Tire size questions

I'm confused. What affect is larger tires supposed to have on the speedometer?

I went to 14" SBP rallys on my 66 Barracuda and then later fixed the speedometer with a new cable. I expected the speedometer to be off because of the tire change and expected it to read slower than I am actually going. Bigger tires cover more distance with the same rpms.

It is actually reading about 15-17 percent off in the other direction. 70 mph on the speedometer is right at 60 mph with the GPS.

When I bought the car the 904 tranny wasn't installed. I had it checked out by a tranny shop. It seems to be the right tranny with the shifter mounts and all. I guess it could have come from a floor-shift B-body or something.:read2: