With an Arduino and some custom interface, I made an onboard diagnostic tool for your car, it can display various PIDs and also the current fuel consumption as well as average on tank and trip.
I'm still working on the hardware to have a custom PCB and a kit one day.
All information are here
Hey dude, i've been looking in to this for my car! but my car (Honda del Sol (1994)) is not OBD 
i dont think, its certainly not OBD2. Talking to the chaps on http://forum.pgmfi.org/ recon i need to get it chipped to be able to do stuff like this!
this is awesome information! This is amazing, considering an obdII scanner of this depth would cost $300 retail. Definitely going to use some code from this for my next project.
Phill: a '94 del sol would definitely be OBD, just not OBD2. OBD doesn't do nearly as much fancy stuff as obd, though. If you're keen on electronics, what I would recommend doing is building yourself a new standalone ecu called a megasquirt, its a very cheap performance ECU that just comes with a PCB and various bags of components. Its really cool to have your whole car run on a little box you built 
phill, you can use the mpguino if you have a fuel injected car. It only does distance/time/fuel consumption based calculations (i.e. miles per gallon, gallons per hour) but works on non obd cars, and can be tweaked to look at any sensors you want (but you have to hook them up and tweak the code).
But a lot of folks will certainly like the "plug and play" aspect of the obduino, as well as the access to all the different parameters.
I dont fancy building my own ecu, i can see distastrous concequences! Also i dint reli want to chip my ecu becuase insurance would be a pain to get it costs £1200 for fully comp already! And as its a legal requirement over here!
what do you mean "build your own ecu"? the mpguino just taps into the speed sensor and an injector wire.
sorry, was replying to N3v, i couldnt find the quote function! I will look in to mpguino when i get a bit of time, cheers!
Thank you Magister for your work.
Where can I get Arduino code?
The code (GPL) is hosted on the google project page
So do you need both the 'interface for iso' and the 'interface with elm'? Or does that depend on what your car has?
You only need one of them.
My car is CAN so I tested only with the ELM327.
It seems the ISO program has some problem being "softserial" so I changed the code to use the UART and will change the diagram to reflect it (pin change from 2/3 to 0/1).