My favorite insert is a 7.5mm. Its got great bottom mid top and a little better mileage than an 8mm. 8mm most situations decreases your traction because of the sheer power output/wheel spin, it's too much of a handful. If you like to drive the car run a 7.5, if you like the car to drive you run an 8. On anything but super high traction 7.5 is the greatest of all worlds.