Since the US gov't decided to turn off the 'selective availability' quirk built into the original GPS satellites, even consumer receivers are extremely accurate. Moving at high speeds, they only get more accurate. So they become useful tools for checking actual speed, in my opinion.
Even assuming a really poor accuracy of 30ft, that's only .5% off. If I'm wrong, please correct me.
Anyway, I've checked my speedometer against my GPS and found that they both agree within 1mph up to about 110mph. I'm not going to watch TWO instruments inside my car while going any faster than that. And I guarantee that my wife isn't going to assist me for the sake of science.
It should be noted that the tinting in the glass inhibits GPS reception. I had the sunroof opened for this experiment, and the GPS was estimating an accuracy of less than 9ft. In the Jusy and the Legacy (and even occassionally in the SVX), I can get better reception than that. I still plan on mounting a remote antenna inside the trunk - I just haven't gotten around to it.
Anyway, Ice, I don't know what MY your SVX is, but '92s and '93s are drag-limited. In '94 Subaru started electronically limiting the SVX.