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Differential Gear Swap?


Devin O'Conor

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My 1991 DeVille has the 2.73 final drive gear ratio as did all except the Touring Sedans which got 2.97 gears. Seville's got 2.97 and STS's got 3.33. After reviewing info for different vehicles and driving in my 1991, I have to wonder why the 2.73 gears were used.

It seems to me that with the 2.73 gears, when driving against a wind or up even small hills, the engine lugs and the gas has to be pressed harder to keep up highway speeds than if it had 3.33 gears. I figure this is why the gas mileage is worse than 4.9 Cadillacs with the 3.33 gears.

What was the point of 2.73 gears in this case? With the Northstar, the 3.11 gears were to achieve better overall gas mileage in the Northstar lineup. With the 4.9, the lower 2.73 gears seem to hurt gas mileage. They certainly hurt power output over the 3.33 gears also. I would love to have the 3.33 gears in my DeVille.

Does the whole transmission have to be removed to change the gears? The computer is no problem. I have already contacted a company who can make the necessary computer changes if I change the gears.

Any thoughts on this? Bruce? Bbobynski?

K.O.T.

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This subject has come up before and I think the issue was the computer seeing the wrong ratio, if you can get the computer reprogramed or a new chip, its seems that would solve the problem. I am curious about the difference in gear ratio also and why its done..

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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What was the point of 2.73 gears in this case? With the Northstar, the 3.11 gears were to achieve better overall gas mileage in the Northstar lineup.

This is true -- but the STS model got 3.71:1 gearing, for better performance. The 3.11:1 gears were used in the SLS, Eldorado, ESC, Deville, and Deville d'Elegance for improved fuel economy. Most reports indicate the STS and other models with the 3.71:1 final drive get anywhere from 2-8 mpg lower mileage than comparable cars with the 3.11:1 final drive. As Bbobynski says, in general, the 2.73:1 gears you have should produce much better economy than the 3.33:1 gears will.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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In theory, yes, the 2.73 should help raise fuel economy. I have observed otherwise at times though. If you look at what Bruce got with his 1992 STS, he got between 25-27. I have never gotten that. I have had two 1991 DeVille's and a 1993 DeVille. On all three, I have noticed a fuel economy of 21-23 at best. I have also noticed that if I put the transmission in 3rd gear when driving at 55-60 mph in hilly areas or against winds it gets better fuel economy than in 4th gear. It seems to me that in these conditions the engine is turning too few rpm's to keep up speed. When using cruise it practically holds the pedal to the floor up hills or against wind to keep up speed. When it's put in third gear, it has an easier time holding speed and doesn't have to open it wide up. Does this make any sense?

Here's another example. I have a friend who has a Dodge Dakota with a 3.9L V6 and 4 speed automatic. It has a 3.55 final drive ratio. Driving at highway speeds, the engine isn't turning fast enough to keep up speed. It constantly shifts in and out of overdrive. For this reason it gets poor fuel economy. Other Dodge Dakota owners who had the same problem and switched to 3.91 gears. Because they had no problem holding speeds on the highway, the trans doesn't shift in and out of overdrive all the time anymore. For this reason, the fuel economy goes up.

I of course would like to switch to the 3.33 gears for the extra power to the wheels. Does the trans have to be removed to change the gears or can it be done in the car?

K.O.T.

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Even prevailing winds affect the fuel economy significantly. I drive to the western side of Michigan and northern Michigan frequently. I always get better fuel economy eastbound and southbound compared to west bound or north bound. By 2 or 3 MPG sometimes...!!!

Can't underestimate that! When I was in college (in southwest Virginia), and my folks lived in Knoxville, TN, I drove down there and back probably 5-7 times. Virginia Tech was 2200 ft. in elevation, and Knoxville was around 800 ft. Despite driving downhill to Knoxville, I would always get measurably better fuel mileage driving to Virginia, because it was ENE, and prevailing winds are always from the southwest in this area.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Jason

Didn't the STS get the steeper gears over the SLS because the difference in the camshaft profiles, or was that ... as one would say; " just one piece of the puzzle?"

Also, just recently I averaged 28.0 MPG per the DIC in my 01 STS using regular fuel on a 200 mile trip with a few moderate hills. Of course, a couple of WOT's would drop that number I'm sure.

Thanks!

Jim

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Didn't the STS get the steeper gears over the SLS because the difference in the camshaft profiles, or was that ... as one would say; " just one piece of the puzzle?"

