Doing 55 mph with 47 indicated is 14.5% error, not 10%. Also there may be some error associated with your motorcycle in following her, which will compound the problem. Are you using a different size rear tire than the 140/80H15? A 140/90H15 is larger in diameter and will give an error, but it should not be 14.5%, it will most likely be around 6%. (There are variances between theoretical size and actual manufacturing tolerance.)
One way to check is along a stretch of highway in which the mileage markers are reasonably accurate. (Believe me, I've seen stretches of road particularly state roads where it was off, I'd try a US highway.) At 55 mph, you should be hitting each mileage marker at 65 seconds. At 60 it is 60 seconds. At 65 mph - 55 secs, 70 mph - 51 secs, 75 mph - 48 secs.
A check of both your bikes will let you know where your bikes stand. Then speed up or slow down at the particular speed you are trying to maintain until your speed times right with the mileage markers for that particular speed. Glance down at your speedo, this will be the speed you should be traveling in order to maintain the proper speed. At least then you will have established "Kentucky windage" on your motorcycle and be able to stay out of harms way with the LEO's and theoretically people following.
- George
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