Speedometer Gear for 3.55 gears?

[mustanggt94]

I am helping a friend out on her restoration of her 68 Coupe (289 with C4) and the guy restoring it seems to know more about body work than anything else and hasn't dealt with the speedo gears much, so I'm doing the research for her.

Original: 16T driven gear, 2.79 gears, standard 205/75-14 tires

Resto-modified to: ?? driven gear, 3.55 gears, standard 205/75-14 tires

We didn't get to find out for sure what the number of drive teeth are before the tranny was rebuilt, but we are assuming it is 7 teeth since that appears to be the most popular with C4s and this car was bone stock original before the resto. Also, backwards calculating says it would need a 15.07 tooth driven gear, which I know they don't make a 15T, so 16T would be the closest. Thought it could be an 8T driven since that calculation puts it at 17.2 driven, which is not as close to 16T but still almost as close as 15T!

For now, lets assume it is a 7T drive gear. Now when I calculate what tooth we need for the new 3.55 gears we come up with 19.2. My question is, since the original calculation comes out to be almost exactly 1 tooth less than what is actually in the car, should I round up to 20T for the new configuration to be consistent or stick with 19T that is the closest whole number? Anyone with experience that can chime in I'd appreciate it!

 

[woodsnake]

http://www.cougarpartscatalog.com/speedocalc.html

 

[mustanggt94]

Now that confuses me even more! haha....That says C4 has 8-tooth drive gear, but all the forums I've searched on here people have been saying most C4s are 7-tooth. The calculator on that site is giving me the same numbers I've calculated above, so I guess the real question is whether it is 7- or 8-tooth drive gear and which way I should round (up or down) when calculating.

I'll have to try and get them to count the teeth through the hole since the tranny is off the car but already re-assembled, just to be sure.

 

[woodsnake]

removing the tail shaft is no big deal. I don't know how tall your tire's are in inches, but I would think this answers the question...