Put Brian Preston's rules through your numbers
Two rules JMM has located in a dated source, rebuilt as a working calculator, and taken a position on. Three outside parties have published a figure on them, and those numbers are on the page next to the rule.
- Sources located
- Money Guy
- Source years
- 2026
- JMM verdicts
- holds up, overstated
20/3/8 car rule calculator
What car price does 20/3/8 actually allow you?
holds up The 3 is the rule. The 20 and the 8 are how you survive it. Money Guy, 2026Open model
Wealth multiplier calculator
What is a dollar invested at your age worth at 65?
overstated The arithmetic is honest to the dollar. The number it produces is not the number people hear. Money Guy, 2026Open modelNot every rule here survives its own arithmetic
JMM does not grade these rules as a set. One of them, wealth multiplier calculator, is where the position turns critical: The arithmetic is honest to the dollar. The number it produces is not the number people hear.
Each page separates three things people usually run together: what the source states, what JMM's model adds to make it computable, and where the rule itself breaks. The last one is not the calculator's limitations list. It is the failure of the idea, which is the part the person who published the rule has no reason to write down.