So I sat down and figure this out, since I don't have a tach, I figured out how to calculate engine speed based on on vehicle speed. This won't work with 1st and 2nd gear, as the torque converter slip throws everything off.

Assuming circumference (N) is 25 inches (195/xx/xx tires), Assuming final drive ratio (Z) is 3.58:1, and assuming 3rd gear ratio is 1:1 (applicable on both 3T40 and 4T40 transmissions) you can figure out engine rotation at certain vehicles speeds like this. (Assuming torque converter lockup)

Engine RPM= D
Vehicle speed= Y

( Y x 5280) / 60 =L (L / N) = A (A x 3.58= D)

So rounded, the chart looks like this.

Vehicle Speed Engine RPM

35MPH = 1700RPM
45MPH =2200RPM
55MPH =2650 RPM
65MPH =3150 RPM
75MPH =3600RPM
85MPH =4100RPM
95MPH =4600RPM
100MPH = 4850RPM
109MPH (Limiter) = 5200RPM

There you have it, if you have any questions, please let me know!