G60-15 replacement question

I have these G60-15 tires but they don't seem to grip well. So I am looking to replace them with the Mickey Thompson ET Street Radials. My question is which size tire is closest to the G60-15's I have now. The 255/60-15 or 235/60-15. The G60-15s I have now clear the leafs by about 1/4 inch because the backspace on my steelies is 4-1/2. Ideally it would be best if MT made 245/60-15 but they don't. I think I can get by with a 10" section width.
Any experience or suggestions would really help since these tires aren't cheap. Here is picture of the car with the tires mounted.