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I'm not real sure what your question is but I'll try to answer. In all cases speed within a gear selection depends on your trucks rear differential gear ratio. If you mean what is the lowest speed you can shift to third gear you should be able to drive on flat surfaces as low as 20 mph. If your truck seems to run well you are OK. If your truck jerks or seems to be laboring, you probably have a very high gear in the rear end and may need to wait to about 25 mph.

If you are talking about the maximum speed you can go in third gear you will know when you reach it because you will have also reached maximum effective RPM on your engine and the truck will simply not go any faster in that gear.

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14y ago

What else can I help you with?