Speedometer Gear 1967 RUG-S

[Mike James]

Guys,
I am having difficult getting my speedometer to read anywhere close to actual speed.  It's reading about 20-25% under the actual speed.  I know my differential ratio as it is newly rebuilt (all new) and I know my tire size.   I used the online calculators and it told me to buy a pink tooth gear (19 tooth).  What am I missing?  With 3.50 ratio differential, 4 speed, and 235 60 15 tires.

[1967 eight barrel]

You have to measure the tire diameter, know how many teeth are on the drive gear and the gear ratio.  You can't just plug in tire size i.e. 235 65 15. That doesn't work.   They also vary slightly manufacturer to manufacturer.

If the drive gear is black it would be 7 teeth. ( what it should be)

Black drive gear. ( Inside the trans on the output shaft) 7 teeth  ( Make sure of the tooth count!) Color determines teeth.
Tire diameter 235-60-15 industry standard 26.10 tall
Rear gear ratio 3.50
It shows to use a 19 tooth driven gear.

When was the last time the speedometer was serviced?  Are you sure the differential isn't 3.25? That would make the combination you have about 20% off.

                                                                    -Keith

[Mike James]

Speedometer is restored by the top speedometer shop in AZ.
Yes I used 26.1 tire diameter as you and got the same answer.   I need to confirm the drive gear.

[67350#1242]

This is probably obvious, but if you pull out the speedo cable from transmission you should be able to see the color of the drive gear on the output shaft.   I think either 6 or 7 tooth drive gear will work with 3.50 gears (there is some overlap),  but of course will require different driven gears.   The difference between 6 and 7 tooth drive gear could explain close to 20% error.

[1967 eight barrel]

This id probably obvious, but if you pull out the speedo cable from transmission you should be able to see the color of the drive gear on the output shaft.   I think either 6 or 7 tooth drive gear will work with 3.50 gears (there is some overlap),  but of course will require different driven gears.   The difference between 6 and 7 tooth drive gear could explain close to 20% error.

^^^^ Note my reply. The only factor here is the drive gear.  He likely has a 6 tooth gear. Black is good to 3.50.