"free" 20% gas mileage gain, why doesn't GM do it?

[TheWraith]

I've been thinking about this for quite a while, but the gear spread of the 6L80E transmission is such that it could be used with a 2.75 ratio rear axle, and be directly comparable to a 4L65E transmission for 3.42's.

here would be your ratios:

4l65e/3.42:
1: 10.47
2: 5.57
3: 3.42
4. 2.39
------------
6L80e/2.75:
1. 11.06
2. 6.49
3. 4.21
4. 3.16
5. 2.34
6. 1.85

as you can see, you have extra torque and power multiplication in the first 3 gears, an added gear in the middle, and the same 2.3X overdrive, and then a completely extra overdrive after it. it's a win-win situation all around.

You get the same sort of effect if you put a 6 speed manual transmission in place of the 4L65E transmission, no gear ratio change required.

And what would that do for highway fuel mileage?

A LOT.

I'm going to quote some fuel mileage figures - they're all under the NEW adjusted epa rating system, so they are direct comparisons. You can make your own judgments...

2006 GTO, 3.46 rear, 4l65E transmission
14 city, 19 highway

2006 GTO, 3.46 rear, 6 speed manual transmission
15 city, 23 highway.

2008 G8, 2.92 rear, 6L80E transmission
15 city, 24 highway.

All 3 have a 6 liter LS series engine. The G8 does have active fuel management, but it's also over 200 lbs heavier than a GTO, so factor that in. So that 24 highway figure vs the 19 on the auto gto is in no small part due to the transmission and gearing.

Now consider this - every v8 light duty truck and suv uses a 3.42 or 3.73 axle ratio and a 4L65E transmission. And those rare few that do have the 6 speed auto, like the cadillac escalades and denalis retain the 3.42 axles.

Why? If GM is so concerned about the new 2020 mileage laws and such, why are they still throwing 4 speed automatic transmissions whose design is now 25 years old into their vehicles? Especially since a very viable alternative is out there for several years now.

This decision is hurting everyone - it's costing GM opportunities for one thing - if all the v8 trucks and suv's got mid 20's in highway mileage, they would drastically outsell competing trucks. and for those people who do buy them, we're the ones taking it in the wallets, when it's wholly unnecessary.

[F14CRAZY]

As I understand it GM does not yet have the production capacity to put the 6 speed auto in all SUVs and trucks

[TheWraith]

Originally Posted by F14CRAZY

As I understand it GM does not yet have the production capacity to put the 6 speed auto in all SUVs and trucks

i had heard the same, in late 2005 when the 2007 tahoe and the other GMT900 suvs came out - as their reason for not doing it then. and then again in later 2006 when the 2007 silverado and GMT900 pickups came out. it's Q1 2008 now, not only is it well established that people have been asking the questions for over 2 years now, it also should have been more than ample time to solve the problem. Especially since gas and gas prices have been major news topics as well since 2005, when things first hit the $3 mark.

[likearock00]

Maybe you should write a letter to GM and let them know of your discovery. I'm sure they'd be impressed and may wish to give to a job....

[Randiglio]

[big swede]

Well for one you cannot compare a 6 speed manual to a 4 speed automatic and give all the credit for any benefit to the extra two gears. Manuals are typically 5% more efficient than automtics to begin with. Secondly you cannot compare a 4 speed auto to a 6 speed auto in two different cars with two different engines and give all the credit for any benefit to the extra two gears.

The manufacturers claim about 6% for the 6 speed over the 4 speed which is similiar for D.I. It's kinda like D.I. when everyone thinks it makes a massive difference with no downside. For example look at GM's 3.6 with and without D.I. From the outside D.I. looks like a massive increase in performance for a minor change but in all reality when GM went with D.I. on the 3.6 several other upgrades that contributed to the improvement were done at the same time but everyone sees a 40 h.p. increase and thinks it all comes from D.I.

Even if it's only around a 6% improvement for the 6 speed it is worth it for not only the improvement in mileage but also in driveability. You can have tighter gear spacing which makes the car drive more smoothly because there isn't such a large rpm fluctuation between gears.

Personally I wish GM would have just skipped the 6 speed auto's and went straight for the Dual Clutch Transmissions because they have the efficiency of a true manual with the other benefits of extra gears.

[JBsZ06]

I wish GM would offer a dual clutch sequential too..

