Another way to check (roughly)
With the trans in drive (OD locked out) I should be driving 1:1 +/- converter slip (converter lockup not wired yet).
That should be the same as the original trans in 4th.
If I could find the stock speed at 3500 rpms I could calculate the ratio difference. present reading 3500=67mph.
8:20 tires (new)
