I was wondering if i change rear end gears do i need to change the speedo gears at the tranny or can you just change the road speed constant in the prom to correct the speedo reading for the ip cluster.

 

ahh which ecm ?

 

sorry it is the 7730 ecm.

 

what kinf of car ?which code mask

 

91 camaro with $8d mask

 

have you checked the diagrams over at the gmecm at diy-efi.org site. there schematics i dont have time top puruse them myself. but if it has a vss input and a dash vss output then you could just the input pulse counter to arrive at the correct dash speed. go look at the schematics let me know whats in there. and maybe traxion will pop in and lend some $8d know how.

 

There's info here on changing it for a T56,
but it may be less complex on a T5 w/gears...

http://www.eecis.udel.edu/~dzug/92z2...0-vss4T56.html

mike

 

With Tunercat you can change the Road Speed Constant in the ECM Constant Tables to get the ECM to read the actual speed but the speedometer will still be off unless you change the gears to match. From 3.08's to 3.73's I went from 4007, which is what every binary file I've viewed was set at, to 5453 to get the ECM to calculate the correct speed.

There is also a Instrument Panel Vss Pulse Divisor but changing that only got me a speedometer that didn't work at all. I'm not sure what it does.

There may be a way to edit the binary file to get the speedometer correct too but I didn't. Just called a GM dealer and they sold me the correct gears.