Interesting. I offered you information (actual data that I gathered from vehicle instrumentation) that shows how fuel economy can decrease while engine efficiency is increasing. Do you need clarification of the data? buttumptions? Math?
My data shows that your premise is wrong. Furthermore, it also shows (quite clearly IMHO) that engine efficiency (BSFC) and fuel economy (e.g., mpg) are *completely* different.
You don't seem to understand that engine efficiency (i.e., BSFC) is a function of both load *and* rpm. In fact, over an engine's normal operating speed range, BSFC varies at least as much with load as with rpm. But you choose to ignore varying loads and base your conclusions solely on rpm. That's not valid!
Here's a simple question: Do you think that your engine has the same efficiency at 40% of redline in 1st gear as it does at 40% of redline in top gear? If not, why not?
IMHO, you need to understand two things:
1. Engine efficiency (BSFC) is a *strong* function of engine load, as well as engine rpm. 2. Vehicle road load (i.e., the power required to hold a steady speed) increases quickly at highway speeds. For typical pbuttenger cars and highway speeds, required power is roughly proportional to speed squared (the exponent is usually slightly larger than 2, actually...).
Items 1 & 2 can be used to show item 3:
Maximum engine efficiency is *very different* from maximum vehicle fuel economy. -- Chuck Tomlinson
