It allowed people to use engines with better gas mileage, what is a different thing.

Just adding the lead addictive to gasoline reduced your gas mileage. But it made better engines work.

It surely reduced it by a tiny amount compared to just straight octane, but ethanol reduced it by something like 10%. So using TEL instead of ethanol gave you about 10% higher gas mileage.

If you are comparing it to pure ethanol, yeah, that's the ballpark.

Leaded gas compared to ethanol mixed gas with equivalent octane numbers should have something around 1% or 2% difference.

https://en.wikipedia.org/wiki/Energy_density#Chemical_reacti... says "Gasohol E10 (10% ethanol 90% gasoline by volume)" is 33.18MJ/liter, while "Petrol (Gasoline)" is 34.2MJ/liter. That's a 3% difference, which is much closer to your 1% or 2% than to the 10% I had believed. E85 is lower still in energy density.

All of this is assuming the engine has the same efficiency on both fuels, rather than, for example, using a much higher compression ratio on the ethanol.

At worst to get equivalent octane rating you lose about 4 percent in power density. In practice you won't notice due to the slightly more efficient burning of alcohol.

I don't think that is correct.