What's the Best Combination of Gears?
 

I've read snippets of info about taller gears, closely spaced, various features for a transmission, but I have yet to find a thread (or outside article) that concretely states what the best combination would be (at least economically speaking). So I'm starting this thread to hopefully try to hash out the best option.

Here's my best preliminary guess:
1st, 2nd and 3rd gears are all short and closely spaced. (This allows for quick acceleration, allowing the driver to get to speed as quickly as possible.)
4th and 5th gears begin to get taller and are spaced out more to allow efficient operation while at the lower end of the top speed (say, between 40MPH and 60MPH).
6th gear is extremely tall and will allow for most efficiency possible at a higher speed (say, 65 or 70+).

This of course assumes a 6 speed, but maybe there is a better amount? More gears? Less gears? Also, is there a reason why a manual and auto would require something different to maximize efficiency?

well in theory, if you were to have the BSFC map for your engine you would be able to see how wide, in rpm, the "island" is where you acheive your lowest BSFC. on my Ford Focus zetec it is from 1500 to 3000rpm.

So the minimum ideal closeness is gear spacing would be where when you upshift at 3000 you dont drop below 1500 for the next gear. Of course the closer your ratios, you can stay closer the centre of that sweet spot.

as a rule of thumb i think more gears are better. That why there are 7, 8, and 9 gear transmisison coming out, so you can always be in the BSFC sweet spot.

30-60-90-120-150 @ redline in each gear.
50% of redline at 75 MPH.

regards
Mech

Depends on the car, specifically the weight, the engine, the aerodynamics, the final drive, and the desired cruising speed. Thenorm was right on the money about needing to know the BSFC of your engine. The problem you run into is that most readily available manual transmissions you only have so much spread between your lowest and your highest gears and you have to compromise somewhere. For example my Toyota Celica has a c60 six speed manual, gearing as follows:
1 3.166
2 2.05
3 1.481
4 1.166
5 0.916
6 0.725
Final 4.529

The super low final drive makes takeoff quick and peppy, but it bites me when my crusing RPMs are high. I plan on regearing the final drive on my car to a 3.9 when the transaxle needs rebuilt, but I will lose the peppiness on takeoff to gain my better cruising rpms. If my car had 5th at .725 and 6th at 0.50 or if 1-4 were lower and I had a taller final drive would also be pretty ideal for my car, but in the world of just picking between drop in parts you have to make compromises . Different cars with different RPM ranges, power bands, weights, etcetera will require different transmission options. If I was starting with a blank sheet of paper and could have any transmission I wanted, I would get one with the widest range between first and last gears, and then adjust the final drive to suit the car and driving style.

The automatic transmissions have a torque converter, which basically acts like a lower gear on high torque loads, thus you can get away with a lot taller gears and still have some of the peppiness down low. The disadvantage is that automatics have more losses, so you are giving up HP to get that better gearing.

Moonwell's Gear Ratio Calculator

that is the gear calculator i've been using for years now. it's nice to see all of the math done automatically and graphed.

I see what you're saying. The best gear set is whatever keeps it in the 'sweet spot' of the BSFC map. For this exercise, I was more trying to determine a generic rule of thumb for the best gears. But I am a visual and kinetic learner, so let's say we use the BSFC map of my Corolla.

http://ecomodder.com/wiki/images/6/6...1zzfe_bsfc.JPG
I started a thread about that already. Decoding & Breaking Down the 1ZZ-FE Engine BSFC

Also, I've heard there are issues with having more gears, especially with efficiency. I guess that can be offset by keeping the engine in the best range, right?

Thanks for that link. I'll play around with it some more. I already tried aardvarcus' gear ratio numbers and messed with it, but for consistency I will try my Corolla's numbers when I get a chance.

So would my original idea work like what you're saying, where you would have short closely spaced gears in 1-4 with a tall FDR, then you would still have peppiness with higher efficiency? Would having the last gear (say 6th) be way out there help or hinder, since that would make the range wider? Or would it just make more sense to have an extra gear in between 5th and 6th?

Regarding the TC, would that be when it is locked or unlocked? I would imagine if it is while locked, and having taller gears like you say, that would be more efficient or equal to a manual?

