I'm afraid the percentage method won't work.
5 mph error at 60 mph is 8% 5 mph at 25 mph is 20%
You just need to drive a measured mile and record the odometer error. If there aren't any "measured Mile" sign markers left around you, use mile markers on the main roads. I see that they are marked in tenths now so I would assume they are very accurate.
If you changed tire sizes over the years, you can get the mfg data for rolling diameter of each size you had and correct for the percentage difference to what you have now.
