I was once off 7 tenths in the spring and forgot I had backed timing off for cool fall air street driving. was at 29 back to 38 and it went from 12.2 to 11.5.
Just this week I tried throttle stopping to the twelves went back to full throttle and 11.21 [Edited by Moparts - Keep it clean]?? oh timing went from 35 to 40 and 10.82!!
It happens

backing plates on rear axles rotating and binding up the brakes?