It is my understanding (and I'm about to test this) that the speedo is PRIMARILY driven by the speed sensors in the front wheels.

I say this because, when I went with the oversize tires on the rear, my speedo originally disagreed with my GPS by 1 mph. Now, it seems to agre with it.

But my TACH, at 80 mph, shows about 300-350 less rpm - and THIS cannot be denied. I don't think oversize REAR tires make any noticeable difference in the Speedo.

But again, I'm about ready to jack up the rear and turn the wheels and see what I get... (As the R&D manager at work often says: Why are we discussing it? TEST IT!)