Speedometer adjustment

I have a 69 with a FMX. I had to change out my speedometer and now it is running 5 MPH slower than my GPS reading. The cable has a 19 tooth gear on it now. What size gear do I need to have to speed up speedometer by 5 MPH?

Thanks for any help.

Here’s a link to a calculator that will help you find the right gear:

Speedometer Driven Gear Calculator at West Coast Classic Cougar :: The Definitive 1967 - 1973 Mercury Cougar Parts Source (cougarpartscatalog.com)

Need to know at what speed it’s 5mph slow, then you can calculate the new gear. To increase the indicated speed on the speedo, you need fewer teeth on the driven gear. Some basic 5th grade math is all you need to do.

indicated speed / actual GPS speed = X

19 x X = new tooth count.

Round up or down to the nearest whole number.

Thanks for the help.
50 MPH / 55 MPH = .909

19 x .909 = 17.27

17 tooth gear


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