Finally put a used Volvo large tach in my car (86 DL, M46) and ditched the ugly ol' clock that hasn't kept time in eons. I'm surpised at some of the readings that I'm getting. For instance, at a steady 70mph in 4th gear with the OD on, it reads exactly 2500rpm. I know that this car has a 3.31:1 rear axle, but that works out to 28mph/1000rpm. Seems like awfully tall gearing for an engine that develops very little low-end torque. What's interesting is that, at 80mph, the tach reads just a hair under 3,000rpm. The engine has to rev up nearly 500 rpm to accelerate the car only 10mph? Or is this not as "linear" as one might think?
If I disengage the OD at 70mph & 80mph, it reads 3,200rpm and 4,000rpm respectively. At idle, somewhere around 700-800rpm. Is it possible for a tach to be accurate at low rpm but not at higher rpm's? Can this tach be re-calibrated?