i have a 390 rear.how many teeth do i need on the speed o gear.its in a 55 but im using a 58 trans so had to go to a newer cable.thanks

There is a formula that takes account of the gear ratio and tire size, but I don't have it. 

Have you tried it with he current gear? how much are you off ?  If so, count the teeth on the current gear.  More teeth slows the speedo, less teeth make the speedo faster.  Each tooth represents about 4-6 mph..  New gears are available - try ebay - lots of them on there.


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

64F100 57FAIRLANE500

This is thre best I can do, maybe someone could size it a little better.   It's the formuila from an NPD truck catalog. NPD also has many Ford speedometer gears.  Hope this helps.......

64F100 57FAIRLANE500

im fast so i need more teeth.i found that out yesterday.ill start from there.thanks

                    Maybe this is clearer - and right side up


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

Thanks Paul and BTW, I have used that formula twice - very handy.

64F100 57FAIRLANE500