3000RPM @ 55MPH With 3.54 Rear

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72 GMC 402/400. My tach reads 3000 rpm while cruising at 55 mph in 3rd gear, all the online calculators show this to be accurate with a 4.56 rear end, my spid says i have a dana60 with 3.54's and i turned over the rear wheels to verify, the driveline turned 3 1/2 times for one revolution of the wheel. with this combo i should be turning 2150 rpm at 55mph.

could my tach be off by almost 1000rpm? its the factory tach and it seems to be pretty accurate by the sound of the engine and how smooth it moves. the speedometer is accurate verified by GPS. Im not sure what it could be, any help is appreciated.