If you're planning on running 35 tires on your Silverado, you'll likely need to regear your truck in order to maintain optimal performance. Depending on the size of your engine and how much power you're looking to get out of it, you may be able to get away with not having to regear. However, if you want your truck to perform at its best with 35 tires, re-gearing is definitely something you'll want to do.
There are a few things that contribute to why re-gearing is necessary when running 35 tires. First, the increased tire size leads to increased rolling resistance. This means that your engine will have to work harder in order to keep the same speed as it did with smaller tires. Additionally, the larger tires also add weight which can further decrease performance. Re-gearing helps offset these effects by increasing the ratio between the rotational speed of your driveshaft and wheels and the number of revolutions per minute (RPMs) that your engine produces. In other words, it makes it so that your engine doesn't have to work as hard while still maintaining decent speeds.
The most important thing to keep in mind if you decide to re-gear is that you must do both front and rear axles at the same time. This maintains proper balance between both axles and prevents any potential issues from occurring down the road. It's also important to note that re-gearing is a fairly involved process so it's best left to a professional unless you're confident in your ability to do it yourself.