My speedometer is 3.5mph fast at 60-70mph. This gap is consistent with multiple GPS's and just about every rader-based "your speed is" sign I pass on the highway. I think the radar/GPS technology is much more accurate.

I disagree that speedometers are "very accurate". Many manufacturers intentionally design them to report high. They never want to read slow, putting a customer in position of a ticket when the speedo says you're OK. So they stack all of the tolerances available, and assume full tread, max inflated tires. Then throw in a safety factor.

Yes, when calibrated, a police speedo is accurate and can be used to issue tickets (yes, I know from experience). But they must be calibrated on an individual and very frequent basis to account for individual variances and assorted wear items. The average car on the road doesn't have this level of attention.

There was an article in one of the car magazines a few years ago. I believe one of the regulators (DOT, European?) has +0/-10% requirements - you couldn't read slower than actual but had tolerance to read higher. I think BMW has actually published specs that their standard is something like -3mph, +/-5% at 60mph - aiming for 57, allowing for 54-60. My 56.5 would fit right in that target.

Many of the mags also report on actual/indicated speed and they are almost always 1-3mph high.