[vitus]

A gallon of gasoline back in 1930 (or 1925 for that matter, pre-depression) cost about 20 cents. So, even 2 cents per gallon would be a 10% savings.

https://pubs.acs.org/doi/10.1021/om030621b has some points of comparison (as well as a lot more historical context of the research that led Midgley & co to leaded gasoline if you're interested).

For what it's worth, per Table 1, ethanol is not anywhere near as effective as tetraethyllead -- it has an effectiveness of 0.1 per mol, vs TEL having a value of 118. Even when normalizing by molar mass (since TEL is a much heavier compound), TEL is still ~100x more effective than ethanol.

"... iodine had a few slight drawbacks. It added over a dollar to the cost of a gallon of fuel and we were afraid that would be considered a trifle excessive by most motorists."

"Further studies with more dilute solutions showed that a one-fortieth of one percent solution of tetraethyllead was equivalent in knock inhibition to 1.3% aniline in kerosene."

"The first public sale of Ethyl gasoline (the name given it by Kettering) occurred on February 1, 1923 in Dayton, OH at 25 cents per gallon (regular gas cost 21 cents per gallon) (Figure 8) before large-scale production had started."