I sure do want that in my next Vette! (or anything else I buy)

[TheWraith]

big swede:
agreed on the manuals being more efficient in terms of power transfer.

and again, agreed on the dual clutch. the standard style torque converter is an antiquated technology that's entirely too much acceptable drivetrain power loss for the year 2008. hell, according to back to the future II, we should all have flying cars in another 7 years but i'm only expecting realistic, off the shelf part solutions from gm at this point..

honestly if people are really serious about every penny counting on gas mileage, then the manufacturers and buyers should both be taking manual transmissions far more seriously. I personally have 3 vehicles i drive, and 2 of the 3 are 6 speed manuals. One of which is a supercharged, modified camaro i swapped from a 700r4 automatic to a T56, and went from 18-20 mpg highway, if i babied it, to a consistent 26-29 mpg. the one automatic vehicle i own was a compromise - it's actually a 5.3 liter AFM 07 trailblazer, with the lowly 4L65E, one of the reasons why this argument has a personal impact.

Back to the subject though, that 6% for 6 speed vs 4 speed figure is for the same, or similar gear ratios. My example called for a pretty substantial numerical reduction in gear ratio, since the 6 speed does not need a steep gear. And that will make for a much more drastic comparison in highway mileage, while, as i laid out pretty thoroughly, absolutely no compromise in driveability/performance/towing/etc compared to a 4L65E. that's my primary point.

randiglio:
yes, obviously the bean counters are a major factor, but after all, isn't GM's goal to actually sell vehicles? And wouldn't full size trucks and suv's knocking down mileage in the mid 20's do exactly that? I sure as hell think it would....as much as there are die hard purists, with $3 fuel at the pump, if GM's trucks are getting 20% better fuel economy, that's going to sway a lot of people over to GM. the save every penny mentality only works if you're not compromising what the consumer actually wants..

likearock00:
i would think that people there at are certainly aware of this. the G8 is evidence of that - i don't think GM of even 2-3 years ago would have considered putting a "lowly" 2.92 gear in a vehicle...or for that matter, a 2.56 axle in an automatic equipped corvette. but i would certainly take a job offer

[63GrandSport001]

If GM made the moves that it needed to make in order to make its gas Tahoe's get much better fuel economy then people would pretty much stop buying full size truck based SUVs from all other car makers.

Here is one problem with that though, GM now has its hybrids out which are starting at like $50,000 so they cant afford to have a much cheaper SUV that gets better mileage. If you can at $35,000 get the 4.8L V-8 with direct injection, VVT, and 6 speeds and it gets 25MPG highway then who would buy the hybrid?

[TheWraith]

Originally Posted by 63GrandSport001

If GM made the moves that it needed to make in order to make its gas Tahoe's get much better fuel economy then people would pretty much stop buying full size truck based SUVs from all other car makers.

Here is one problem with that though, GM now has its hybrids out which are starting at like $50,000 so they cant afford to have a much cheaper SUV that gets better mileage. If you can at $35,000 get the 4.8L V-8 with direct injection, VVT, and 6 speeds and it gets 25MPG highway then who would buy the hybrid?

interesting argument...i honestly hadn't given that much thought. and it makes sense, but only strictly from a certain viewpoint (granted the decisions makers at gm might have that viewpoint).

but i honestly think there are two very strong arguments against it.

1.if they can cannibalize sales of an overpriced low volume product like the hybrids, to vastly boost sales of some of their biggest "core" vehicles, I'm sure it would be a smart business move.

2.a conventional 6 speed transmission truck and the hybrid both cater to different audiences. the hybrid mostly boosts city mileage.

for arguments sake we're talking 4x4 tahoes. as a rough estimate, let's say a 6L80E/2.75 final drive tahoe would get 15 city / 24 highway. The hybrid gets 20 city / 20 highway, and the standard truck gets 14/19 now.

given the choice between a 6L80e/2.75 tahoe, and a hybrid:

if you drive mostly city, the hybrid still has a major upper hand.

if you drive mostly highway, no one is going to want the hybrid true. But that's also true now...who is going to move up to a much more expensive truck just for an extra 1mpg?

if you drive roughly 50% city/50% highway, well that's where sales are going to be compromised for the hybrid i suppose, but there are still benefits to each.

[mikmak]

Best comparison I could come up with HERE. Although the 2 engines are slightly dissimilar, the effect is quite minimal. 11.4 vs.11.7l/100km. Every time a powerplant is changed, Holden changes the availability of the Transmission. ie you can't get a VE 6M 3.6 HFV6 without VVT (hence the power increase to 195kw).

[TheWraith]

thanks for the link, but not really a good example of what i propose. that looks like a 4 speed auto vs a 5 speed, 2 different vehicles, 2 different engines, and as far as i could tell, it said nothing of gear ratio changes.

the best example as far as i can tell is still the 2006 GTO auto vs the 2008 G8 auto, which i referenced above. if anyone can think of a better example, please post it. the 2005 corvette had the 4L65E, and the 2006 had the 6L80E, but the gear ratios did not really change significantly, so there's no helpful data there unfortunately.

