GRT rearend centering and 14'wheels verses 12' wheels

[racer21]

got a 97-98 grt latemodel and wanting to know the measurment to locate rearend in car from side to side,also been running 14 inch wheels with 5 inch back
spacing and am switching to 12in (track rules)and was wondering if i needed to stay with 5 inch backspacing or change it,thanks for replys

[mqdirtracer57]

When I had to make a change to 12" from 14". I used a 4" backspace 12" wheel