Biggest tire on 1969 dart

I want to run a 15 x8 but don't know what backspace
Have seen a few guys with a 5.5 backspace

I'm not going to run a street tire the car won't hook

I run cheater slicks
Most times it won't hook on the street with cheaters

Hooks awesome at the track

I need to know about backspace as i have seen posts with people getting 8" rims on darts with shackles moved in