This is for the truck in my sig. I ran the tire size and gear ratio though a gear ratio calculator because I thought my speedo was off. Acording to that I should be around 2000 at 65mph. But my tach is reading more like 2250 @ 65. Next I checked my speedo with gps and it appears to be reading correctly. So my question is it possible my tach is that far off?

 

The tach isn't that close, but I would be surprised at 250 Rpm out. I set my idle using a phototach and the needle didn't move on the dash as I moved the idle up 100 Rpm.