That doesn't seem to bad, all considered. 5% would be the natural number for any one year out of 20, to get to 0.4% from 5% in 20 yrs should be about 8 yrs of m

That doesn't seem to bad, all considered. 5% would be the natural number for any one year out of 20, to get to 0.4% from 5% in 20 yrs should be about 8 yrs of mean life per car, assuming exponential decay. I'm fairly sure this thought process is missing some fudge factor to make it exact, but seems about the right ballpark.