speedometer driven gear

[pw520a 19]

I have a 69 390 c6 with  3.25 axle ratio and stock14 inch rims on car, the speedometer cable was missing the  driven gear so I purchased on eBay a 16,17,18,19 and 20 tooth set ,does anybody know which one would be correct or which one to start with for correct speedometer reading?

[aslanefe 333]

1 hour ago, pw520a said:

I have a 69 390 c6 with  3.25 axle ratio and stock14 inch rims on car, the speedometer cable was missing the  driven gear so I purchased on eBay a 16,17,18,19 and 20 tooth set ,does anybody know which one would be correct or which one to start with for correct speedometer reading?

Don't know which one is correct, but I would start with 18 (the middle one) and check speedometer reading with GPS then go up or down if needed. At least it is easy to change the gear.

[pw520a 19]

makes sense Thanks

[jmlay 80]

Take a look at these:

https://www.428cobrajet.org/speedo-gears

https://www.428cobrajet.org/speedo-calc

[bigmal 225]

As said above using the GPS. Plenty of cheap Speedo Apps for your phone that work vey well.

As a rule of thumb I found each tooth equates to about 5% change. 

[det0326 179]

Depends on what u want. If u don't care about your odometer being correct then changing the gears to make your speedo correct is fine. If u want the odometer correct as said put the 18 tooth one in and check the odometer reading for 1 mile. If it proves to be correct and speedo still not correct then u have a speedo problem.

[pw520a 19]

GREAT INFORMATION JMLAY THANKS!

[jmlay 80]

Anytime!