I used to stick to that rule and get a sensible mid sized Toyota Corolla or Honda Accord that’s a few years old as needed. But in 2013 (the last time I needed to get a new car) the cost savings were really low compared to new. I think the used market was particularly hot so you’d save less that 10% of the cost, so I ended up getting my 1st and only (so far) brand new car ever.