re: '97 F250 HD differential gear ratio
I have been considering tires sizes and gear options so I happen to have thrown together a little spreadsheet...
Based on 4:10 gear, the ZF-5 OD ratio of .77, and 255/85-16 turning 629 revs per mile, the theoretic rpm at 70 mph is 2,316 rpm.
Reality when compared to the figures my spreadsheet generates has shown that if I add about 3% to the claculated rpm, I come real close to what the tach shows. Your calculated 2,316 ends up at 2,385. So if you are on the low side of 2,500 on your tach, you are probably calibrated about right.
For reference, if I plug in 4.56's with your OD and add the 3% factor I come up with 2,653 rpm...?
Last thoughts, the PSOM is supposed to be adjusted when you change tire size, though you say you have confirmed your speedo's accuracy.
Finally, you could jack up the rear end and (since the "C" in C% represents a locking dif you shouldn't need to block the opposite wheel) rotate the tire one revolution and see whether the driveshaft rotates 4 time (for a 4.1) or closer to 4.5 times (for a 4.56).
97 F350 XLT, 4x4, Toreador Red, CC, PSD, MLE0, ZF5, 3.55 l/s, LUK clutch,Velvet Rides, ISSPRO gauges, v/cover vented to tailpipe, DPPI downpipe - modded to 4" right below firewall seam, open element intake using Duralite filter, coolant filter mounted between washer fluid bottle and jack, gutted EBPV, 99+ I/C installed using early 99 Y-pipe and custom pipes, 4-pos chip, BigBopper Seat Mod, Bilsteins, 130cc injectors, homegrown 4" exhaust, uses 36" Magnaflow muffler (Model #12773 body is a full 30" long - nice and quiet!) Custom Fuel System using Carter electric pump, Aeromotive regulator and frame-mounted filters...
2003 Tahoe MS27TT (27' Lite travel trailer), Prodigy Brake Controller.
1991 5.0 LX, runs 12.57 @ 109 mph
My Intercooler/Fuel System Install Webpage (section on CCV Mod too): http://members.localnet.com/~boys3/