I have a 98 ISB 4. 10 and a auto at 55 in third it will turn 3000 rpms so yours is working right. With the 4. 10s in third 60 mph is all you will get at red line.
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1998 2500HD,reg cab,4x4,auto,4. 10LSD,ISB,trailer tow, boss 8'2"v-plow,pyro,boost,trans temp gauges,up graded...