'57 Speedometer calibration

[dickday0]

I downloaded and app on my phone that shows me, using GPS, the speed I am going. As I thought, the car is registering about 10mph less than the actual speed. And, this I don't understand, the odometer is advancing way too fast. I typically go about 20mph on the road we live on. I can see the odometer 10th's dial moving. It's clicking away the miles way too fast.

The car has 20,588 original miles, I hate to see that climb prematurely.

Is there a way to calibrate the speed?

Thoughts?

Thanks.

 

[Dale Leonard]

You have to change the gear that turns the speedometer. If I remember correctly it is on the end of your speedometer cable where it fits into the transmission. They used to calibrate them at the local Ford dealers as most of them had a dynomomitor to test them on. They would change the gears til they were close to correct. Yours is reading slow on miles but fast on Tenths which does not compute correctly. Measure off a quarter mile and see if the tenths are correct. If they are then something else is wrong with your speedometer. You can or should be able to use the mile signs on the highway as they are set up one mile apart to check your tenths of a mile.

 

[handyman]

our 57 tbird reads 7 miles per hour faster than we are going, I don't know if I need a gear with more or less teeth to bring the needle down a bit, not sure how the ratio works, anyone know? thanks handyman

 

[Dale Leonard]

our 57 tbird reads 7 miles per hour faster than we are going, I don't know if I need a gear with more or less teeth to bring the needle down a bit, not sure how the ratio works, anyone know? thanks handyman

I believe more teeth would slow it down as it would take longer to rotate the gear. Like screwing on a fine thread nut versus a coarse thread nut on a bolt.