[MelvinJ]

Originally Posted by TheWraith

thanks for the link, but not really a good example of what i propose. that looks like a 4 speed auto vs a 5 speed, 2 different vehicles, 2 different engines, and as far as i could tell, it said nothing of gear ratio changes.

the best example as far as i can tell is still the 2006 GTO auto vs the 2008 G8 auto, which i referenced above. if anyone can think of a better example, please post it. the 2005 corvette had the 4L65E, and the 2006 had the 6L80E, but the gear ratios did not really change significantly, so there's no helpful data there unfortunately.

I really don't think you can apply your GTO-vs-G8 comparison results to the GMT900. Look at the frontal area of the two cars vs. the frontal area of the GMT900's. They have far less wind resistance than the GMT900s. The GTO automatic with the 3.42 axle had a LOT of excess torque available at the RPM it was turning at highway speeds because of relatively low wind resistance (the engine is running in an inefficient small-throttle-opening, relatively high RPM condition). So when they cut the RPM's way back with the 6-speed, of course there was a big increase in mileage. RPM's dropped, and throttle openings increased, reducing pumping losses in the engine, and in these relatively light cars (1000+ lbs lighter compared to most GMT900s) with relatively small frontal areas (again, compared to full-size trucks) it pays off big.

The trucks don't have nearly the reserve torque at highway speeds because of increased wind resistance. It's already operating at wide throttle openings, particularly in 4-cylinder mode. Decrease RPM's any further with the taller axle/6-speed combo, and the small range of highway operating conditions that the AFM engines are actually able to operate on 4-cylinders goes away entirely. So the AFM benefit is gone, although I don't believe they are all that substantial for the way most people drive anyway. But, a 1.85 overall ratio on a full-size truck (with 30+ in. diameter tires) is going to downshift on virtually every hill, annoying the hell out of the driver, increasing transmission wear, and cutting mileage some anyway.

One more note: The Tahoe Hybrid, with all it's aero tweaks, Atkinson cycle engine, 3.08 axle and .73 overdrive (2.24 overall ratio, the tallest geared GMT900 there is, as near as I can tell), manages 22 highway. Simply dropping the overall ratio down to 1.85 is NOT going to get a full-size gas-engined truck into the mid-20's on the highway cycle.

So, in short, your GTO-G8 example provides an a very extreme example of what the taller overall ratios can do and its improvement percentages cannot be applied to full-sized trucks.

[TheWraith]

excellent point melvinJ, and it was something i did take into consideration, and full expected that to be contended.

I would never expect that a truck would have the same highway mileage as the G8 would just based on the same gearing. and while i realize i did use that figure in the previous post, it was solely for arguments purposes just to make an argument that it wouldn't totally cannibalize hybrid sales. you're talking completely different rolling resistances and aerodynamics.

But i still think there is a large window there for improvement, and while it's not necessarily in the weight class of a GMT900, i can attest that there is plenty of room for highway gear reduction in my 2007 trailblazer with the 5.3 AFM. I believe would have no trouble maintaining highway speeds on moderately flat grades with 15-20% lower rpms personally. Especially if i was talking 65-70mph vs 50-55.

And the points you make about AFM not being active, for the relatively small amount of good that does, i actually believe you could still have a market increase in highway mileage without it active. lower rpms mean less fuel burned.

And in terms of hills, yes, another valid point, but given an mostly ideal flat highway (which there is certainly no shortage of in every area of this country), it would produce.

But you're right, perhaps 2.75 gearing is too drastic a reduction then for a GMT 900?

the idea then that i call for may require gearing that allows for a "sweet spot" in a given vehicle, with a given engine.

Maybe in a GMT360 that could be 2.75, and it would need to be increased somewhat across the board until you get all the way up to the suburban, which you may not reap much benefit from. I'll post more later. I'm on the way out the door to work.

[MelvinJ]

The GMT360's with the 5.3L like you have could probably pull some fairly tall ratios. My brother had an '88 GMT400 pickup with a 4-speed automatic (.7:1 overdrive) and a 2.76 axle for overall gearing of 1.93 - not far off of what you are proposing, so it can be done. His truck was a half-ton, 2WD regular cab with a 350, so it worked as well as that gearing ever could have back then. I'm sure his pickup weighed less than your Trailblazer.

GM will continue implement AFM on most of it's truck V8's (I'm not sure the 4.8L will ever get it, but the 6.2L will have it soon), so they must feel there is some benefit to AFM even when 6-speeds are used. It probably makes some difference at suburban-type speeds of 45-60. Given small real-world mileage difference between the 5.3L/4L60 and the non-AFM 6.2L/6L80 (about 1 mpg according to Consumer Reports), I think the 6-speed will be more effective in the real world than AFM.

I wouldn't be surprised to see them use a 3.08 axle with the 6-speeds which would produce some very tall crusing ratios and perhaps one more mpg than the 3.42. Beyond that I think they'll shrink engine sizes rather than go for taller gearing since they'll need to bring city mileage up also, and the taller ratios can't help much there.