Jim, I think that's one piece of the puzzle. The LD8 engine (standard Seville, Eldorado, Deville) has a better torque curve than the L37 engine does (STS, ETC, Concours, DTS). Partly for that reason, the 3.71 gears were installed with the L37 engine. I think Bbobynski has said before that if you combined an L37 engine with the 3.11 transaxle, it would be slower than an LD8 car. The torque curve of the LD8 engine can handle the 3.11 gear better than the L37 engine can. The benefit to SLS owners is slightly better mileage (I get 20/30). The benefit to STS owners is slightly better acceleration performance (mine is 6.7 seconds...most STS's are a tick faster than that).

In the end, as we know, the two drivetrains are so close in performance, any one vehicle might perform better than another. But for this discussion, yes, those are the differences, and are just a piece of the puzzle as you say.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Pretty much what I figured.

I prefered the grunt of the SLS at first, but when you drive the two you do notice that the SLS ran out of steam before the STS.

It was really the exterior look and the suspension differences is what sold me on the STS.

Both are good automobiles.

Jim

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I prefered the grunt of the SLS at first, but when you drive the two you do notice that the SLS ran out of steam before the STS.

It was really the exterior look and the suspension differences is what sold me on the STS.

While I've never driven an STS, the styling had the opposite effect on me. I prefer the more subdued, traditional look, of the SLS, with the brightwork accents, the stand-up hood ornament, and the "cleaner" look (to me) of the lower body panel treatments (I don't like plastic cladding).

I guess those comments really only apply to the 92-97 generation Sevilles. The 98-04 generation really tightened the two cars up, and most of the differences lie solely in interior trim and powertrain options I believe.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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There is some obvious confusion here regarding gear ratios! If you have a 2.73 ratio that means that the drive axles turn 2.73 times to make the tires do one complete revolution. A 3.33 gear ratio per se is going to HURT gas mileage because the engine has to do more, to get the same from the tire. The car with the 3.33 ratio will be QUICKER because it will make the engine rev sooner in it's powerband, but not FASTER. There is a clear difference bewteen quick and fast. The car that has the 3.33 ratio will be slower by terms of top speed only because that ratio will "red line" the engine sooner. Think of horsepower versus torque. Torque is for quickness, and horsepower is for speed. The circumference of the tire makes a huge difference as well. Those 20 inch wheels will help your fuel economy and top end, but will sacrifice the 'quickness" of the car as those wheels reduce the final drive ratio. As far as I'm concerned, none of us needs to travel at speeds over 100 on the roads we have today. I want the higher gear ratio and smaller tires because it gives me a boost in the 0-30, 0-60, and to a lesser extent, the 1/4 mile times of my car. The NS is not an engine that has a wealth of low end torque. So I want to find a way to get the engine to rev to it's torque band sooner, and a higher gear ratio does that. My roots lie in drag racing, so I know all about gear ratios and tires. E-mail me if you have any questions.

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The car with the 3.33 ratio will be QUICKER because it will make the engine rev sooner in it's powerband, but not FASTER. There is a clear difference bewteen quick and fast. The car that has the 3.33 ratio will be slower by terms of top speed only because that ratio will "red line" the engine sooner.

This is true only if the top speed is redline-limited, unlike most of our cars. Our cars are either drag-limited or electronically limited.

I believe the STS is electronically-limited to 150 mph. In general, I think it's drag-limited to pretty much the same speed. Numbers of 145-150 mph seem to float around. If you put a numerically-lower gear in the transaxle, like a 3.11 vs. the stock 3.71, your theoretical top speed would be higher, but the engine doesn't make the power to get there anyway. Your theoretical top speed would be like 200 mph with the 3.11 final drive, but I bet your drag-limited top speed would actually be LOWER than with the 3.71, because you're not letting the engine get into its "sweet spot" as you near top speed.

On the old board Bbobynski has mentioned that if you removed the electronic limiter from an SLS (which has a 3.11 final drive), its top speed should be between 130 and 140 mph. The STS (3.71 final drive) has a top speed, as we know, of closer to 150 mph. In theory, what you say is true, but you have to account for aerodynamics.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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I do not think that either Seville, in stock form could hit 150MPH, but it is just my opinion.

I remember an article from Motor Trend I think it was from probably a decade ago. It was a top speed test of a bunch of different cars. They did test a mid-90s STS and a mid-90s ETC. They both crested 145 I think, but I don't think either quite hit 150 mph. On a downhill stretch, you'd probably see 150, but I think in general, they're good for an honest 145 mph.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Do you personally think they can hit those speeds? I was thinking 135 best. I ask for your opinion because I trust it more than Motor Trend. I have often questioned the "numbers they post" because quite often, they don't match up to what they claim versus what we know, and that is taking into account the numbers they give themselves. They often make claims that just don't match up to their own numbers. Not to mention the fact that this mag has been "anti-American" regarding cars, for as long as I can remember. So what is your opinion on it, based on your knowledge? For that matter, what is the forums take on this? I trust your opinion moreso than that of journalists.

