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If you are at 2200RPM at 65mph, you do not have stock 3.73 differential gearing. You must have a 4.30. Your delta is never going to match up with the stock boys, even if you weren't pulling a load in 100 deg heat.
You might try running unloaded on a flat run at 1900RPM to get a more useful measure of your delta. But... even then you might find that your cooler is fine and you still have those high temps when loaded and in heat at highway speeds. In that case, it's air-to-oil time.
Edited to add...
It's air-to-oil time if you can't live with the temp. An EOT of 240F (250F-265F in the pan) is fine for a synthetic. So, really, this is more about what you are comfortable looking at than what the oil can stand.
I am assuming that you have a Ford OEM oil cooler. If you have a Dorman/Sinister/eBay/whatever in there, swap that out and the EOT will come down.
Since you have a relocation, one other option is that you could swap the existing oil cooler out with something higher than 100,000BTU/hour. That suggestion is purely theoretical, though. I have considered it out of pure science. As far as I know, nobody has ever done it.
2004 F550 CC Lariat
BPD oil cooler relocation and upgraded EGR cooler (spooled), BPD oil bypass filtration, aFe Magnum Force CAI, Mishi radiator, 7.3L fan clutch, EC1 ELC, XDP coolant bypass filtration, '08 trans pan/filter, PTP direct clutch solenoid, blue spring, 6.4 starter, MBRP y-pipe, BD boots/clamps, '05 intake elbow, ccv reroute, Torque Pro + OBDLink™ LX, Autometer EGT
Last edited by djmaguire; 08-14-2016 at 08:34 PM.