I just bought my first mustang, it’s an 89 mustang gt. 5.0 5 speed bone stock except for exhaust and rear end gears, unfortunately I don’t know what the ratio is. At 60 mph in 5th it’s right at 2k rpm. I originally thought it has 4.10s but after doing some reading I think it might be 3.73s.
There is some math involved with using rpm/tire height/speed but since I suck at math I mark the driveshaft, spin the tire 10 revolutions counting the turns on the driveshaft gets you the ratio, just turn the tire slow some someone else can count. You can do it in one revolution but that will be an approximate, 3 1/2 revolutions would be a 3.55 gear, 3 3/4 would be a 3.73, either way will get you in the ball park. My dad told me to turn the tire 20 times but I think he was just messing with me!