Speedometer issues?

[AALynch]

Hey guys,

I've got a '69 Camaro w/ an LS1/T56 in it. I'm using a Speartech new harness and Stewart Warner electrical gauges...I have HP Tuners for it if that helps in this any.

My speedometer seems to work just fine with the car, except for one thing. When I am anywhere from ~70% throttle to full throttle, the speedometer will spin to 120-140, and stay there until I let off the throttle. I'm not too knowledgeable in wiring, so...any ideas? The Speartech harness had a speedometer-labeled wire, so it runs straight to the input on the gauge...

Thanks,
-Adam