Mark, you should be getting better than that with the '57. You didn't mention what transmission or rear-end gear you have in it, but if it's completely stock and the engine isn't worn out, 16-18 is a realistic figure.
As for using high octane fuel helping with your milage, no, it isn't going to change that, just cost you more for fuel. The compression ratio of an engine is the determining factor as to what octane rated fuel to run. The higher octane fuel is meant for high compression, high performance engines, your 235 should run just fine on regular.
Needing to use the lead substitute additives as a valve lubricant is a myth. The TEL (tetraethyl lead) was added back around WWI as a cheap way to boost the ‘octane rating’ to prevent premature detonations. TEL made a lot of money for the investors in the Ethyl Corp. Octane contains 8 carbon atoms linked together, and it will spontaneously combust even at low compression ratios. An 87 octane rated fuel is 87% octane and 13% other blends like heptane or other fuels used to adjust the rating. This is more costly than simply adding TEL. Also Alcohol blends will also boosts the octane rating.
Denny Graham
Sandwich, IL
