PeachParts Mercedes-Benz Forum - View Single Post - the effect of gear ratio on fuel economy in spark ignition and combustion ignition en

Richard Wooldridge · #2 04-11-2006, 10:59 AM

Hi Tom,
Very interesting, but I believe you should also mention two other innovations that have also greatly improved mileage. The first is the overdrive transmission - same as reducing the numeric rear end ratio, but with the od transmission you still have a good low gear for starting. The second, and possibly more important with an auto. trans, is the locking torque converter. This invention eliminates the heat generated from a constantly slipping torque converter and thus adds to the efficiency of the drive train. A good modern automatic trans with overdrive and locking torque converter will add up to 30 percent to the mileage on a car with no disadvantages in operation. They also allow lower numeric gearing, as the engine can run at a lower speed without the torque converter becoming "lossy". This is one reason I converted my '82 300D to the 4.3L V6 with the T700R OD transmission with locking torque converter. When in "high" OD and torque converter locked, the auto has very little parasitic loss. My '82 will produce 28MPG at 75mph with 4 adults in the car and the AC on full blast.
Modern American engine design takes advantage of this by designing the engines to be very torquey at lower RPM's where the engine is running most of the time. The newer 4.3L engines actually change the length of the intake runners at different rpms to provide the best charge of air at lower speeds.
Of course, none of this is new info, the late 30's/40's Studebakers and others used an overdrive transmission for fuel economy. Intake tuning has been around for years, mid 50's Buicks used a variable pitch torque converter, etc...

By the way, I just yesterday drove from Vancouver, WA to Seattle, WA and back with my '82 380SL with an '86 560SL engine installed, and it produced an average of 19.2 mpg for the 317 miles, not too shabby, I'd say! Traffic was moving around 80mph for most of the way.