Speedo Calibration With Larger Tires - Why?

[Farmerj]

275*0.6 = 165
285*0.6 = 171

171-165 = 6

6/2 = 3mm. Barely a 1/10 of an inch. ~0.113 inches in radius.

considering the manufacturer error allowed in electrical components, rather insignificant.

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[780]

I've had my 35x11.5r20 Toyo AT3's for a few weeks now (2"ReadyLift).

Never recalibrated the speedo.

Buuuut.... On 4 sperate occasions, at 2 different locations I've driven past those speed signs that look like this:

Each time the speed that was displayed on the radar sign matched the speedometer reading on my digital speedo...... How is that possible?