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Nice to know my car can go that fast. Now all I have to do are get some good, Z-rated tires. Guru, I asked my service dude at my dealer about reflashing to PCM for the Z-rated option (provided I had the tires) and he said they could do it. Do they use the Tech 2 to do this, or something else. Just curious. Thanks.

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The Motor Trend top speed article was from the summer of 1995. The ETC did 147, and the STS did 148. A comment was made of the STS handling better at high speeds; feeling like the rear of the car was more stable. I have ridden in an 01 STS that had 152 on the digital speedometer. Conditions were cold and high baro. I have driven a 96 STS that buried the speedometer, however it wasn't quite in stock form. If the 98 STS's did 145 + /- 2mph on a hot, humid, low baro day, why shouldn't the 1st gen models, and later 2nd gen models do better in ideal conditions? A 98 STS of all years for top speed testing!..for goodness sake, that has been the slowest year of the Northstar!

As for the gear swap in the Deville: Using a 3.33 may make you even more aware of the torque of the 4.9, in fact you will enjoy the extra snap! You should SERIOUSLY have a limited-slip differential installed at the time of the gear swap, to alleviate the problem of the right front tire spinning out of control!! Maybe your car has a limited-slip setup, but if it doesn't, get it installed. I drove a 92 STS for a while. It had a 4.9 and generally gave 24-26mpg.

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You know they say that the 1998-99 models were the slowest, but mine seems pretty fast compared to other years that I've seen. I get more compliments on my acceleration from people driving Camaros, and Mustangs than I do from anyone else. It really depends on the particular car, and the condition of the drivetrain, more than anything else. If the car was broken in properly, and well maintained, it will be faster than one that was not broken in properly and had the crap beaten out of it. Maybe the car wasn't broken in properly when they tested it, or it was made slightly less powerful due to variations in manufacturing. This could all be possible, you never know.

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The Motor Trend top speed article was from the summer of 1995.  The ETC did 147, and the STS did 148.

Ahh, yes, thanks for that. I knew it was from the mid 1990s sometime.

The '98 STS, on average, is really as fast as anything else. Look again at the chart:

http://jadcock.oldsgmail.com/cadsls/images/caddyperf.gif

There are four '98s tested. 1 is 6.7 seconds to 60, and 3 are 6.8 seconds. That's right smack in the middle of the average for a Northstar Seville. Some are faster (like the '95 STS @ 6.5 seconds), and some are slower (like the '93 STS at 7.3 seconds). But again, theoretically, any 93-94 STS SHOULD be exactly the same. Any 95-97 STS SHOULD be exactly the same. Any 98-04 STS SHOULD be exactly the same. But there are always variables out of our control, so you really have to look at averages. In addition, every single Northstar engine (1995 and later) is claimed to produce 300 hp and 295 lb*ft of torque. There really isn't a "slow" year...they're all the same...save for manufacturing and maintenance variables that are hard to control, or know.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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That certainly is a neat chart for acceleration! I am purely surprised at the 93 being the slowest. Again maybe simple variables, but my experience driving the 93 STS's felt as though they gave the most aggressive acceleration, torque steer, etc. The 95-97's certainly have been proven powerful. I was just generalizing the 98's to be the slower year, from reading posts on the board, along with seeing them accelerate. That's all. I remember a 95 STS race a 98 STS numerous times, and the 95 consistently won by a considerable margain. Likewise, a 97 STS took a 99 STS by a good amount, too. I just thought about those cases and compiled those experiences with people writing on the board making comments of how the 98-99 years didn't feel as strong. The Guru technical explanations to these comments were 98+ cars have: extra tire rolling resistance, extra wind drag, 200lbs more weight, motor mounts that 'soaked' up more power, 98+'s being designed to feel more refined than 92-97, etc. I'm sure it becomes a pick-of-the-draw considering tires, maintenence, actual weight, weather, etc. All things considered, should it not be possible for an STS to achieve 150mpg in ideal conditions? I have seen how the same STS (not mine) would do 153 one night (cold, possible tail wind, high baro), and only 143 a few days later. At any rate, they certainly do well, considering the mass. I drove a 94 STS between Mack, CO and St. George, UT at speeds reaching 145mph at times. (I know it was stupid, yes) It was loaded and following a Jaguar S-type driving at similar speeds. But it did have enough power to run like that. Thankfully still alive, it was a true STS experience, lol. They sure can be fun vehicles! Now, a parent like you, Jason, these driving styles are out of the question. You agree?

