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.