Speedo Gear Info Needed

[TonyMadrid]

I am running a 1975 Ford C-4 Tranny with a 3.50 rear end gear ratio and BF Goodrich Radial T/A's (295R50/15). The Tire Diameter is 26.7.

Two questions:

1) What size drive gear should be in the 1975 C-4? A seven tooth or an eight tooth?

2) Assuming the seven tooth drive; what speedo gear would I need. When I do the calculations it results in 18.54. Should I go up to the 19 or down to the 18.
Assuming an eight tooth drive gear the calculations come out to 18.9.

Thanks for any response you may give,

Tony