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speedometer help needed

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I have an 88 superliner 350hp with a mack 9sp and mack rears. The problem is that the speedo reads about 22mph off of actual speed confirmed by gps. It is a consistent doesn't vary going faster or slower speedo doesn't move until you are running 22+ now when I bought the truck was told it had 3.86 gears was also told of the speedo issue and that it was suppose to have 22.5 talls and he put low pros to gain clearance pulling a van trailer. So I requested the build sheet from the museum and the truck originally had 5.73 gears so what part do I need to change to correct the speedo and odometer? If it was a show truck or purely personal farm truck wouldn't care put will be running either leased to my father in law or the place I bought it from pulling flatbed and hopper bottom running midwest states so true cost of operating per mile is important and so is accurate ifta reporting thanks. Sorry it's a long post

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Ok I'll give them a call. Not sure how good they are around here. It looks like the previous owner has attempted to fix the situation because there is a lot of new parts coming off of the transmission and the speedometer head is newer

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