Speedometer help.

[71LS5Formula]

Having issues with my speedometer. The car is a 71 Formula 455 and a new speedo from classic industries. I am running a T56 with a Cable X box to drive the factory speedometer. I have the box set to the proper settings for my tone ring. I took the car for a test drive to compare it to gps. The odometer is just about dead on. The issue comes from the speedometer part. When I'm hitting about 35-37mph as indicated by gps; the speedo reads about 20mph. I'm thinking there's an issue with the speedo, but I am not sure considering the odometer reads correctly. Any thoughts or suggestions will be greatly appreciated.

[andrewb70]

If the odometer is spot on, how can the speedometer be off?

Andrew

[71LS5Formula]

Tell me about it. Confuses me.

[130fe]

Which T56 do you have and what reductor wheel setting are you using?

[71LS5Formula]

Running an lt1 t56 with all the viper upgrades. Tone ring is the viper 10 tooth. The cable x box is set right as the odometer is reading right.