Tire size can also throw off the speedo, but usually not that much. My bet would be the trans gears are not matched to the rear end. Could have had some trans work done in the last 27 yrs.Thanks! I found one "for an '87-'88" so I just bought it. Now I hope it will fix the problem of the old one being erratic and running about 30% fast! This is a very stock Elky, so I can't imagine anyone has messed with drive/driven gears or rearend ratios.