@Old Mechanic, I'm not sure what those numbers are or what you mean, can you explain further? Thanks!

speaking in Old Mechanic's place: MPH at redline and cruise at redline/2 (?)

Your exponential gear spacing would be strong off the line, but suffer in passing situations. How about a low that's low enough to allow normal 2nd gear starts with a "stump-puller" gear in reserve. Or an inline overdrive or electric motor replacing the transmission tailshaft, and if you have a live rear axle, ultimate control with one of these:
http://image.streetrodderweb.com/f/t...tersection.jpg

I'm not so excited about the close-ratio gear ratings which have been prevalent in the manual transmissions. No wonder the automatics, which have been retaining wider ratios, are getting their mileage disadvantage reduced.

Me neither. The bias is towards maximum acceleration capability. I skip-shift my 5-speed m/ts, usually 1-3-5 or 2-4-5. As a flatlander with a car that has abundant torque, I could do away with the extra two gearsets and save the weight and loss of transmission efficiency by having a nice 3-speed. Perhaps my desire for only three gears would change if I lived in hill country and/or had a less torquey engine.

Back in the '80s the Ford "High MPG" transmission was a 4-speed, not the 5-speed which was also offered.

Also I think overdrive gearsets lose power transmission efficiency- the reason they work is that the loss is smaller than the engine efficiency gain. I'd like a "straight through" top ratio with no gear meshes paired with an appropriately tall non-hypoid final drive gearset.

By the same token I believe gearsets have improved efficiency the closer they are to 1:1 ratio; another plus for a tall final drive.

So would my original idea work like what you're saying, where you would have short closely spaced gears in 1-4 with a tall FDR, then you would still have peppiness with higher efficiency? Would having the last gear (say 6th) be way out there help or hinder, since that would make the range wider? Or would it just make more sense to have an extra gear in between 5th and 6th?

Regarding the TC, would that be when it is locked or unlocked? I would imagine if it is while locked, and having taller gears like you say, that would be more efficient or equal to a manual?

Yes, as long as the first few gears are low enough a tall final drive would make the most of peppiness and efficiency. The spacing is more a function of how big your engines sweet spot of BSFC is, if you have a big wide area you don't need them spaced as close, versus if you have a narrower peak you would want them closer. For example, looking at your 1ZZ FE chart, your BSFC is pretty darn good from 2500 to 4000 rpm. Ideally the spacing on your transmission should be such that if you shifted in any of your first few gears at 4000 you would land at 2500 or more, so to still be in your BSFC sweet spot. Different engines will have different requirements.

For mainly open highway driving done with the cruise control set (not P&G), having 6th gear way far out is fine. It is not that bad to be out of your ideal range for a few seconds revving up the engine a little higher in the previous gear before you shift to your overdrive gear. For example, on my car for highways I typically accelerate first through fourth and will get going the speed I want in fourth and then shift it into sixth and set my cruise. No fifth involved unless I am getting on the interstate. When I need to make a pass or accelerate back up to speed, I pull the car out of sixth down into a lower gear 2nd-4th, complete my pass or acceleration, and then put it back into sixth. My car barely ever sees 5th gear, it just isn’t in a usable range for acceleration and my gears are so low that 5th isn’t good for cruising unless I am doing 40mph or less.

That is with the torque converter unlocked, when it is locked up it is basically like a direct drive just like a manual, but in most cases even with the torque converter locked up there is more internal friction in an automatic. That being said, with present gearing choices you can see in the EPA estimated numbers the punishment for the extra friction of the automatic is less than the punishment on the manuals of the super low gears when driven normally. The real advantage of the manual is the controllability, the ability to react and plan ahead when and how to accelerate and the ability to put it in neutral and just coast along. Pair that with “correct” gearing and you have a recipe for success.

The ideal gear ratio for any condition depends on how fast you want to travel and how quickly you want to accelerate to get there. Both of those dictate the power requirement. You use the gear to operate the engine at or near the most efficient load and engine speed that makes that power. (You need to measure, guess or calculate how much power is required to achieve the acceleration or maintain a particular speed.)

@Jeff88: We covered it in the other thread but those curves aren't a full BSFC map. They are a slice from one edge of one. A guide for what the map looks like, at least close to WOT, but not the full story.

