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Piston speed is o.k. as a rule of thumb, provided the ratio of road load power requirements to displacement stays the same but when we're looking at small'ish vehicles with big engines, or vice versa, it isn't very accurate. If we say a small car with a two liter engine that needs 25hp@55mph has an ideal piston speed of whatever in terms of economy, then if we have a four liter engine in a truck that needs 50hp@55mph the ideal piston speed for economy will be more or less the same.
Otoh, and six liter engine in a vehicle that's only about five hundred pounds more with the same or lower CdA than that of the small car would need to cut engine speed proportionally to the difference in displacement and increase (or decrease it if it needs less power) it proportionally to the extra power required, so it would be moving maybe 2.5 times slower to see optimal efficiency.
There's a problem however if the drop in speed required puts a decent load(torque) level at or less than idle. The solution is to shut off half of the cylinders and all of a sudden a 6L becomes a 3L, with the ~50% increase in mileage mentioned earlier. A large vehicle at 55mph can see a ~40-50% increase in mileage, and something like a vette should be able to see better w/ the right gearing. Something else indicative of poor gearing in terms of efficiency is if mileage doesn't change much w/ speed. ~27-28mpg@75mph is common w/ a vette, so if ~32mpg@55mph is the best that can be done, then we know engine efficiency increased a whole lot because if it was optimal at 55mph it would've been cut in half at 75mpg instead of dropping by ~12%.
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