Two things. First, and foremost, you have an automatic transmission. Input speed to the convertor does not equal output speed. You will have loss. Some converters more than others. If you have a cheap "Saturday Night Special" verses a well built efficient converter, say a Coan for example, you could pick up a few mph in the quarter from that one change alone. Even with a good converter you will have slippage. And, the more torque you input, the more slip you will have.
Second, the roll out on your tire. Did you measure it, get the number from the MFG, or just assume based on the diameter?
