actually, the actual 4th gear (or whatever top gear) of the trans has little to do with overall milage. The reason the 5 speed auto gets better milage than the 4 speed auto is that the PCM can keep the engine in its most efficient powerband much easier. The overall ratio spread is much more usable.
In the case of the mitsu 5 speed, the top gear is exactly the same in the 5 speed auto as the 4 speed auto. The other 4 gears are lower (especially 1st), and the final drive is higher to allow for lower RPMs in top gear but still allow for a lower overall ratio spread.
An example is this.. Only the montero in this country has had the 5 speed for a while (since 2000). the cheaper version of the montero had the 4 speed in it. whilst both transmissions are basically identical (the 4 speed basically has a gearset yanked out of it, with 90 percent of the parts being interchangable), the ratio spread on the 5 speed is much greater.
here are the ratios for the 4 and 5 speeds in the montero (the front driver transaxles are nearly indentical).
now, the 4 speed has an axle ratio of 4.636 to 1
the 5 speed has an axle ratio of 4.3 to 1.
do the math, and you get this:
the 4 speed has a ratio spread of 13.18 to 3.388 to one
the 5 speed has a ratio spread of 16.29 to 3.14 to one. So the 5 speed has a MUCH larger spread of ratios, making it much easier on the engine to keep it in a usedful powerband, whilst at the same time making it perform better, AND get better milage.
the new 6 speed in the outlander (coming out in fall) has even a WIDER spread of ratios... making it the most useful of all for this stuff.
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