This gem has been updated so that it should support the Sparkfun OBD II UART board (https://www.sparkfun.com/products/9555) without modification.
It should also work with any other OBD II scanners that follow the ELM327 standard as well.
gem install obd
Connect your OBD-II adapter and pass its device file to OBD.connect:
require 'obd'
obd, err = OBD.connect '/dev/tty.obd' 9600
obd[:engine_rpm]
=> "4367.25rpm"
# Retrieve error codes
obd.send("03")
=> "43000545"