I have stock sized tires, stock transmission, etc. I have "calibrated" the speedo with those "Your Speed Is" boxes on the side of the road, and I'm apparently reading abot 10% too high at 50mph (reads 55). That works out to - yup - 20mph per 1k in 5th. (Assuming the TACH is accurate!) But the speedo isn't linerarly off, I've noticed... at 70mph (3500rpm) the speedo reads 80.

Basically I just go by the tach. 10mph per 500rpm in 5th, 8mph per 500rpm in 4th. So if I'm cruisin' at 3k in 4th I know that I'm rolling along at 48mph or so. 3rd gear doesn't work out to a round number, and 1st and 2nd gears aren't cruised at important speeds, so I just guesstimate. Really, I don't look at the speedo much anyway, just go off of the tach. Besides, on the highway, the speedo is just pegged anyway when I'm cruising at 3800 or so.

Note that 5th gear changed ratios in '84 and again in '85, and depends on 12A or GSL-SE, but 4th is ALWAYS the same.

 

According to my GPS, my 85 SE stock @150k miles reads about 76mph @ 80 indicated. My two newer vehicles (1999 & 2000) read about 78.5mph @ 80 indicated.