You can't tell how fast you "should" be with a 60'. There are way too many variables especially with a truck being much harder to hook than a car.

All things considered, nothing seems out of the ordinary. Your trap speed is too low to even think about 13s at this point, and the 60' isn't bad, but nothing to write home about.

I don't mean to come off negative, but all the variables to your timeslip add up. I don't see you dropping more time without a bigger change somewhere.