[justme1969]

My gear change was done with the tire circumference. there is a calculator to figure rotations per mile I will copy from other thread should help.
As for sprag and gear changes seek a trans shop that build/s racing trans for these cars.
I worked at Ford when they first came out and there was a solid following of tuner crowd working these cars.

215/17/45 tire
Originally Posted by vrmilionzx
I converted to mm (so everything is in the same form of measurement), and used 25.4 mm in one inch, so:
Old Wheel Dia = 17x25.4 = 431.8mm
Old Tire height = 215mm x .45 (aspect ratio) = 96.75mm
Old combo total Dia = 431.8+(2*96.75) = 625.3mm

185/15/65
New Wheel Dia = 15x25.4 = 381mm
New Tire height = 185mm x .65 (aspect ratio) = 120.25mm
New combo total Dia = 381+ (2 x 120.25) = 621.5mm

divide a mile by your outcome for the rot per mile.

stepping up either tire height {215 to 225/235} or aspect increased .67 to a .70 or .75 increases circumference without changing wheels.