With all the attention being put on fuel mileage, why don’t car makers use overdrive transmissions like they used to in the old days? – Bert in Aurora, Ont.
They do – literally every transmission installed in a new vehicle today has what you refer to an “overdrive.” It has become common practice and thus the word is no longer used.
In fact, many of the current crop of transmissions has two or more “overdrive” gears. As we accelerate from rest, we go through the gears from first to sixth, seventh or eighth. The lower gears are used to multiply engine power or torque. With each gear change, the ratio of road speed to engine speed decreases. To put it another way, at 2,000 engine revolutions per second in first gear, the car might be going 15 km/h while at that same engine speed in top gear it would be travelling at 115 km/h.
In the continual quest for improved mileage, manufacturers are increasing the number of gears in a transmission to ensure that in each gear engine speed is kept within a specific and efficient range. At higher and constant speeds, there is no need for torque multiplication so the gears employed will often let engine speed drop so it uses less fuel.
My experience has been that some smaller engines need to run at 2,500-3,000 rpm at highway speeds while larger engines with plenty of torque can run as low at 1,100 or 1,200 rpm at the same vehicle speed. In that latter vehicle, with a seven-speed transmission, at a steady speed of 100 km/h, the engine may be running at 2,500 rpm in fifth gear, 1,500 rpm in sixth and 1,100 in seventh.
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