Quote:
Originally Posted by racprops
I am thinking that a second overdrive added to a stock 4 speed auto with a OD gear can work and in fact greatly increase a stock car/van’s MPG, just by lowing the cruse RPMS from around 2400 RPMs to 1700 RPMs.
This seems to be the best of all worlds, stock gearing until the added Over Drive is engaged then the advantage of super highway gearing.
Your thoughts and knowledge is invited.
Rich
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It would basically be impossible to add a 5th gear to a 4 speed auto. However, it would theoretically be possible to swap the 4th gear to a taller one, although I don't think that would work well and you may not gain anything for several reasons:
1. The PCM would have no way of knowing that the gear ratio has been changed, so it would likely shift to overdrive early and then lug the engine or shift back and forth between 3rd and 4th, so you would probably need to lock out overdrive by putting the shifter in 3, pressing "OD OFF", or similar (if your vehicle even has this option) until you are at a high enough speed to shift to 4th gear. The PCM will likely also set a code, likely for an incorrect gear ratio.
2. The torque converter clutch will probably not lock very much or for very long since lockup is controlled in large part by engine load, and the load would probably be too high for the TCC to lock, even cruising down the highway at a steady speed on flat ground. The converter is also likely to lock and unlock quite frequently, which is inefficient and will quickly destroy the clutch. You could get around this with a manual lockup switch, but again, this will probably set a code, and may even cause the PCM to enter limp mode.
3. The trans will also frequently need to downshift to 3rd gear to accelerate a tiny bit or climb a tiny hill, which will eat into your savings, especially if you manually lockup the converter. Unlocking the TCC may provide enough power to climb a hill or accelerate without downshifting, but again will eat into your savings.
4. When driving in the city, your speed will probably be too low to cruise in 4th gear, especially with the converter locked. I have this problem with my car when I manually lock the converter in 4th gear below 35-40 MPH, which is a speed that I drive at a lot around town. A taller 4th gear would make this much worse. So you will need to cruise in 3rd gear a lot at lower speeds, which will also eat into your savings.
So for those reasons, I would advise against installing a significantly taller 4th gear on an automatic transmission vehicle unless the vehicle is driven on flat highway almost exclusively, you have workarounds for those other issues, you are okay with the vehicle being miserable to drive around town, and you don't care if it sets a code.