Cruising at about 62mph (100 km/h) what RPMs are you guys achieving?
I'm about 1800rpm and that is high for my car.

I have disconnected my overdrive from my TH700R4 and what a difference. The computer has changed the auto's shifting. The exhaust note is better and my mechanic tells me it is better for performance and mileage when travelling mostly below 55mph, I can hear and feel the performance gain.

Now that I am highway driving a lot, I am going to reconnect the overdrive module. I'm interested in the change in fuel economy.

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British Columbia Enthusiast
1989 Formula 305 TBI

 

I'm at 2500 (on the tach) at 100k with 3.73 gears.

Not sure what you mean about disconnecting the overdrive. As far as I am aware you can choose not to use it but you can't disconnect it.

 

What I did Cosmic Debris was unplug a wiring unit a top the transmission.

 

I'm thinking you either unplugged the torque converter lockup or the tran cable. I don't like either of those reasons. If you want it to be "faster" then just keep the trans in "D"/3rd gear and don't put it into OD. If you don't let the torque converter lockup then the trans fluid could be getting pretty hot.

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, Jon (350 TBI!)
91 Red My website

 

1) Explain how an engine is supposed to get better mileage by turning more RPMs.

Avg drop is around 300RPMs. Going on the 300RPM idea, thats 150 less gulps of 305ci of air the car makes. 150 less gulps would seem a bit more fuel efficient.

Im confused about that.

2) Unless your tranny is different than mine, you have two connectors. 1 is the computer interface, a 4 pin connector, then 2 is the speedometer connector. If you disconnected the computer one then the computer has NO say about the shift points as you think. The transmission is now a purely hydraulic actuated transmission, pressure alone is affecting the shift points. As far as I know, disconecting the connector should only affect the TC lockup. But as pointed out, thats going to heat up the transmission fluid.

Clayton

 

This is news to me.
This connector was disconnected by my GM certified mechanic.

Originally, the car was shifting early and "bogging down." Ever since he disconnected this connector, the shifts have returned to normal (what they were before the problem) and the car runs great.

I will bring this too his attention.

Thanks everyone for their input.

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British Columbia Enthusiast
1989 Formula 305 TBI