I'd test with a gps device.  My speedometer happens to be the oddball
and is accurate with stock tires.  As for the port-a-cop, most I've
seen were accurate but get confused by surrounding traffic.

On 10/13/08, Shawn Wright <vwdiesels@gmail.com> wrote:
> Just got back from a week in Ucluelet, BC (not camping, but lots of hiking
> and day trips in the
> Westy), and passed through one of those radar-check speed warning signs
> twice, and both
> times, noted my speedo read 10km/h high - an indicated 70km/h showed 60km/h
> on the
> radar. As I have slightly larger than stock tires on the front (Yoko Y370
> 195/75R14), this
> surprised me a bit. I expected maybe 5, but not 10km/h over. Does anyone
> know if those
> radar signs can be considered accurate?
>
>
> Shawn Wright
> http://members.shaw.ca/vwdiesels
> '88 Westy 1.6TD 5 speed
> '85 Jetta Diesel 1.6NA
>

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Chris S.
Disclaimer: "Death and serious injury may occur"