I recently went from 3.08s to 3.73s with the stock 15 in wheels/tires on the car. The GM dealer told me that they couldn't calibrate the speed for 3.73s but could get it close by putting in gears for 3.42s. What do you all with 3.73s do to recalibrate your speedo?

Help is appreciated,

Jason

 

You have a 5-speed right? That helps if you tell us. For a 3.73 ratio and 16" tires, its a 7 tooth drive gear and a 21 tooth driven gear. I dont know if its the same for 15" tires.

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1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.25 @ 107.18 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

 

16's and 15's have the same diameter if they have the stock size tires on, the 16's were lower profile to keep the same diameter.

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1985 Camaro IROC-Z LB9

1989 Pontiac Firebird
Formula W6S LO3 TBI 305
-180 degree stat
-Hooker Cat-Back

[This message has been edited by Keith5 (edited January 05, 2001).]

 

If you have an electric unit, non-cable driven speedo, do you calibrate with gears or with the computer?

 

You still have to switch the speedo gears. You can alter the "Pulses/mile" in the eprom, but it has no effect on the speedo itself.

Also, NOT changing your speedo gears will affect the running of the ecm if you have not AT LEAST altered the "pulses/mile". There are many tables that are affected by the vehicle speed.

If you swap the gears, the ecm will know that you are travelling the correct speed (since you will still be sending "4000 pulses/mile), but you should adjust some of the tables to add fuel a bit quicker. This even more important with the automatic which controls the locking of the TC based on your vehicle speed.

 

Ok, I understand now, I was under the wrong impression, the signal comes from the tranny via the Speedo gears/VSS and then goes to the Speedo and the ecm independently? Right?

And after a gear swap it would be a good idea to tune the chip for the gears but it isn't absolutely necessary right?

 

Yes it is a 5-speed.

Jason

 

No, with the manual, it is not as critical to adjust the eprom as it would be for an automatic (due to the locking of the TC).

But, a customized prom, programmed by your own hands, would definitely help. Nobody is going to put as much time and effor to get that elusive "perfect" chip as you will.