Second try to reply, Walt, the site has been mostly unavailable for weeks, and lost my post after I wrote it. Tach revs come from an alternator pulse, if the pulley has been changed to a different size, the tach will read wrong. The speedo gets it's signal from a module near the ECM. Yours is 10% wrong, the difference between a 3.73 diff and a 4.10. Someone may have changed the module, GOOGLE how to connect the right resistors in it, to correct the speed.
Grant H.
1995 6.5 turbo GMC K2500 extra cab 4x4 pickup, PMD under bumper, mechanical wastegate, 228,000 miles, rebuilt engine, 4.10 diff, 16 mpg
1994 6.5 turbo GMC K2500 4x4 suburban #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, 3.73 diff, 16 mpg
1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, since 1992.
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