Tires vs Rear end?

[buttugly]

OK here's a question. My 2001 had 353 rear end driven by 285-75R-16's. At 70 mph rpms were 1900 in overdrive. With these tires I calculated my final drive at about 324. My 2004 with stock tires (265 70R-17) and 373 rear end at 70 mph is the same 1900 rpms. What's up???

 

[Badunit]

Difference in diameter between the two tires is less than 4%. 4% increase from 1900 rpm is about 1975 rpm. Not a whole lot.