Quote:
Originally Posted by t vago
I currently read about 2050 RPM at 72 MPH. I figure that with a 0.78 overdrive, I'll go down to 1600 RPM.
Friction? I figure that friction from the propeller shaft back to the wheels should not change, and friction before the overdrive should actually go down. The overdrive itself, though, will add an element of friction. Whether that friction is significant enough to affect FE is another question entirely.
I'm not that concerned with lugging at 1600 at part-throttle while going down the highway, which is the only time when I would engage a second overdrive.
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You are adding another gearing system to the driveline, so the drag that it imposes on the driveline adds friction. And unless your engine produces the same torque level at 1600 as it does at the torque peak, you will not be operating at it's sweet spot, therefore a loss in mpg. Since you didn't say what vehicle it's going in, I guess it's your Dakota, so the aerodynamic drag is high especially at 72 mph. And you are using a gas engine with a throttle blade in the induction system so the pumping losses are high. It'd be better to do a ring and pinion change to get to where you want to go, a far sight cheaper and more expedient. But based on your numbers, the vehicle is geared pretty tall anyway (2050 rpm at 72 mph is slow for a gasser).
If you want to pull 1600 rpms at 72 mph and get great mileage, put a turbodiesel in that thing and shape the torque curve like a semi truck engine; or maybe alter the cam timing, ECM calibration and gearing to do the same thing with your existing engine.