This is a interesting project - as we have one car with ODBII (A Landrover) :

OBDuino
From Wikipedia, the free encyclopedia
OBDuino is an open source trip computer design based on the Arduino platform. An OBDuino may be assembled and customised by an electronics hobbyist, it displays information such as instantaneous fuel economy (e.g. miles per gallon or litres per kilometre), engine tuning parameters etc. on an LCD.
As the name suggests, the OBDuino utilises the On-Board Diagnostics interface found in most modern cars.
Contents [hide]
1 Features
2 Design
3 Project
4 Variations
5 Related Alternatives
6 See also
7 References
8 External links
[edit]Features

Most OBD-II PIDs or derived values may be displayed. Common values include
Fuel consumption, measured in mpg or l/100 km,
Fuel used or remaining,
Fuel cost,
Fuel wasted while idling,
Distance,
Engine RPM,
Vehicle speed,
Tank distance,
Remaining distance that can be travelled on the current tank of fuel
Engine load,
Coolant temperature,
Air intake temperature,
Throttle position,
Battery voltage,
CAN status, for CAN protocol only, display TX and RX errors
Instantaneous values can be displayed, and average, maximum and minimum values calculated over a trip, an outing or per tank of fuel.
Menu system for configuring parameters
Relatively cheap compared to commercial alternatives
Customisable and extendable
OBDuino does not currently display or reset engine fault codes (which are available over the OBD interface)

[From OBDuino - Wikipedia, the free encyclopedia]