using a GPS will confirm that your speedometer is accurate on plain old highways. My 1984 speedo is off by 2-3mph at 70mph, which also translates into nearly a half-second difference in 0-60 times (need to clock to an indicated 0-57 or 0-58). Oddly enough, it reads a little fast at 25mph, is dead-on around 45, and reads low above 55. My one 87 is dead on all the way to 100mph (haven't tested beyond that), and the other is 4% slow (due to larger tires). You don't need to buy one or have it in the car all the time, it's a one-time check to see how close your speedo is to reality. Find a friend who has one and borrow his for a weekend to play with.
Now for the Silver State Classic, that's a different story. A top-notch GPS is almost required for that kind of thing, unless youre just out there for the fun of driving fast legally (like I would be).
