Originally Posted by Granpa

Wow, lots of variables here starting with track surface and layout. Generally tho, the softer the insert, the more grip. I'd start with Tamiya
hard foam inserts all around. If the car is loose, start packing the tires firmer. Where I run, most of the guys running S-Grips are using foam in the rear and a medium soft tire insert combo up front.

To get a medium soft tire, double stuff with a cut down touring car insert and the soft Tamiya foam insert. You can vary the hardness or softness by the length of the insert. Try starting with a soft TC insert cut to 160 mm. You can make the tire softer by shortening the insert. As you can see there are many combinations you can try by varying the length of the insert, the type of TC insert, the hardess and thickness of the foam ineer insert.

Try starting with a Hpi yellow or a Muchmore pink cut to 155 to 160mm on the outside and the Tamiya soft blue foam on the inside as a starting point. Unfortunately, it's all trial and error. Frankly, if you don't have to run Tamiya tire, why bother. There a tons of tires on the market that are much better on most surfaces.