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Originally Posted by Gregte
Another advantage big trucks have over our American cars is that the diesel engine is more efficient in producing power than the gasoline engine. this is due to the higher compression ratio that a diesel can and does run at.
If larger engines that didn't have to work so hard were advantageous FE wise then our large engine cars would get better mileage than our small engine cars, but alas the opposite is true. An engine that is working harder is more efficient. For instance, my GMC with its 4 cylinder engine has to work harder than the same pickup with the 6 cyl engine but my smaller, harder working engine, gets better mileage.
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It's also due to not having throttling losses.
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3. all things being equal, diesels engines get better mileage per gallon because diesel fuel has 10-15% more energy per gallon than gas - not because diesels have high compression.
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And that.