Is 1:1 gearing really more efficient?

[Ecky]

Let's say you have taller tires, or a taller final drive, or something else to make up the difference - in other words, all else being equal - what makes 1:1 gearing more efficient than any other combination? I hear this repeated a lot, but I haven't been able to find an engineering explanation for why a gear reduction is lossy.

[rmay635703]

That’s been the claim since the dawn of time

“Inline” gears (fwd) are also considered more efficient

The trouble is modding your final drive to support 1:1 ratio without “big gears” a 2.8 rear has much more robust gears than a 4.10 which is problematic in a transaxle

[Drifter]

Direct drive removes the countershaft from the power/torque delivery:


Direct drive is typically 98-99% efficient vs ~94% for overdrive.

[oil pan 4]

But then how much gas are you burning to go 70mph with say 1:1 at 3,000rpm versus over drive and closer to 2,000rpm?
I tested it once, in my my six speed firebird it made about 4mpg to 5mpg difference. I noticed I went from full to 3/4 a tank in less than 100 miles and I was like "that's enough testing for that". Went and filled up saw the catastrophic loss of fuel economy and didn't do that again.

[Drifter]

You spec a different final drive ratio to end up at the same rpm. 10 years ago our semi trucks that ran direct drive had 2.47 rear ends. I think some of the new Freightliners run 2.16 final drives!

From a fleet management perspective, I found that you wanted your transmission to be in direct drive at whatever speed you consumed most of your fuel. For flat land and cross country highway trucks, that was top gear. But for some local applications, especially those climbing grades, you would want them to cruise the highway in overdrive and be in direct when they were chugging up Donner summit.

[Ecky]

I hadn't considered that 1:1 gearing might bypass the countershaft altogether, but it makes sense. You can't really do that in a FWD transmission, to my knowledge.

[Isaac Zachary]

So FWD transmissions have an input shaft that you choose gears from that to the main shaft which is geared to the ring gear on the differential. So a 1:1 gearing would still turn the same amount of shafts and still send the power through the same number of gears.

UNLESS, you had a chain drive going from the input shaft to the differential. But then the question would be if a chain drive could be just as efficient as going through gears. The main shaft would also still be spining unless you could disconnect the final gear that goes to the ring gear. Not to mention, it would just be more complicated.

[Taylor95]

My Sequoia gets terrible mpg in most conditions except for when it is in overdrive. In 6th gear (0.588) I can get ~23-24 mpg
on flat highway surfaces at 60 mph.

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

UNLESS, you had a chain drive going from the input shaft to the differential.

The only cars with a chain drive from the transmission to the differential that I remember right now are some FWD Cadillacs from the '70s and the Oldsmobile Toronado.

[Isaac Zachary]

Originally Posted by cRiPpLe_rOoStEr

The only cars with a chain drive from the transmission to the differential that I remember right now are some FWD Cadillacs from the '70s and the Oldsmobile Toronado.

And the GMC Motorhome.