rpm = (trans drive ratio x rear gear x mph x 336)/ tire diameter inches.

Add about 150rpm for a stock non-lockup torque convertor, or more (up to 300rpm) for a high stall. If for some reason it's quite a bit off, borrow a GPS and check your speedo.

If I plug in 26" for the tire, it looks like your running about 10% higher rpm (even considering torque convertor slippage).