Proper measuring techniques are very important in order for your Torque-Tight to fit properly. The easiest way to do this is to utilize a high quality 6 point Socket Wrench, however, there are some things to keep in mind.
First, when sizing up a nut, always remember that metric and standard (inch) nuts may be used. With that in mind, be sure to check that you are using the proper size, for example, when you are working with large nuts, a 1-5/16” socket will fit and work okay on a 32mm nut… but a 32mm will fit properly. Always try to size up nuts to the smallest size socket that will fit.
Second, if nuts have been replaced over time, be sure to check multiple nuts throughout your truck(s). The ASME B18.2.2 Spec for Heavy Hex nuts allows for a large variation for the Min-Max of the “across the flats” dimension. Be sure that your nuts are all commonly sized!
If you do not have a socket large enough, simply use a pair of calipers to measure across the flats of the nut, as seen in this image.