I use it 2 parts water to 1 part tire cleaner. It works very well if you let it dwell for a little bit. I also use a tire brush. IMO, itd be okay to dilute it a little more, however, I wouldnt water it down a whole lot more... I'm planning to try 4:1 next time.
I dont really know about its safety around wheels. Its pretty easy to keep it just on the tire, except when rinsing. And then, its at its weakest.
Overall, I love hi-temp products. They work extremely well and very economical! Tire cleaner followed by Grape Tire Dressing... beautiful!