RPM on the highway

[squidley63]

I want to get an idea on what rpm that others are seeing on the highway without overdrive as I'm pretty sure my tach is off or something as the rpm seems to be lower than indicated at anything above idle. I have a small block in it with msd6al ignition with vdo tach and have been stuck between their tech departments on if I have an issue. Msd thinks I need a tach adapter, but vdo says my tach will work fine with an mad box. Neither will tell me what the symptoms are of needing an adapter and not having one.

My rpms indicated at 55 mph is almost 3500 rpm with the stock 3 speed and 4.11 gears with 235 75 15 tires. Calculators are telling me I should see about that rpm at 65 mph. I know our trucks arent very aerodynamic but I think it shouldnt cause this much difference from calculated.

I have checked the simple stuff like slipping clutch and any bad connections between the tach and the mad box and the dip switch setting on the back of the tach. Any info others could give me would be greatly appreciated.