Of course, there are no examples of Pawnee ASIs being incorrect!
Many of us have towed fully ballasted behind 235HP Pawnees at Ely, NV. Field elevation at Ely is six thousand five hundred feet MSL. Predictably, the main problems there seem to be ground roll length and climb angle.
Short of calibrating the ASI or adding a radio so you easily communicate without the awkward "speed up" signal, perhaps make a higher standard tow speed for anything that doesn't look antique?
Jim