It might be easier to make an instrument that measures miles per hour more accurate than an instrument that measures kilometers per hour.
Think of measuring distance with a ruler using units of half and inch. You can only be accurate to within half and inch.
Compare the accuracy of that same ruler with one using units of a thousandths of an inch. You can be accurate within a thousandths of an inch
Miles and Kilometers are a similar situation:
A speedometer that is off at 100 mph by 5% will only be traveling at 95, an error of 5 mph.
A speedometer that is off at 100 kmh by 5 % will only be traveling at 95 km/h, an error of about 3 mph
Both will have the same percentage of error, but one is significantly more accurate than the other.
Don’t confuse the term accuracy with resolution or precision.
No one said we were limited to whole numbers. On the speedometer, simply display either the English or the metric speed to a resolution of one decimal place and the English units have no practical advantage over metric units. Accuracy (displayed vs actual speed) on the other hand, is all in the hands of the speedometer’s design and implementation.