Your 44's aren't really 44's. The tires aren't "round" when they're on a vehicle. If they were, you'd get a straight line of rubber touching the ground across the surface of the tire. You don't. You get a patch. So while the distance from the TOP of the tire to the center of the axle will be 22 inches, the distance from the center of the axle to the street will be less as the tire is "flattened". A lot of it depends on your air pressure, weight, tire sidewall stiffness, but you're probably closer to 42 and maybe even 40 inches for your calculation. Use a similar percentage decrease for smaller tires. My calculation shows a REAL 44 inch tire (that's a true 22 inch radius) would show 2547 rpm assuming 1:1 drive through your trans (no slip) and transfer case. A REAL 42 would show 2669, and a 40 would show 2802. In theory a drop to a 42 at the same MPH shown would increase your RPMS by 4.76% (3457 rpm). A drop to 40's would increase you RPM by 10% (3630 rpm). Your speedo is most likely way off as it thinks you're running 15's through 3.50 or 4.11 gears. It assumes 15 inch tires are spinning and the revolutions = 65 MPH, but a 44 inch tire goes a LOT further on each revolution, offset by your higher gear ration. A 44 inch tire goes 138.23 inches per revolution. A 15 inch tire goes 47 inches per revolution. Your original differential could have been a 3.50 or 4.11. Assuming it was a 4.11 and you moved to a 5.13 your motor is spinning 25% faster than it was but you're going 2.94 times further. You're going a LOT faster than 65, closer to 78. Not bad for 44's, Regardless, assume the drop in tire size will raise your REAL rpm by 5% for 42's and 10% for 40's.
