When I bought my car the guy said that he had put in a 3.42 rear gear in it. It came stock with a 2.73 (GH3) and the speedo gears haven't been changed to fix the speedo so when the speedo says 120 KPH I'm actually going approxametly 100. Beside being on extra miles on the odometer does this effect the ecm and any calculations it makes???

 

Yes. There are many items within the ecm that are affect by the vehicle speed. The TCC locking/unlocking speed are just one example, but there are many others.

Once you change the speedo gears, some of these you may wish to change in the eprom, some you may not.

 

So with the speedo gears not being calibrated correctly My torque converter is probably locking up too early right. This could cause slower acceleration couldn't it.
I knew that there were parameters in the prom such as tc lockup dependant on vehicle speed but I wasn't sure if it had it's own seperate sensor independant of the speedo.
How does it work?? Is there a sensor within the actual speedo that sends the info to the ECM. This is on an 89 iroc.
Thanks Glen

 

All is driven off the VSS from the tranny.

So what happens is your ecm is seeing the speed higher than it really is, and invoking functions sooner than you may really want it to. For example, when you converted from 2.73s to 3.42s, your speedo (and ecm) is reading everything 25% higher than it should (or 20% soon (1/1.25).

Thus if, your TCC is suppose to lock up at 40 mph, it now locks up at 32 mph. Whether this is what you want, is up to you. With lower gears, you probably would want your TCC to lock up a little sooner (for fuel economy) but maybe not that soon (for performance).

But a bigger concern may be your speed limiter (if you have one). If it is set for 125 mph, now it is only 100 mph.

 

Thanks Glen