Is 1:1 gearing really more efficient?
 

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.

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

Direct drive removes the countershaft from the power/torque delivery:
https://www.eaton.com/ZS/ecm/groups/...ct_1114002.jpg

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

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.

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.

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.

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.

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.

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.

And a lot of the hybrids (if you consider the power split device to be a transmission)

https://priuschat.com/attachments/pr...ted-jpg.33414/

Which BTW resorted to the same setup. Presumably that was the easiest way to set the transmission back while keeping the axle more forward, avoiding to turn it excessively nose-heavy.

Talking about 1980s trucks.
I've been in half ton trucks ranging from 4.55s to 2.73s for axil gearing.
Comparing 2, 2 door short bed 2wds.

I'd expect mid to upper 20s from a inline 4sp with 2.73s with smaller/lighter then stock tires/rims, with no cargo.

The same truck with the same inline engine, with 4.11s and a 3speed auto will be hard pressed to get over mid teens. Mix in some larger tires and try and run 75+ your going to be lucky to get over 14.

At least in fords 1980/1996~ the 2.73 are 2wd only. 4wds are limited to 3.08 and up.

Tire size does have an influence. I remember some Korean trucks which had 15" tires all-around and single rear wheels, while the DRW versions had smaller 12" rear wheels. As the smaller wheels equated to the effect of a lower gearing, the DRW versions had a higher differential to overcome this issue.

Even though full-size trucks often had the same riding height and tire size for either 2WD and 4WD models, in theory a lower differential may compensate for a transfer case with an overdrive high range whenever that's the case. Not sure if that was the case for most full-size trucks, yet I remember some 4WDs of other types had it.

Honda's S800 (and similar variants) drove the rear wheels with a chain, though it also had a driveshaft.

https://img.hmn.com/900x0/stories/20...00_06_1200.jpg

I think 1:1 is more efficient for any fixed gear strength as equal diameter gears have less sliding friction at the teeth for any given pitch. If the ratio is not 1, the smaller gear has to have a bigger angle between the teeth.

I think this matters a little less on hypoid gearing when the ring gear is driven, but when the ring gear is driving the pinion gear (e.g. engine braking) the friction is higher than with a bevel/helical gear. That's what I remember reading.

1:1 6th isn't popular because then your 1st has to be a very high ratio which is harder on the gears (gotta make it beefier).

1:1 gearing
 

There was a 2% mechanical efficiency penalty for overdive, compared to 1:1, with normal mineral gear lube.
Since the difference was in hydrodynamic churning losses in the extra gear set, that relationship is no doubt less now, with contemporary all-synthetic, lower viscosity lubrication packages.
As of 2006, the National Academies of Science published that ICE vehicle powertrains absorbed 5% of transmitted power.
A 1:1 transmission would knock that down to 4.9%.
28.1% of available power would be available at the traction wheels, instead of 28%. A 0.0035% gain.
It's my opinion that, in a capitalist nation, you wouldn't want to even waste the time thinking about the difference it would make, let alone act on it.;)

I think that's gotta be transferring power through a set of gears vs. direct drive with no gears. There's no way e.g. 0.8 and 1 have a 2% difference in efficiency.

set of gears
 

Correct. The overdrive is running power through an additional set of gears, to get the output shaft spinning faster than the input shaft in top gear.
That's by definition, an 'overdrive.'
Any gear set costs a 2% loss in heat, from viscous shearing of the lube.
On top of the transmission loss, must be added the loss at the propeller shaft, ring and pinion, spiders, and wheel bearings.
General Motors measured their 1:1 powertrain under power transfer, reporting an overall 94% mechanical efficiency.
You'd have the main shaft bearing losses, propeller shaft losses, differential and rear wheel bearing losses.
Had it been an overdrive, it would have been 92.1%. Delta- 1.9%.
And all that was with mineral oil.
With full-synthetic gear oil, the loss is less than 1.9%.
The aerodynamic load hasn't changed at all.
When you add aero and rolling resistance, the difference in the powertrain begins to get lost in the noise of both aero and RR.
You'd be splitting hairs.
If you're going to lose 500-rpm at the rear axle, how are you going to do it? And what's it going to cost for the unknown quantity you'll attain?

I have seen a similar setup on ancient trucks, and some tricycles which had the chain drive but didn't feature a driveshaft.

The question seems to asking about mechanical efficiency of the transmission. perhaps confusing it with the overall economy

A lot of older transmissions would have the mainshaft run continuous from the clutch or torque converter to the output. Lower gears would use two sets of gears. Going to the secondary shaft and back to the primary shaft . Top gear is always 1:1, direct. The lower gears would have more losses as they would be transferring power to the secondary (lay) shaft and then back up to the main shaft

Most newer manual transmissions will have an input shaft and output shaft. So the transmission losses are similar across all the gears. Power is transferred across both shafts in all gears.

Varn you make a good point about the two sets of gears vs one. I was wondering what a ND Miata transmission looked like since Mazda bragged about the 1:1 6th gear being more efficient, and I noticed that there are a lot of transmissions out there that have 3 shafts, one pair which is inline and locked together for the 1:1 ratio. I had always thought transmissions came with just an input and output shaft.

I think in automotive applications however due to gear strength limitations there is a drop in efficiency from the gear ratios that deviate further from 1 as bigger gear pitch for strength = more sliding friction on the gears. I wouldn't be surprised if the typical 1st gear or some of the taller overdrives (like 0.6 ratio) were under 95% efficient.