Need Help 2nd Opion

I recently changed my rear end gears from 278 to a 391 in my 8.25 rear end. I am running 17" rims. My spedo is off about 30MPH at 55. I am being told that the only way to correct this is with a custom built gear reduction box because they don't make a spedo gear large enough. What do you think?:glasses9: