Aftermarket tach questions

[Keyser Soze]

I have an aftermarket tach in my m1028, and I believe it is wired into one of my alternators. My truck has 4.56 gears, 400 turbo transmission, us gear dual range, and 37" tires. My question is to the accuracy of my tachometer. Technically speaking it would have to be calibrated to compensate for the difference in rpms due to the pulley ratio. When I am driving, and my speedo says 45 (i'm sure it isn't accurate) with the dual range in high my tach says anywhere between 2200 and 2600 depending on throttle position. My transmission shifts firm, and consistently. So is my tach off, is my transmission slipping at higher speeds, or am I just paranoid?

 

[Barrman]

Does the dual range give you anything other than 1:1 in high? Plug your numbers into a site like this:
Randy's Ring & Pinion. RPM Calculator.

If your truck is still stock electrically, then the STE/ICE tester will give you a rpm reading. Using pulse sensors in conjunction with an aftermarket timing light, scanner or rpm sensor will also tell you engine rpm. Most diesel shops that have worked on 6.2/6.5 or 6.9/7.3 Ford trucks in the past will have a way of checking rpm. I would suggest you take it to them for tach calibration.