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2001 Seville STS
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16 Posts
Discussion Starter #1
I have an 05 STS 4.6 Rwd. I saw that later years of the sts have a 6 speed. I was curious if I swap the tcm over and shifter, if I could slap a 6 speed trans in my car? Now I looked at tcm and shifter images on ebay, the connections look about identical from what I can tell. I was wondering if I'm the only one thats had this thought. I know there will be a lot of nay sayers, "why do you need it, it doesn't help much" blah blah blah. I want the 6th gear to have a lower rpm while doing 80-85mph with traffic these days, maybe get that extra 1 mpg its rated to provide, boost from 18 to 19 or 19 to 20. And maybe it would help more in that 70-75mph mark. Anything helps, most of the time I'm lucky to hit 300 miles per tank, thats just not enough, makes for short road trips. My trans isn't the silkiest smooth thing anymore, just curious before I swapped it out if I could do a worth while swap, bring it into the future a couple years.
 

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08 STS4 V8 1SG & 04 SRX4 V8 & 01 Monte SS
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5,017 Posts
the best answer you'll get is "nobody knows"
I'm not aware of anyone doing this kind of swap, many would argue that the cost of the work would take a really long time to get recouped by your 1 mpg extra.

The ticket for mileage would be to swap in the 2.71 gear ratio differential assuming you have the common 3.23 ratio rear end. This would net you the most gains at likely the least cost.
The big con here is you'd have to get the speedo corrected as the car would not be calibrated for that ratio so it would be off by a lot.

FWIW at 67mph my 6spd is just about 2,000 RPM, anything above 2k rpm and your mileage drops really fast.
 

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2001 Seville STS
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16 Posts
Discussion Starter #3
Thanks for the input. A trans is less than $600 and a tcm is relatively cheap, figured it would be worth a shot. If nothing else sell the components and put it back to stock. I found out on a road trip that putting non ethanol gas increased my mileage and power by almost 2mpg cruise set at 76. With my 5 speed 80mph is roughly 2.6- 2.8k rpm. I believe 70 is almost 2200rpm, so anything lower would be great, I just don't want to lose any off the line speed and such. I have a YouTube channel going "2005llyduramax". If nothing else, the swap might make for interesting content. I wanted to do head studs over the winter since this is only a summer car for me as rwd in Minnesota is pretty useless in the winter, and the fact the car is pretty rust free I want it to stay that way. I guess if no one has anything else to contribute to the topic, I'll keep ya'll posted as my project progresses.
 

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2008 STS Northstar RWD, 2000 Trans Am WS6 6 spd, 1968 LeMans convertible 6.6
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268 Posts
But still, how long will it take for aprox 1 MPG to recoup that $600? Run pure gas like you found. But hey, go for it.
 

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08 STS4 V8 1SG & 04 SRX4 V8 & 01 Monte SS
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5,017 Posts
Indeed. Pure gas has “more gas per gallon“ and less filler so you should expect a gain in MPG.

the final drive ratio from a rear diff Swap will reduce rpm at speed much more than the 6th gear from a trans swap will get you.

why do you want to do head studs? The LH2 Northstar does NOT have any head gasket issues like the FWD variants of the engine did prior to 2002 model years.
 

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2005 STS V-8 1SF Magneride HUD/ACC, 2008 Escalade ESV 1SZ Y91 Luxury Edition
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1,474 Posts
I drive my RWD STS in the winter not far from you. It's far from useless with just a quality all-season tire on it. As far as rust, run it thru a car wash with underbody flush every couple weeks (just like you SHOULD be doing with ANY vehicle in these climates)
 

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2001 Seville STS
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16 Posts
Discussion Starter #7 (Edited)
I really don't trust the northstar head gasket holding capability after cracking the block on my 2001 Seville. And after seeing all the Cadillac DTS models blow gaskets all the way up till it changed over to the ct6 or xts which ever took over for it. I love the engine and just want a bullet proof platform that I don't ever have to worry about that dreaded engine hot or low coolant to come on do to head gasket failure, plus I'd like to find a tuner to go in and add a bit more power, maybe from the guy that sell the head stud kit since they started to offer tuning for other models.
To go with driving it in winter, I just want my car to stay its 95% rust free that it is. She's really shaping up to be the best $700 car I've bought.
For those that haven't heard, I saw this car I have now on FB marketplace for $1000 with a blown engine and estimated 130k on the clock. I went and checked it out and it was clean as a whistle from what I could tell. So I offered $700 and he took it. I believe the timing let loose from the sounds it was making cranking over. I found an engine with 109k for $800. The car had like new tires on it so that was a nice score. Plus when I threw a battery in the car the odometer read 103.5k. So now I've got a screaming deal of a Cadillac, clean low mile and just a ripper and a trooper. Now it has 110k in just 2.5 months. Drove to Michigan and back, raced my cousins BMW wagon and have done a nice 110mph down the freeway keeping up with traffic like it wasn't even trying. Man I miss things of my Seville, but this thing is just fun to drive. Lol
So far I put new rear mag shocks on the rear, newer front tires cause front ones were weather checked and flat spotted horribly after sitting for 2 years with a bad engine. A back left tail light, a bunch of Led's
A new motor mount, air filter, fuel filter, spark plugs. Had to change out the radio as it went bad, $200 total between dealer and cost for radio. A new knee bolster panel $10
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589030
(someone busted the mounting tabs off old one) and a new drivers seat button panel (tabs were busted on that also) $35. And for the rare find,, I got some body molded mud flaps made for the chinese model SLS of our lovely STS ordred from China and installed nicely for $38. Someone backed into my beauty a month after I got it, Did $5500 worth of damage, almost got totaled but I settled for $4560 from insurance. Had an in with a local body shop, let me buy all my own parts, They did phenomenal job and I came out with literally a couple dollars in my pocket, Now she's like new again and better than ever. Pic go as this 1. After Accident
2. After Repair
3. Before Accident
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2006 STS 1SG & 2007 STS 1SE
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13 Posts
I have an 05 STS 4.6 Rwd. I saw that later years of the sts have a 6 speed. I was curious if I swap the tcm over and shifter, if I could slap a 6 speed trans in my car? Now I looked at tcm and shifter images on ebay, the connections look about identical from what I can tell. I was wondering if I'm the only one thats had this thought. I know there will be a lot of nay sayers, "why do you need it, it doesn't help much" blah blah blah. I want the 6th gear to have a lower rpm while doing 80-85mph with traffic these days, maybe get that extra 1 mpg its rated to provide, boost from 18 to 19 or 19 to 20. And maybe it would help more in that 70-75mph mark. Anything helps, most of the time I'm lucky to hit 300 miles per tank, thats just not enough, makes for short road trips. My trans isn't the silkiest smooth thing anymore, just curious before I swapped it out if I could do a worth while swap, bring it into the future a couple years.
I wouldn't even try. My 2006 has the 5-speed & it's superior.

My wife's 2007 has the 6-speed & it does get great mileage, but it's nowhere near the 5-speed in performance.

Opinions vary & the two cars are very different in other ways. But given the choice, I prefer the 5-speed.
 

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2001 Seville STS
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16 Posts
Discussion Starter #9
I wouldn't even try. My 2006 has the 5-speed & it's superior.

My wife's 2007 has the 6-speed & it does get great mileage, but it's nowhere near the 5-speed in performance.

Opinions vary & the two cars are very different in other ways. But given the choice, I prefer the 5-speed.
I appreciate the input. Are you saying the 5 and 6 speed shifts differently like different gear ratios. Do both your cars share the same differential ratios? I was hoping the 6 speed was just the 5 speed with an extra gear at the end for cruising.
 
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