I have a similar problem, it's due to the sizing of my tires, probably the same case with yours, I also have wondered how hard it is to adjust it, scan tool required? I've got a 95 RS and I'm pretty sure it's OBD1 tho I have not checked yet
mine is off by 5 mph cause of tire size but a friend of mine works in the honda dealership told me if i want to fix it, he said he has to go in and remove pins in the cluster. thats the answer i got unless somebody know a different way will be great.
I'm curently in a automotive trade school right now and one of my teachers said that in the older domestic trannys you can swap out the speedo gear for a different sized one or something to change the accuracy of the speedometer. Oh yeah I have stock sized tires on it. Thanks guys.