Does tire size really affect speedometer?

[Matt Timion]

Okay, I've heard time and time again that putting the non-stock tire size on your car will alter the speedometer. This would alter your mpgs as well if you didn't adjust for the difference.

but i'm not sure if it's true or not.

I recently went from 175/70R13 tires to 185/70R13 tires. While the difference isn't too great, there should be at least a few percentage points difference in the speedometer.

Not a single change. When I cruise at 3000rpms in 5th gear it is still 65 mph. When I drove past one of those digital speed displays near a school the other day at 39mph it also said 39mph.

So does the ECU compensate for tire size? Any theories?