Hot Rods buicknailhead st400 auto trans

My 401 with the st400 tranny shifts into high at 30 M.P.H.
My 425 with the st400 tranny shifts into high at 20 M.P.H.
I believe the gear ratio's are the same, why the difference in
shift points. Any Advice here

 

Vehicle weight, tire size, linkage adjustment....etc., etc.

 

Recently I worked on a 1966 Wildcat with a 425 and an automatic transmission. It shifted about the same points. I think I was in high gear around 20-25 mph

 

My ST400 in my '35 pickup will shift at various car speeds depending on how much the accelerator pedal is pushed down........more pedal = higher shift points.