Have a look for a BSFC map, even one from a different engine from yours, that includes lines of constant power. If you understand what that is showing, you will have a better idea of what different gear ratios do. Each line of constant power will pass through a number of efficiency islands. You would like to be in the high efficiency ones.

eg. here: http://ecomodder.com/wiki/index.php/..._2ZR-FXE_2010-

and read the comments under the Volkswagen Jetta TDI 1.9L ALH 1999.5-2003

The ideal transmission is a CVT, which holds the engine at the most efficient operating point for the requested power. Multi ratio transmissions are an approximation of that. The more gear ratio choices available, the closer the approximation.

For acceleration you probably want something close to an even spread. There is a case to close up the ratios (less difference between ratios) as you get to higher speeds, drag becomes important, and higher power is required to achieve the same acceleration.

If you spend a lot of time at particular speeds i.e. the speed limits, it may be best to select gear ratios for high efficiency at those speeds. It's not going to be perfect because real roads have rises and falls in elevation and wind direction varies, so the power required to maintain a speed varies. You need that CVT or have to accept that you wont always be in the efficiency sweet spots.

Depending on how wide is the gap between 2 random gears, there is virtually no advantage in getting a bigger amount of gears. Automakers are doing so because of some folks who buy a manual because it's either cheaper or the only option available for a certain model, but are too lazy to downshift when required :turtle:

Absolutely wrong. The increase in the number of gear ratios over time is due to the manufacturers chasing fuel economy (and possibly emissions).

Extra ratios cost real money. They also add weight - more than just the extra ratios would suggest because longer gear shafts deflect more so have to be made stronger or supported better - and some friction. For them to be specified there must be a tangible benefit.

As I posted the ideal is an infinite number of ratios so the engine can be held at the optimum load and rpm for a particular power demand. Pick any power demand. There is one engine load and engine speed at which that power is achieved most efficiently. That is where you want to be operating that engine. If there are a finite number of discrete gear ratios you will be somewhere away from that engine load and speed for most of the time the engine is generating that power output.

With too many ratios it is possible to spend so much time shifting that there isn't enough driving time remaining to justify them. That is ameliorated with the robotic shifting also more common in recent years.

The recent increase in gear ratios in automatic transmissions is designed with fuel economy in mind.

Most manual transmissions available from the factory in small cars in the US in recent history are not designed with economy as the most important element; instead they are geared for performance or for cost. The manuals are either the “cheap” option or the “sport” option. They don’t have a wide enough spread between first and last gear, so the designers have to choose between performance and economy. So they put a low final drive in the car, and let the RPMs suffer at higher speeds.

Just as an example, the reason my car has the gearing it has is that you can go exactly 60mph in second gear at redline of 7800RPM. Thus when they advertise 0-60 times they have the lowest possible gearing to maximize acceleration while only needing to shift gears one time.

The “solution” is to put a lower first gear, taller overdrive gear, and a middle of the road rear end gear in the car, which nets you the best of all worlds. The downside is that now you have to increase the physical size of the transmission to fit these larger gears, the stresses on the gears are higher, and you end up with a much more expensive transmission that consumers today aren’t willing to pay for. Imagine a world where the manual is a more expensive option than the automatic, that would cut out all the “cheap” sales and leave just the “sport” sales.

As pointed out by aardvarcus, having more gears is not so absolutely right if their ratio is too compromised, either for performance or fuel-economy.

In the context of:

If there is a choice between only having gears 1 and 3 (say) and gears 1,2 and 3, where ratios 1 and 3 are the same, and 2 splits them, three gears will allow the engine to be operated closer to its highest efficiency than will two of them.

If your aiming for a 10-second quarter mile, and each shift takes 1/2 second?

But when the intermediate gear is actually not so intermediate, thus too closer to the previous gear and with a longer gap to the next (or with a longer gap from the previous and too closer to the next), which is what has been more usual, it ends up not increasing the efficiency so much...

Doesn't matter. The overall ratios will be different but the spacing will not.

OK, the closer any two of the ratios are together the more like 2 (vs 3) ratios they are.

The flatter the BSFC map is, the less is the benefit. There is a law of diminishing return too, where shift/"idle" time is required for the ratio changes. Maybe 7 speeds is somewhere near enough?