On a side note, maybe not even an interest on a cadillac board...I have been lucky enough to find two 135mph limited Stratus cars, a 96 (smashed) and 97 (replacement) both having 41TE transmissions (like your van) and 3.91 gearing. Both were daily drivers, having Mitsubishi 2.5L 24V V6 engines-the 97 having more output with larger OEM exhaust, bigger OEM TQ converter, etc. In good states of tune and ideal conditions, they both have cleared 130mph. Again, conditions play a variable. The weights were 3320 and 3460. I was surprised they were able to conquer these speeds! Is the deep 3.91 gearing the factor?..low wind resistance..any ideas? Oh well... What was the top speed of a 92 STS 4.9? What would a 91-93 Deville theoretically do with 2.73 gears? w/ 3.33 gears? This is an intriuging thread. It sure is fun to discuss these things! Back to the Motor Trend acticle from 6-95. The writers/drivers commented that the top end power of the Northstars pulled away from the helicopter (no clue what kind) they were using, leaving the copter in the distance! Isn't that something for a luxomobile?!

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Yes, the '98+ cars are *generally* a touch slower because they are heavier with larger tires and the same engine output. As such, any 98-04 Seville should be equal to the others. But manufacturing variables are certainly out there. For instance, check out that one SLS that did 0-60 in 6.7 seconds. That's quicker than a good many of those STS cars...which "should not" happen, on paper. But that SLS was just at the quick end of the "tolerance", and those particular STSes were probably at the slow end. For any given car, it's best to get actual track times, or it's just bench racing.

For hearing of how poor the 41TE transmissions are, I'm thoroughly impressed with ours. It's a fully electronic and adaptive transmission that was introduced ahead of its time. In fact, I believe it was the first fully adaptive automatic transmission. It was introduced back in 1990 -- 15 years ago. There have certainly been refinements since, and since 2000, with the use of Chrysler ATF+4, reliability is supposedly up to prime time. Ours shifts very nice at 40,000 miles. WOT upshifts are quicker and more positive than they ever were with the 4T80-E in the Seville. As far as how those Statuses got that high, I don't know. I'd say those 24V engines are probably pretty high revving. I'm sure the 3.91 gearing helps a lot. If they were saddled with 3.62 gears (like from our 3.3L minivan), I'm sure the top speed would suffer. We have an '04 Stratus at work with the Chrysler 2.7L 24V V6 and it's a really quick car. Not as fast as the Seville, but it'll still rip off great acceleration. Upshifts are positive and firm at about 6200 rpm.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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I think the earlier 41TE transmissions may have received some of that bad rap from thw incorrect fluid being used, too. The recommended fluid of ATF+3 was often times ignored for Dexron/Merc. I have had good luck with them, when the proper ATF was used. Yes, the ATF+4 is a wonderful group-III base, which helps a lot as opposed to the ATF+3 group-I. There was a recent article that certain Mercon-V brands can be safely substitued in place of ATF+3. I did this on the Stratus, and it has been working better, especially in the cold, to my amazement! It is a group-II/III blend (syn blend), and it has been working swell! The low temp performance and high temp heat transfer are much better, amongst much stronger HTHS and oxidation stbility. I've read and tested the performance of the 2.5 Mitsu V6 compared to the Chrysler 2.7, in the Stratus cars it is nearly the same. The current Stratus has a best 1/4 of 15.5@92mph, where the other never went below 16.2@88. Both in stock trim with K&N panel. Anyway....I've read a long life is easy to get from a 41TE, with lots of fluid/filter changes and a trans cooler for heavier usage. This Stratus has Autostick, which is a novelty-like fun feature.

From driving LD8 cars and L37 cars, the performance is really a toss-up. They give far different acceleration feelings, but the end result is nearly the same. I currently tend to like the LD8, because of the added MPG. The L37 cars I've driven usually returned 18-21hwy, yet the LD8's 26-31hwy! That is hard to beat, for the performance, size of the car and economy.

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The only serious problem Chrysler has with the 41TE is that is does not have OEM Limited-Slip, although that is something the blasted auto makers have failed to do.....they don't even OFFER Limited-Slip an option on FWD vehicles. Save money?....whatever. It should have been an option for the past 20 years. That's just my steam. LOL. Then we have to find an aftermarket limited-slip maker and have it installed for our aweful wintery conditions. Guru has even agreed in some context to this, and he has said there are some good aftermarket units available. There are.

As you said, the high revving of the Stratus must help. It shifts around 6600rpm's. It's power delivery seems very much like a Catera-not much off the line but a lot of top-end/high rpm pull. In this application, a K&N panel replacement has shown to be a benefit. Hmmm, I think Catera's were given 3.91 or 3.92 gears, too. They have good top-end speed and performance. Know what the top speed of a Catera is? I thought ~135mph, maybe more. It's made for the autobahn, right? I kind of see a trend with the deeper gearing related to high top speeds. So, that is what Guru meant by saying the 4.11 gears in an STS would be best for top speeds. Huh?

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