Not exactly sure what transmission you have in your units. My 1985 has an Allison 4-spd and at 2800 rpm I am doing over 60 mph. At 3250 I am doing over 70 mph. If you don not have an Allison you might consider it. They are a very popular transmission and should install right onto an Isuzu.
Which model Allison 5-speed will install right onto an Isuzu?
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