Even with the proper collection of components, a stock speedometer is probably only a half decent rough estimate of speed, especially after 50 years.

But your PV most likely came with a 4.56 rear end, the B16 engined cars were geared lower. They needed a little more oomph in the lwoer speeds to accelerate, and probably couldn't easily push the car up to higher speeds anyhow. The B18 engined cars came (mostly, probably exceptions galore) with 4.1 rear ends, trading a little acceleration for more top speed. Assuming the speedometer outputs were matched to the rear end ratios to arrive at some standardized RPM input to a speedometer (itself a bit of an assumption on my part) then you should expect your speedometer to be reading too high by 11 - 12%.

Another way to check it is with a stopwatch and lightly traveled highway. Go an indicated 60 mph. Hit the stopwatch as you pass a mile marker. ime how long it takes you to get to the next one. Anything over 60 seconds, speedometer reads high. Anything under, it reads low. There's an adjustment on the speedometers, although I'm unsure of the range. It basically adjusts the spring tension on the needle. More tension, the more it resists the magnetic pull of the speedometer input, and the lower it will read.

Mine reads approximately double, probably due to the time I took it apart to fixe the trip odometer. I probably put it back together with one tensioning turn of the spring too few. I've just learned what speed goes with what RPM in each gear.
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'63 PV544 rat rod, '93 Classic 245 + turbo