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7 segment led display with I2C driver interface

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I need help on driving my led display which are 4 seven segment led displays. They are common anode. Led display datasheet>>KINGBRIGHT|SA15-11SRWA|LED DISPLAY, 1.5", RED, CA | Farnell United Kingdom

I'm gonna drive it with a 4-digit LED driver with I2C-bus interface.
Driver's datasheet>>http://www.nxp.com/acrobat_download/datasheets/SAA1064_CNV_2.pdf

The driver will be connected to PIC 18F4480 since it has the I2C thingy as well >>http://www.farnell.com/datasheets/70710.pdf

The input to the PIC will be from a hall effect sensor >>http://www.farnell.com/datasheets/34203.pdf. The sensor is used to calculate speed from a toothed gear. The gear has 26 teeths.

My question is:
1) How to calculate the speed of the rotating gear. I want the speed to be in mph. fyi, the speed of the gear is constantly changing.
2) after ive calculated the speed how to send it to the displays thru the I2C. I have no idea what is I2C so any explanation is welcomed.

all helps are very much appreciated.
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