Engine efficiency outside of peak.

[Hersbird]

I was playing around with the economy calculator on the aerodynamics page and was wondering where to find numbers to use for engine efficiency. I can often find peak numbers but what would be a common number to use outside of peak? Basically how bad does that number get if say operating at a low load, low rpm condition where some motors are way outside of BSFC. How does a diesel compare with a standard gas. How does direct injection compare with regular multi-port FI?

It started with trying to convince trailer towers to slow down. Many think their setups get the same fuel economy no matter how much they slow down. That would only be true if the somehow are way outside of BSFC. I agree my truck (a Cummins)"feels" great at 2000 rpm, smooth as a baby's butt with power to spare. But it gets better feul economy towing my sail at any speed slower even if it means using a lower gear.

I think people just want it to be true and towing 11 mpg doesn't seem that much better than 7 mpg even if the percentage is huge MPG wise.

[Hersbird]

So here is one chart (the only one I have found) of a new Toyota with the highest thermal efficency. It starts at 22% and goes as high as 40%. It is a direct injected gas, non turbo. So how much worse than 22% does a more basic motor get or do these fancy motors just get better peak thermal efficiency?