I'm suspicious that you can tell that the mileage is off by such a significant amount in only a few hours. How are you doing the calculation? What mileage are you referring to? Fuel consumption? Speedometer error?

The change in speedometer reading is practically insignificant between the two tire sizes that you mention.

I agree with the other poster that ride height is also slightly different when comparing "new" tires. All bets are off for comparing a set of worn 195s with a new set of 205s.

For what it's worth, I always purchase one size larger (like you were sold) because I have found that the tires look a little better on the car. The original size always look like baby carraige wheels to me as they wear.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig