There are 300million adults buying 17million cars per year. That's an average of 17 years per car, not 4. Rules of thumb don't tend to work well at the extreme

There are 300million adults buying 17million cars per year. That's an average of 17 years per car, not 4. Rules of thumb don't tend to work well at the extremes, since discretionary income matters more than total income, as far as debt is concerned.