You have to realize that these diesels are not high rpm machines. Whereas you might be able to wind a 351 out to 4500 rpms if need be, you simply cannot do that on a stock diesel. Not that its not reccomended, but its simply not even capable of it. I take that back, if you threw insane amounts of money into a diesel, you could wind it out then. But in stock form, most diesels redline at 2500 to 3000 rpms, and thats maxed out, pegged out, all the way to the governor. So if you have a towing ratio rear end, with a final gear in the tranny of 1:1, then you talking 45mph at redline in top gear. You are almost required to get an overdrive tranny, or change the rear end to a highway ratio.