https://www.sounddeadenershowdown.com/ good reading here. Some people cover every square inch of the interior. I personally covered mine in strips and squares strategically placed to cover around 30% of all the larger areas. Remeber, the butyl based stuff dampens harmonics, it does not block sound!! Question: How much of a cymbal do you need to cover with your hand to make it go "tink tink" as opossed to "crash, bang, clang"? Answer: not very much. After a certain point you begin to get diminishing returns. You spend more money and add more weight to the truck just to obtain that extra little bit. If it's worth it to you to blanket the whole cab, then go for it but just remember that most harmonics get dampened with strips and squares. True sound deadening is a series of steps and products that all work together. Edit: the inside of the doors are quite important to dampen too. I covered the inside of them at least 50%. They are a huge flat panel exposed to lots of vibrations.