My factory deck did the same thing, you can adjust the volume level that it increases too or just turn it off altogether. My ultimate solution was to just yank the deck out and replace it with a Pioneer headunit.

 

Yup, im retarded, that last option on the menu fixed it... Had the truck since November of '03 and been wrong about it the whole time.

 

This is a good post overall btw, I have the same problem in my Lariat w/ Audiophile. Given its happening w/ every type of radio and even w/ replacement radios makes me think this problem has nothing to do w/ the radio and there is a fault w/ the speedo sensor..