Sources? I could be argued that due to "hedonic" improvements CPI understates inflation (viz, the entry level Apple laptop today costs about as much as the entry level Apple laptop 20 years ago, but is vastly better).

*overstates

One could plausibly claim it over the course of a few years, but look at 20-30 year average already makes it entirely implausible. Just because of compounding, that would mean 30-year old prices being 5x mistaken, and 60-year old prices 25x mistaken.

Only reason why some people think CPI is wrong is because people psychologically tend to ignore prices on things that are going down, only mentioning those that are going up. Prices on almost all food, all clothes and most energy (surprise!) are down vs 1921 CPI-adjusted price, and prices on housing are up per unit but down per square foot.

>Prices on almost all food, all clothes...

But can we even compare food and clothes from 1920s to today? Of course prices have gone down, and the quality has diminished greatly along with the prices. None of our food is nearly as nutrient dense as it was in the 1920s. Large yields is what the large producers care about. What does this mean for most foods mass produced these days? More water added. You're paying less, because you're getting significantly less.

...and the leaded gas in 1921 smelled so much better

Hehe. I don't care much about the smell, but I can tell you the leaded AV gas I use in all of my small motors, directly leads to me not having any engine knocks, easier starting, cooler at run-time, and in two decades of chainsaw operation I haven't had to clean or "rebuild" a carb. Yes, leaded gas is a whole lot "better" technically, than all of the additives we put into "gas" today. Gas really should be called something completely different these days to differentiate it from what it once was.