Well, nowadays even some big rigs are getting trannies with less gears, is not so uncommon to find some with only 9 gears...

Occasionally6 -- My point was more that two extra shifts would cost 1 second at the traps. Not a point I should expect to resonate on a forum about eco-modding. :)
_____

VW transaxles have a larger gap between 2nd and 3rd. When I got a 5-speed conversion, it lowered third and had a 'low 4th' and a 'high 4th'. Fifth wasn't much use below 65-80, but it was a joy around town because you spent more time with two gears to choose from against 35-45mph speed limits.

aardvarcus you're spoiled with a C60. I have C56. 3000rpm in 5th is only 55mph.

More gears is better yes, and I would love to see 7 speed manuals everywhere. However being more realistic, a 6 speed like the C60 is a pretty good compromise for a small 4 cylinder engine. The 6th gear is a proper cruising gear that lets you follow traffic without going too much over 3000rpm (where a lot of engines start to really decline a lot in efficiency at low load due to friction), and the first gear is nice and short for starting and such. You can tip the scales more in the economy direction by making the 6th taller (a 2ZZ powered small car can hit 160mph in 6th with some drag reduction) and spreading 1-5 a little more.

I would be extremely happy with a slightly destroked engine and a C60, it would leave some gas mileage on the table but it would be pleasant to drive and still do okay on the highway. Yea if you wanted maximum gas mileage you could spec a gearbox that lets you do 55mph with 1500rpm (assuming you have a motor that can pull this off, I doubt my 1ZZ can), 5th put you at like 1500rpm for 45mph, and have 4th place you right at like 1200rpm for 25mph, but the giant gear spacing would drive me nuts when shifting and having to downshift to 3rd to accelerate hard would be really weird.

Yeah, my RPMs aren’t quite that high, but still pretty high especially on the interstate. Just as an FYI if you ever have to have your C56 worked on there is a factory Toyota 0.725:1 5th gear that is a direct replacement, definitely worth considering.

Oh, as far as the crazy downshifting goes, imagine pulling the car out of sixth and putting it into second. The peak power band on a 2ZZ engine is 6200-7800 RPM, which is 49-61 MPH in second, so if I need to pass someone doing 45 or less in the 55 and I only have a small window to pass it comes out of 6th and into 2nd.

The options for final drives in C60s are 3.941:1, 4.312:1, and 4.529:1 all US spec Celicas got the 4.529 from the factory. I would love to drop in the 3.941 gear set, my only concern is making it hard to take off in first. I have not yet found anyone making a lower first or a taller sixth for the C60.

Yeah- especially if you're driving a tractor.

In the context of the real-world trip- for example when I go out of town- I'm accelerating through the lower gears for mere seconds then droning on for hours in top gear. With, say, 3 gears and a 0-55mph operating envelope, the engine might fall outside of it's "perfect" rpm range for what- .001% of the trip? A minimally geared transmission that suffers less internal friction losses would be a fe benefit to me the other 99.999% of the time. Heck, maybe two gears would be enough... or a smaller engine (thinking of the 2.3 torque monster in the Tempo).

More gears maybe would have some benefit in trip scenarios heavy with urban, heavily loaded vehicle, or mountainous conditions... but I don't have those.

The act of shifting is itself quite inefficient: the flow of power and thus acceleration is interrupted, the throttle snaps shut and all the engine's whirly bits have to decel then "recel", similarly the flows in the engine's tracty bits have to slow then restart. Best to avoid that as much as possible, I think.

Yea I know about it but they don't make the special gear puller anymore so it's really annoying and I'd have to pay someone to do it, and I would never be able to recoup the cost in gas savings. I'm just going to buy a C60 when this drivetrain gives out. The synchros are pretty much totally shot so it's only a matter of time.

I actually like the 4.529 with 0.725 6th gear setup, maybe my 1ZZ isn't so healthy (it is a 2000 model year afterall) but pulling from a stop it feels like it struggles a little, I think the slightly shorter 1st would be nice, and I would be pretty content to be able to cruise at 3000rpm without feeling like I'm going to be the victim of road rage.

No wonder the AMTs, including DCTs, are usually based in a non-synchronized gearbox, thus allowing them to be shifted without the use of the clutch.