The 19.5 tires are part of the answer. Your tires are quite tall. If you can measure the height of one of your rear tires you can use that and assumptions about what rear gears you have to estimate RPM at Speed.

If I had a height for a 19.5 I'd already have those numbers in a table in this reply.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...