lean burns quicker and thus internal engine pressure is much higher. My thinking is sure ethanol has higher octane, this just lets big oil use less octane boosters in plain jane fuel. My HX lean burn lost about 15% or more when ethenol was introduced into Texas, the MPG loss is even greater with lower octane fuel. yes my car is in perfect shape, yes I keep pen and paper of every fill up. I've gone from about 43 MPG to 36 MPG on cheap stuff, 38 or so on 93 octane. The car does ping with the advance I run on cheap stuff, if I set the ignition advance to factory spec MPG takes even a bigger dive. Ethanol was a big downer for me and my HX.
Question for people with a ScanGauge, does it show you when the car is in leanburn? Most foreign cars have a MPG light that lets the driver know but it does not come on a US spec HX or any other lean burn US spec car that I know of... bummer.