Quote:
Originally Posted by mkoesel
...Yes and no. There's an optimal RPM that trades consumption at a steady speed vs. consumption while accelerating or while under load changes. We could look at the extreme as an example - if you put in a gear to let the car go 70 mph at idle RPM the car would bog terribly on the slightest elevation change, grossly decreasing efficiency. In ther words the consumption vs. RPM curve is not linear.
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This is mostly incorrect, I think. In point of fact higher gearing (therefore lowering rpm at cruise) nets you better mileage, because there is less overall friction with reduced rpm, and more importantly, pumping losses are reduced. Pumping losses are associated with manifold vacuum (meaning when a cylinder is firing, power is lost because it must drive another piston down on the intake stroke against a vaccuum), and these losses increase when an engine is making more power at cruise, because you use less throttle to maintain a given speed, resulting in higher intake vacuum. When you use a taller gear, rpm is reduced, power is reduced, and you have to use more throttle to maintain a given speed. Therefore pumping losses are reduced.
An M3 with Vette gearing (say, 1500-1600 rpm at cruise) would be a pig out on the highway - but it would get a good deal better mileage. BMW has been saying since the eighties that to drive for best mileage, use max throttle and minimum rpm through the gears. The M3 would get best highway mileage if you had to use full throttle just to keep it going - but of course it would be more or less undriveable because you'd have to downshift for every hill or any puff of headwind.
Quote:
Originally Posted by mkoesel
Did BMW hit the sweet spot? Maybe, maybe not. I suppose that depends on many factors, some of which will vary between driver, environment, etc. My guess is that they spend a lot of time on this stuff and probably arrived at the current gearing after much experimentation, data collection, and deliberation...
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They said the hell with mileage, and went for a strong and responsive engine at cruise.
Bruce