Well now, let's see, new speedo gauge, new cable, "new gear"? So....assuming all that is really true and all are correct parts...something is not right, right? Or something is slipping. Don't think that the tires would cause that much. Are tranny and gauge original to truck? The exact tooth count gear for what trans? exactly like what? the old one? What was wrong with the old one? You and I probably suspect the wrong gear, slipping gears, speedo not rebuilt properly, cable square slipping in the female. If everything is "engaged" properly, then it's a ratio problem as Tim indicates. Because the speedo IS being driven. My guess: bad speedo gauge because I know you made sure the cable was engaged properly on both ends. BUT I don't know what trans, from what year, from what model vehicle you have. The fact that the odometer is correct and speed is not means it's the Speedo gauge provided you drove far enough to pick up any mileage error.
