Also, the hit *appears* to be bigger in a cars that gets high MPG readings to begin with.
In other words, you take a 20% efficiency hit in a "50 mpg" car and you drop to 40 mpg. But the same 20% drop in a 20 mpg car puts you down to 16 mpg. The 10 mpg drop looks much worse than a 4 mpg hit, but it's proportionately the same.