Just to clarify, I-Bus is BMW (and BMW-designed cars such as MINI and some model Range Rover/Land Rover) specific. It has nothing to do with the engine, drivetrain, guages, or transmission or the management or monitoring thereof. It's the bus that lets the radio talk to the cd player and the nav system and that kind of thing.
I think what you are asking is for something like OBD-II (A requirement on all 1996 and later cars sold in the US and possibly a requirement elsewhere also) A lot of higher end cars were implementing OBD-II or some parts of it before the standard was finalized so there are some '94 and '95 cars that implement it. Also in the years prior to OBD-II, there were some cars implementing OBD-I.
That said, I do not believe that most american manufacturers were implementing this kind of thing before they had to. If your car has a trip computer (ie a Range button that tells you how much farther you can go on your tank of gas or your current gas mileage) it's likely there's something in there to hack. Also, if it has "digital dash," again, there's probably something in there to hack. But the protocol will not be standard OBD-I or II and you'll need to find some information about your car's specific interface. I do know that cars back as far as 1984 were implementing systems capable of doing this sort of thing because my 1985 BMW 7 series has a diagnostic connector that will interface with a computer (proprietary protocol, though.. I'll leave this project for later
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Ok... That year Trans Am has OBD I. That's OBD (One). The original. I have an 85 Corvette as well as my 97 Formula Firebird. OBD has been with GM since 84. When they implemented the TPI engine (which is the one in that Trans Am) they added the ECM which in turn used the OnBoard Diagnostics system to find errors in the ECM or throughout the wiring harness. On 96 and up GMs, the OBD II was implemented. However, both OBD I and II use a similar connection that sends an output of simple bits. These bits when read through the serial port give you the MPH of the car as well as the RPMs. There is a bit more about this, but that's just about as far as my knowlege goes with OBD I. I most only know about OBDII because that's what my car uses. I don't mess with the OBDI on my Vette because I don't want to hurt the Hypertech Chip I put in it. The Firebirds ECM is completely reprogrammable, so I don't have to worry about messing it up.