a picture of you doing a certain speed.
You get the picture a couple of weeks later, after paying a small fee

[andyb]

You'll notice I said GPS was second to radar.  I've gotten offical written proof of my speed via radar, and not all the king's horses and all the king's men could prove that invalid.  Occasionally, I do hear of GPS readings in error.  Ergo..