The sort of person interested in getting high miles per gallon from a car is likely to own one already.
This means they're already using relatively little gas, and even fairly large increases in economy would save proportionally very little additional fuel, compared to someone moving into a moderately economical car from an inefficient one.
An example: A driver doing 12,000 miles per year in a 30-mpg car will use 400 gallons, costing just under $1,500 at current fuel prices. Should they trade up to a 50-mpg car -- a Prius, for example -- fuel use will fall by 160 gallons, and their gas bill will drop to less than $900, a $600 saving.
But a second driver who trades a 50-mpg Prius for our hypothetical 70-mpg fuel-sipper makes seemingly the same 20-mpg jump. This time, though, the overall saving is only 70 gallons, cutting the gas bill only $250.
It's why we've said that miles per gallon is a stupid way of rating a car's efficiency. It isn't linear, so doesn't illustrate the cost reductions of moving from one car to another.
Granted, a driver trading from say, a 35-mpg car to a 70-mpg car will realize some hefty savings: Fuel use is halved, as are the gasoline bills.
But for those already driving an efficient vehicle, savings could be minimal. Particularly if they already own that 60-mpg first-gen Insight.