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The problem with your theory is that radial tires have a very strong non-stretching belt under the tread. The effective diameter is very constant regardless of pressure over a pretty wide range.

I suspect that there was a prevailing wind, or the temperature was lower on the way back (or hotter and you changed the ventilation settings), or the traffic was different, or there's a significant elevation change. All these can have a very significant effect.
 

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Those high tech cars that flash low tire pressure warnings have little sensor/transmitter thingies on the rims of the wheels, inside the tire, that talk via radio to a receiver on the car body.

The radius doesn't have anything to do with it. The sidewall squishes around as the tire rolls, and the result is that the wheel, the sidewall, and the tread all go around at the same speed. Probably if the tire is at like 5 PSI you'll see a difference, but going from 35 to 45 is nothing.

Getting better mileage from higher pressures doesn't have anything to do with changing the tire circumference, it's because there's less drag because the tire flexes less.
 
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