Hi andrewk, hello all,
Finally, some interest in one of my “off the wall” statements.
88Coupe wrote:
andrewk wrote:
Overdrive is where the transmission will spin faster......... to increase fuel economy.
Not true. But that's a topic for another thread.
As designed, the eighties overdrives are nothing but a way to artificially raise the final dive ratio to a point higher than the ring and pinion design will allow.
Quote:
........ Why isn't it true? Less revolutions of the engine surely has to result in increased fuel economy ........
OK. Why not change your OD ratio from .7 to .2 and solve the oil crisis?
Quote:
........ If I take my lumina, and drive 50 miles on the highway out of overdrive, and then drive that same stretch in overdrive, I see an increase in fuel economy ........
Did your increase come from the OD itself? Did it merely compensate for a poor choice in final drive ratios? Or was the ratio for third gear chosen for use as a “passing gear”?
And would you have seen an even bigger improvement, if you lowered the final drive to somewhere between the two?
Quote:
........ Is it that the real reason overdrive is used is not for fuel economy? ........
Bingo. From ’74 on, mfrs sole focus was on meeting Emissions standards any way they could. Primarily, the ability to pass all tests at 50K Miles. Lowering engine RPM helped them to do it without spending the money to redesign the existing engines. Since going to higher gears (in rear drives) was out of the question, putting an OD in the TH350 was a quick and cheap way of doing it.
Modern EFI systems and slippery aerodynamics have improved overall efficiency, so the ratios could be lowered to address fuel consumption. Problem is, emissions standards still keep getting tighter every year.
Regards, Norm
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