After initial and continuing success with my first WalMart insert, I have found that getting the right match with tires and rims and bikes is tricky. Have quite a collection of useless gear from attempts to get something else to work. Once, measuring partner Dave Rogers had to pedal downhill with mountain bike style tires. It was the same setup where the rig evidently had a "ka-chong" settling moment during break-in and I had to go back and add 9' to a 5K.
Fortunately, though my original setup is doing fine having increased only from about 18,545 to 18,565 cts / mile over 5 years of use. Whether that is wearing off rubber or the insert is softening, I do not know. Fine for a flat 5K but for longer rides and hills, I use a 120 lb pneumatic on a lighter bike.
Oscar